Genetic and Medical Glossary

Glossary

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There are currently 681 names in this directory
-ploid, -ploidy
a suffix that refers to a chromosome set; humans have two sets of chromosomes and are referred to as being diploid Point mutation: a DNA mutation involving a single nucleotide

46,XY complete gonadal dysgenesis
disorder of sex development (DSD) associated with anomalies in gonadal development that result in the presence of female external and internal genitalia despite the 46,XY karyotype

A
the abbreviation for adenine, a purine nitrogenous base found in the structure of both DNA and RNA

Aberration
characteristic that deviates from the norm

Ablepharon
absent eyelids

Acardiac twin
also known as twin reversed-arterial perfusion (TRAP) sequence; rare and serious pregnancy complication in which blood is delivered from one twin (“pump” twin) to the other twin (“acardiac” twin) by retrograde (backward) flow, meaning that blood is being pumped from the pump twin to the acardiac twin which is depleted of oxygen

Acentric chromosome
a chromosome that does not have a centromere and that is unable to participate properly in cell division; often the result of a chromosomal mutation during recombination

Acro–renal–ocular syndrome
characterized by the association of acral, ocular and renal defects; acral anomalies are defects of the radial ray varying from mild hypoplasia of the distal part of the thumb with limitation of motion at the interphalangeal joint to severe thumb hypoplasia and preaxial polydactyly (thumb duplication), sometimes associated with syndactyly of fingers; renal anomalies vary from mild malrotation to crossed renal ectopia with partial horseshoe anomaly; ocular features include coloboma of the optic nerve, ptosis, and/or Duane anomaly

Acrocentric chromosome
a chromosome with its centromere near one end; includes chromosomes 13, 14, 15, 21 and 22 which all have their centromere very close to the outer tip; they have very short p arms and small round appendages on stalks, called ‘satellites’

Acrocephaly
turricephaly (tall head) with the top of the skull assuming a cone shape

Adactyly
absence of all phalanges of all digits of a limb and associated soft tissues

Agangliotic megacolon
Hirschsprung’s disease

Agenesis
absence of

Agenesis of the corpus callosum
partial or complete absence of the area of the brain that connects the two cerebral hemispheres; can be inherited as an autosomal recessive trait, an X-linked dominant trait, or be caused by an infection or injury during the twelfth to twenty-second week of pregnancy

Ala nasi, cleft
notch in the margin of the ala nasi of the nose

Alacrimia
congenital inability to produce tears

Alagille syndrome
rare genetic disorder that can affect multiple organ systems of the body including the liver, heart, skeleton, eyes and kidneys; common symptoms usually develop during the first three months of life and include blockage of the flow of bile from the liver (cholestasis), yellowing of the skin and mucous membranes (jaundice), poor weight gain and growth, severe itching (pruritis) and pale, loose stools

Albright hereditary osteodystrophy
genetic condition characterized by short stature, bone and tooth abnormalities and cognitive/behavioral problems

Alfi syndrome
•9p Deletion Syndrome, usually where the breakpoint is found at 9p22; patients usually have various craniofacial abnormalities, globalized hypotonia (low muscle tone)/delayed gross motor development, seizures, GERD, constipation, cardiac defects, scoliosis, kidney reflux and/or sleep disorders

Allele
a form of a gene at a locus (site); each locus in an individual’s chromosomes has two alleles, which may be the same or different

Alobar HPE
complete lack of separation between the right and left halves of the forebrain

Alpha-globin gene (a–globin gene)
part of the gene cluster located on chromosome 16 spanning about 30 kb; deletion of both genes causes alpha thalassemias

Alpha–thalassemia (a–thalassemia)
blood disorder that reduces the production of hemoglobin, the protein in red blood cells that carries oxygen to cells throughout the body; common symptoms include mild to moderate anemia, hepatosplenomegaly, and yellowing of the eyes and skin (jaundice)

Alphafetoprotein (AFP)
protein made by the fetus and found in the mother’s blood circulation; high level of AFP, either in the mother’s blood or in amniotic fluid may be associated with the presence of neural tube defects or chromosomal problems in the baby

Alveolar capillary dysplasia
life-threatening disorder of newborns in which the normal diffusion process of oxygen from the air sacs to the blood in the lungs (and then to the heart) fails to develop properly; pulmonary vein is malpositioned in a site somewhat different from its normal position

Ambiguous genitalia
external sexual organs that are not clearly male or female

Amniocentesis
a procedure in which a small amount of amniotic fluid containing fetal cells is withdrawn from the amniotic sac surrounding a fetus; fetal cells, found in the fluid, are then tested for the presence of genetic abnormalities

Amniotic bands
constricted strands of the fluid-filled sac (amniotic sac) that surrounds a baby in the womb that can cause a congenital deformity of the face, arms, legs, fingers, or toes

Anal atresia
congenital absence of an opening at the bottom end of the intestinal tract; also called imperforate anus or anorectal atresia

Anencephalus (anencephaly)
neural tube defect characterized by the failure of the cerebral hemispheres of the brain and the cranium to develop normally

Aneuploid
a cell or individual with one or more missing or extra chromosomes

Angelman syndrome
chromosomal disorder that is inherited from the mother (imprinting) and caused by deletion or abnormal expression of the UBE3A gene on chromosome 15; characterized by severe developmental delays and learning disabilities; absence or near absence of speech; inability to coordinate voluntary movements (ataxia); tremulous jerky movements of the arms and legs; patients have a characteristic happy disposition with unprovoked episodes of laughter and smiling—hence, the term “happy puppet syndrome”

Ankyloblepharon
partial fusion of the upper and lower eyelid margins by single or multiple bands of tissue

Ankyloglossia (tongue tie)
short or anteriorly attached lingual frenulum associated with limited mobility of the tongue

Ankylosis
abnormal stiffening and immobility of a joint due to fusion of the bones

Annular pancreas
abnormal ring of pancreas that encircles the duodenum and often causes intestinal obstruction

Anodontia
congenital absence of all primary or permanent teeth

Anomaly
marked deviation from normal, especially as a result of congenital or hereditary defects

Anophthalmia
absence of the eye, as a result of a congenital malformation

Anosmia
loss of the sense of smell, either total or

Anotia
complete absence of any auricular structures, absent ear

Anterio–posterior
front to back direction

Anterior
frontal

Anti-epileptic drug
medication to control seizures

Antibody
an immune protein (immunoglobulin) that is produced by B cells of the immune system and specifically recognizes an antigen

Anticipation
genetic condition that becomes more severe and/or arises at an earlier age as it is passed through subsequent generations (seen in some trinucleotide repeat mutations)

Antigen
any molecule that is capable of being recognized by an antibody molecule or of provoking an immune response

Antisense
a term referring to any strand of DNA or RNA that is complementary to a coding or regulatory sequence

Aplasia
condition characterized by either partial or total loss of the ability to communicate verbally or to use or comprehend words

Aplastic
underdeveloped; defective development or congenital absence of a limb, organ

Apodia
absent foot

Aprosencephaly
congenital absence of the telencephalon and diencephalon parts of the forebrain

Arachnodactyly
long spider-like fingers and toes, a frequent finding in Marfan syndrome

Arachnoid cyst
benign cysts that occur in the cerebrospinal axis in relation to the arachnoid membrane and that do not communicate with the ventricular system

Arhinencephaly
congenital absence of the olfactory bulbs, tract or nerves

Array comparative genomic hybridization (CGH array)
type of technology that allows detection of chromosome imbalances that too small to be detected through a microscope; works by exploiting the ability of a DNA molecule (or strand) to bind specifically to, or hybridize to, another DNA molecule (strand)

Arthrogryposis
joint contractures that develop before and are evident at birth

Ataxia
the loss of full muscle control of bodily movements

ATR–16 syndrome
16p11.2 deletion syndrome; characterized by developmental delay, intellectual disability, and usually features of autism spectrum disorders

Atresia
absence or abnormal narrowing of an opening or passage in the body

Atretic
of, relating to, or marked by atresia

Atrial septal defect (ASD)
congenital (present at birth) disorder of the heart in which infants have a hole in the wall (septum) that separates the two upper-chambers (atria) of the heart

Atrio-ventricular septal defect (AVSD)
congenital (present at birth) disorder of the heart in which infants have improperly developed atrial and ventricular septa and adjoining valves

Atrophy
wasting away or decrease in size of an organ or tissue in the body

Attention deficit hyperactivity disorder (ADHD)
any of a range of behavioral disorders occurring primarily in children, including such symptoms as poor concentration, hyperactivity, and impulsivity

Autism
developmental disorder that appears by age three and that is variable in expression but is recognized and diagnosed by impairment of the ability to form normal social relationships, by impairment of the ability to communicate with others, and by stereotyped behavior patterns especially as exhibited by a preoccupation with repetitive activities of restricted focus rather than with flexible and imaginative ones

Autoimmune response
an immune response of an organism against its own cells

Autonomic
part of the nervous system that regulates key involuntary functions of the body, including the activity of the heart muscle; the smooth muscles, including the muscles of the intestinal tract; and the glands

Autosomes
the numerical chromosomes (chromosomes 1-22); humans have forty-four —one set from each parent; chromosome 1 is the largest and 22, the smallest

Axenfeld–Rieger anomaly
eye disorder characterized by abnormalities of the front part of the eye (anterior segment) in which the colored part of the eye (the iris) may be thin or poorly developed

Axillary pterigyum
wing–like triangular membrane between the chin and neck

Balanced translocation
rearrangement of the chromosomes with no apparent loss or gain of chromosomal material; hence, a person with this rearrangement is not affected in any way; reciprocal translocation

Bannayan–Riley–Ruvalcaba syndrome
genetic condition characterized by a large head size (macrocephaly), multiple noncancerous tumors and tumor-like growths called hamartomas, and dark freckles on the penis in males; many have intellectual disability or delayed development, particularly the development of speech and of motor skills

Base pair (bp)
measure of the size of a DNA fragment or the distance along a DNA molecule between markers

Bechçet syndrome
disorder that causes inflammation in blood vessels throughout the body, leading to mouth sores, eye inflammation, skin rashes and lesions, and genital sores; may have an autoimmune basis

Beckwith-Wiedemann syndrome
overgrowth disorder caused by paternal duplications (imprinting) of 11p15.5 on chromosome 11; characterized by above-average birth weight and increased growth after birth, an usually large tongue (macroglossia), enlargement of certain internal organs (visceromegaly), and protrusion of a portion of the intestines and abdominal organs through a tear in the wall of the stomach or bellybutton (abdominal wall defects)

Bicornis
refers to a uterus that is divided into two parts, usually separate at the upper end and joined at the lower end

Bicuspid aortic valve
aortic valve in the heart that has two flaps (cusps) that open and close, instead of the three flaps of a normal heart

Bifid uterus
an abnormal uterus that may be either a single or a double organ with two horns, or branches

Bifid uvula
bifurcation of the uvula, an incomplete form of cleft palate

Biliary atresia
condition in which the bile ducts inside or outside the liver do not develop normally, causing bile that normally flows from the liver to the gallbladder to be blocked, which can lead to liver damage and cirrhosis

Bipolar disorder
a mental disorder marked by alternating periods of elation and depression

Blackfan–Diamond syndrome
blood disorder, characterized by a failure of the bone marrow (the center of the bone where blood cells are made) to produce red blood cells

Blaschko’s lines
pattern of distribution of skin lesions or pigmentary anomalies, linear on the extremities, S-shaped curves on the abdomen, and V-shaped on the back; thought to result from genetic mosaicism and interplay of transverse clonal proliferation and longitudinal growth and flexion of the embryo

Blepharochalasis
lax, wrinkled, and baggy eyelid skin

Blepharophimosis
fixed reduction in the vertical distance between the upper and lower eyelids with short palpebral fissures; often associated with epicanthus inversus

Brachycephaly
cephalic index (ratio of head width expressed as a percentage of head length) greater than 81%; shortened antero-posterior dimension (length) of the head compared to width

Brachydactyly of the hand
angulation of a digit toward the anterior axis (radial

Brachytelephalangy
short distance from the end of the finger to the most distal interphalangeal crease or DIPJ flexion point (ie, short fingers)

Brainstem
central trunk of the brain, consisting of the medulla oblongata, pons, and midbrain, and continuing downward to form the spinal cord

Branchial fistula
congenital epithelial cyst that arises on the lateral part of the neck due to failure of obliteration of the second branchial cleft in embryonic development

Branchio–oculo–facial syndrome
autosomal dominant genetic disorder characterized by the proliferation of blood vessels (hemangiomatous) in the lower neck or upper chest, low birth weight, retarded growth and degrees of intellectual impairment; patients may have a pseudo cleft of the upper lip resembling a poorly repaired cleft lip, a malformed nose with a broad bridge and flattened tip, blockage of the tear ducts, malformed ears, lumps in the area of the neck or collarbone and/or linear skin lesions behind the ears

Buschke–Ollendorf syndrome
hereditary disorder of connective tissue characterized by benign, connective tissue nevi (hamartomas) in the skin, and the formation of small, round spots of increased bone density (osteopoikilosis)

C
the abbreviation for cytosine, a pyrimidine nitrogenous base found in the structure of both DNA and RNA

Café–au–lait spots
any of the medium brown spots usually found on the trunk, pelvis, and creases of the elbow

Campomelic dysplasia
syndrome featuring angulation of the long bones such as femur and tibiae

Camptodactyly
inability of the DIP joints and/or PIP joints of the fingers to be extended 180° by either active or passive means

Candidate gene
a gene suspected of causing a specific disease

Cardiomyopathy
chronic disease of the heart muscle

Carrier
a healthy individual who has one normal allele and one defective allele for a recessive genetic disease

Carrier screening
specialized testing to see whether an individual carries a mutated gene for a specific condition

Carrier testing
testing of an individual who is at risk due to a family history to determine if he or she is a carrier of a mutated or faulty gene for a particular genetic condition

Cat eye syndrome (tetrasomy 22 cen-q11)
chromosomal disorder in which the short arm and a small region of the long arm of chromosome 22 are duplicated four times; characterized by distinctive eye (ocular) abnormality that is sometimes present whereby there is partial absence of ocular tissue (coloboma), often affecting both eyes (bilateral); other symptoms include mild growth delays before birth, mild intellectual impairment, malformations of the skull and facial (craniofacial) region, the heart, the kidneys, and/or anal region; down-slanting eyelid folds (palpebral fissures), widely spaced eyes (ocular hypertelorism), misshapen ears with abnormal outgrowths of skin and small depressions in front of the outer ears (preauricular tags and pits), and/or absence (atresia) of the anal canal, with an abnormal passage (fistula) from the end portion of the large intestine (rectum) into abnormal locations

Caudal
at or near the tail or the posterior part of the body

Cebocephaly
obsolete, nonspecific term for a craniofacial malformation that has morphologic features of a monkey

Centromere
constricted part (central region of the chromosome that separates it into its two arms (the p arm and q arm)

Cerebellar vermis
structure of the brain located in the medial, cortico-nuclear zone of the cerebellum

Cerebellum
part of the brain at the back of the skull whose function is to coordinate and regulate muscular activity

Cerebral cortex
surface layer of gray tissue of the cerebrum, frequently called the gray matter

Cerebral dysgenesis
abnormal brain development in which the brain did not fully develop, grew abnormally, experienced incomplete division, or developed with incomplete organization

Charcot-Marie-Tooth disease type 1A
slowly progressive demyelinating peripheral neuropathy characterized by distal muscle weakness and atrophy, sensory loss, and slow nerve conduction velocity; often associated with pes cavus foot deformity and bilateral foot drop; caused by duplication of PMP22 located on chromosome 17 at 17p12

CHARGE syndrome
multisystem disorder caused by mutation or deletion of the CHD7 gene located at 8q12 on chromosome 8 and characterized by the presence of colobomas and cranial nerve defects (C) heart defects, especially tetralogy of Fallot (H), atresia of the choanae (blocked nasal breathing passages (A), retardation of growth and development (R), genital underdevelopment (G), and ear abnormalities and sensori-neural hearing loss (E)

Cheekbone underdevelopment
reduction in size of the zygomatic process of the temporal bone of the skull, which forms the middle and lateral inferior orbital margin; flat cheekbone, zygomatic underdevelopment

Chiari I malformation
occurs when part of the cerebellum is located below the foramen magnum; may develop when the bony space is smaller than normal, causing the cerebellum and brain stem to be pushed downward into the foramen magnum and into the upper spinal cana

Choanal atresia
narrowing or blockage of the nasal airway by tissue

Chondrodysplasia punctata
disorder of cartilage and bone development that occurs almost exclusively in males; abnormality that appears on x-rays as spots (stippling) near the ends of bones and in cartilage)

Chorio-retinopathy
noninflammatory abnormality of the choroid with extension to the retina

Chorionic villus sampling
a procedure in which fetal cells are obtained from an embryonic structure called the chorion and analyzed for the presence of genetic abnormalities in the fetus

Choroid
a thin vascular layer between the sclera and the retina of the eye; supplies blood to the retina and conducts arteries and nerves to other eye structures

Chromatid
either of two threadlike strands into which a chromosome divides longitudinally during cell division; each contains a double helix of DNA

Chromatin
the material of which the chromosomes of organisms other than bacteria (i.e., eukaryotes) are composed. It consists of protein, RNA, and DNA

Chromosomal rearrangement
type of chromosome abnormality involving a change in the structure of the native chromosome and encompass several different classes of events: deletions, duplications, inversions; and translocations

Chromosome
the form in which genetic material is found in the nucleus of a cell; composed of a single DNA molecule that is extremely tightly coiled and usually visible only during the processes of mitosis and meiosis

Chromosome breakpoints
locations on a chromosome where DNA might get deleted, inverted, or swapped around

Chromosome map
a diagram showing the locations of genes on a particular chromosome; generated through analysis of linkage experiments involving those genes

Chromosome mutation
a change in chromosome structure caused by chromosome breakage followed by improper rejoining (eg, deletions, insertions, inversions, and translocations)

Chromosome recombination
process or act of exchanges of genes between chromosomes, resulting in a different genetic combination and ultimately to the formation of unique gametes with chromosomes that are different from those in parents

Cleft lip
fissure in the upper lip that is due to failure of the left and right sides of the fetal lip tissue to fuse; correctable by surgery

Cleido–cranial dysplasia
condition that primarily affects the development of the bones and teeth, resulting in underdeveloped or absent collarbones, delayed closing of the spaces between the bones of the skull, decreased bone density, delayed loss of the primary and delayed appearance of the secondary teeth, unusually shaped, peg-like teeth and misalignment of the teeth and jaws (malocclusion)

Clinical trial
an experimental research study used to determine the safety and effectiveness of a medical treatment or drug

Clone
a molecule, cell, or organism that is an exact genetic copy of another

Cloning vector
a DNA molecule that can be used to transport genes of interest into cells, where these genes can then be copied

Cloverleaf skull
trilobar skull configuration when viewed from the front or behind

Club foot
congenital deformity of the ankles and/or feet in which the foot is turned inward and downward (talipes equinovarus), is angled at the heel with the toes pointing upward and outward (calcaneal valgus), or the front of the foot is turned inward (metatarsus varus)

Clubbing
broadening of the soft tissues of the digital tips in all dimensions associated with an increased longitudinal and lateral curvature of the nails

Clynodactyly
bend or curvature of the fifth fingers (“pinkies") toward the adjacent fourth fingers

Coarctation of the aorta
congenital constriction (narrowing) of the aorta that impedes the flow of blood below the level of the constriction and increases blood pressure

Cockayne syndrome
inherited form of dwarfism caused by a defect in one of the genes involved in the normal repair of DNA (ERCC8 gene for CS-type I mapped to chromosome 5 and ERCC6 gene for CS-type mapped to chromosome 10 at locus 10q11); diagnosed by the presence of three signs: growth retardation (short stature), abnormal sensitivity to light (photosensitivity), and prematurely aged appearance (progeria)

Coding region
portion of a gene's DNA or RNA (exons) that codes for protein. The region is bounded nearer the 5' end by a start codon and nearer the 3' end with a stop codon

Codon
a group of three nucleotides in messenger RNA (mRNA) that represent a single amino acid in the genetic code

Coloboma
hole in one of the structures of the eye, such as the iris, retina, choroid, or optic disc

Comparative genomic hybridization
technology wherein a DNA test sample is competitively hybridized with a reference sample of DNA of known sequence to a DNA microarray, used to detect copy number changes in the test sample

Complementary base pairing
hydrogen bond formation in DNA and RNA that occurs only between cytosine and guanine (in both DNA and RNA) or between adenine and thymine (in DNA) or adenine and uracil (in RNA)

Confined placental mosaicism
mosaicism that is seen only in the placenta but not in the fetus

Congenital
present at birth, not necessarily inherited

Congenital defect
a defect or disorder that occurs during prenatal development

Conotruncal heart
general term for truncus arteriosus, transposition of the great arteries, double outlet of the right ventricle, and tetralogy of Fallot; in truncus arteriosus there is a single outflow tract instead of a separate aorta and pulmonary artery

Consanguine
of the same blood or origin; in genetics, the term implies the sharing of genetic traits or characteristics from the same ancestors (eg, cousins)

Contig
A map that shows regions on a chromosome where neighboring portions of DNA overlap; the map provides information about a large segment of a genome by looking at a series of overlapping clones

Contracture
shortening and hardening of muscles, tendons, or other tissue, often leading to deformity and rigidity of joints

Copy number variants
structural chromosomal aberrations - duplications or deletions of relatively small areas of DNA. CNV are present in virtually all individuals. Most CNV do not cause any hereditary disorders but may affect a person's susceptibility to non-hereditary diseases or influence a person's response to some environmental factors

Cor triatriatum
congenital heart defect in which there is a small extra chamber which results in slowing of the passage of blood from the lungs into the heart (left atrium and ventricle) due to the pulmonary veins (returning blood from the lungs) draining into this extra "third atrium"

Corectopia
pupil of the eye is off-center

Cornelia de Lange syndrome
inherited as an autosomal dominant condition involving the NIPBL gene on chromosome 5 or as an X-linked condition involving the SMC1L1 gene on the X chromosome; characterized by delays in physical development before and after birth, characteristic abnormalities of the head and facial (craniofacial) area, malformations of the hands and arms (upper limbs); and mild to severe intellectual impairment; other symptoms include an unusually small, short head (microbrachycephaly), an abnormally long vertical groove between the upper lip and nose (philtrum), a depressed nasal bridge, upturned nostrils (anteverted nares) and a protruding upper jaw (maxillary prognathism)

Costo-vertebral dysplasia
genetic disorder characterized by spinal column abnormalities, dwarfism and a swollen abdomen; also called Jarcho-Levin syndrome

Cowden syndrome
disorder characterized by multiple noncancerous, tumor-like growths called hamartomas and an increased risk of developing certain cancers

Coxa valga
broadening of the head and neck of the femur

Craniofacial
relating to the cranium and the face

Craniosynostosis
a congenital defect involving the skull bones of a newborn, whereby the bones are fused together at birth and require surgical intervention to allow for normal brain development

Cri-du-chat syndrome
also called 5p minus syndrome or “Cat Cry” or syndrome due to the cat-like cry typical of newborns; chromosomal disorder in which a variable portion of the short arm of chromosome 5 is missing or deleted; most deletions are de nova or sporadic; common symptoms include a distinctive cry that resembles a cat’s mewing sound, characteristic facial features, slow growth, and microcephaly; affected children also exhibit delays in the acquisition of skills requiring the coordination of muscular and mental activities (psychomotor disability) and moderate to severe intellectual disability

Critical region
specific portion of a chromosome or a gene that, when altered in some way (deleted, duplicated, or otherwise mutated), produces a characteristic set of phenotypic abnormalities associated with a particular syndrome or disorder

Cryptophtalmos
absent palpebral fissures, with skin passing continuously from the forehead or eyebrow onto the cheek, often with the presence of a tuft of hair; typically associated with a rudimentary or small globe (hidden eye)

Cryptorchidism
absence of one or both testes from the scrotum

CT scan
computerized tomography that combines a series of X-ray views taken from many different angles with computer processing to create cross-sectional images of the bones and soft tissues

Cutaneous
relating to, or affecting the skin

Cutis marmorata
pink marble-like mottling of the skin caused by exposure to cold temperature or associated with various debilitating diseases

Cyclopia
congenital defect characterized by fusion of the orbits into a single cavity containing one eye

Cysterna magna
one of three principal openings in the subarachnoid space between the arachnoid and pia mater layers of the meninges surrounding the brain

Cystic (renal) dysplasia
abnormal tissue development in the kidney that can be confined to a small portion of one (unilateral or both (bilateral) kidneys arranged in a specific pattern within the kidney or randomly present throughout one or both kidneys

Cytogenetics
the study of chromosome number and structure, including identification of abnormalities

Cytosine (C)
a pyrimidine nitrogenous base found in the structure of both DNA and RNA

Dacryostenosis
stricture or narrowing of a lacrimal duct

Dandy–Walker malformation
brain malformation that occurs during embryonic development of the cerebellum and 4th ventricle and is characterized by underdevelopment (small size and abnormal position) of the cerebellar vermis (the middle part of the cerebellum), cystic enlargement of the 4th ventricle, and enlargement of the base of the skull (posterior fossa)

de Grouchy syndrome (distal 18q)
deletion of genetic material from the long arm of chromosome 18, specifically, the distal section of 18q typically extending to the tip of the long arm of chromosome 18

Deletion
a type of chromosomal mutation in which a genetic sequence is lost from a chromosome, usually through an error in recombination de nova mutation: new or spontaneous mutation not inherited from either parent

Deoxyribonucleic acid (DNA)
the genetic material found in all cells; DNA consists of nitrogenous bases (adenine, guanine, cytosine, and thymine), sugar (deoxyribose), and phosphate

Dermatofibrosis lenticularis disseminata
dermatologic condition in which small papules or disks of increased dermal elastic tissue appear in early life; autosomal dominant inheritance

Dextrocardia
congenital defect in which the apex of the heart is situated on the right side of the body

Di George syndrome (22q11.2 deletion)
chromosomal disorder characterized by heart abnormalities, cleft palate, distinctive facial features, recurrent infections, breathing problems, kidney abnormalities, low calcium levels in the blood, thrombocytopenia, feeding and gastrointestinal problems, or hearing loss; many have delayed growth and speech development, and learning disabilities

Diaphragmatic hernia
birth defect in which there is an abnormal opening in the diaphragm, causing part of the stomach, spleen, liver, and intestines to go up into the chest cavity near the lungs

Dicentric chromosome
a chromosome with two centromeres, usually resulting from an error of recombination

Digital Constriction Ring (amniotic bands)
narrow segment of significantly reduced circumference of a digit

Diploid
a cell or organism with two complete sets of chromosomes; humans normally have two sets of 23 chromosomes, making them diploid

Diplopia
a pathological condition of vision in which a single object appears double

Disomy
a case in which both copies of a chromosome come from a single parent, rather than one being maternal and one being paternal

Distal
situated away from the point of attachment or origin or a central point; located away from the center of the body

DNA replication
process of making an identical copy of a section of duplex (double-stranded) DNA, using existing DNA as a template for the synthesis of new DNA strands

Dolichocephaly
elongated head; cephalic index (ratio of head width expressed as a percentage of head length) less than 76%; increased antero-posterior length of the head compared to width

Dolichocolon
abnormally long large intestine

Dominant gene
an allele or a trait that will mask the presence of a recessive allele or trait; requires only one of the genes to be mutated for the child to be affected (50% chance)

Double helix
The coiled structure of double-stranded DNA in which strands linked by hydrogen bonds form a spiral configuration, with the two strands oriented in opposite

Double outlet right ventricle
congenital heart disease in which the aorta rises from the right ventricle (the chamber of the heart that pumps blood to the lungs), instead of from the left ventricle (the normal pumping chamber to the body)

Down syndrome (trisomy 21)
a chromosomal disorder caused by an extra copy of chromosome 21; symptoms include intellectual impairment, congenital heart defects, low muscle tone (hypotonia); a tendency to keep the mouth open with protrusion of the tongue, and distinctive facial characteristics including a short, small head (microbrachycephaly), upwardly slanting eyelid folds (palpebral fissures), a depressed nasal bridge, a small nose, and a relatively flat facial profile

Dravet syndrome
intractable (uncontrollable) form of epilepsy characterized by unilateral (one-sided) clonic or tonic clonic (grand mal) seizures that may be prolonged (> 5 minutes) or progress to status epilepticus (>30 minutes); myoclonic seizures, often called myoclonic jerks, are common but not always present; most cases caused by mutations of the SCN1A gene on chromosome 2q24

Duane anomaly
disorder of eye movement in which the eye is unable to move outward (toward the ear), and in some cases, inward eye movement (toward the nose) is limited

Duodenal atresia
refers to a blockage in the hollow tube that connects the stomach to the rest of the intestine

Duplication
a type of chromosomal mutation in which a chromosome region is duplicated because of an error in recombination during prophase I of meiosis

Dwarfism
the condition of adults of short stature who are less than 50 inches in height

Dysgenesis
defective development of an organ, especially the gonads

Dysmorphism
derived from the Greek dys – meaning abnormal, disease, faulty, impaired and morphology - meaning structure or form

Dysphagia
difficulty or discomfort in swallowing, as a symptom of disease

Dysphonia
difficulty in speaking due to a physical disorder of the mouth, tongue, throat, or vocal cords

Dysplasia
abnormal cell or tissue growth or development

Dysplastic
alteration in size, shape, and organization of adult cells

Dyspraxia
inability to perform coordinated movements

Dystonia
movement disorder in which involuntary muscle contractions cause twisting or repetitive movements

Ebstein’s anomaly
congenital heart defect in which parts of the tricuspid valve are abnormal

Ectoderm
outermost of the three primary germ layers of an embryo, from which the epidermis, nervous tissue, and sense organs develop

Ectopic
abnormal place or position

Ectopic cordis
congenital malformation in which the heart is abnormally located either partially or totally outside of the thorax

Ectropion
outward turning (eversion) or rotation of the eyelid; usually involves the lower eyelid

Edwards syndrome (trisomy 18)
also known as trisomy 18; a chromosomal disorder in which there is an extra copy of chromosome 18; newborns commonly have small eyes, deformed ears, heart defects, and severe intellectual impairment; mortality is high

Emanuel syndrome
chromosomal disorder in which there is a supernumerary chromosome consisting of a piece of chromosome 11 attached to a piece of chromosome 22 in each cell, expressed as der(22); characterized in infancy by weak muscle tone (hypotonia), failure to thrive, distinct facial appearance, and developmental delay; severe to profound intellectual disability is common

Encephalocele
birth defect associated with skull defects characterized by partial lacking of bone fusion leaving a gap through which a portion of the brain protrudes outside the skull

Endocardial cushion defect
congenital heart condition in which the walls separating all four chambers of the heart are poorly formed or absent

Entropion
inward turning (inversion) of the eyelid margin

Epicanthal fold (epiblepharon)
redundant eyelid skin pressing the eyelashes against the cornea and/or conjunctiva

Epicanthus inversus
fold of skin starting at or just below the medial aspect of the lower lid and arching upward to cover, extend in front of and lateral to the medial canthus

Epignathus
unequal conjoined twins in which the smaller, incomplete parasite is attached to the larger autosite at the lower jaw

Epilepsy
neurological disorder marked by sudden recurrent episodes of sensory disturbance, loss of consciousness, or convulsions, associated with abnormal electrical activity in the brain

Epileptiform
resembling epilepsy or its manifestations

Esophageal atresia
birth defect in which the esophagus (the tube that connects the throat with the stomach) does not develop normally and is separated into two parts, an upper and lower segment one of which end in a blind pouch; the two segments do not connect and the normal passage between the mouth and stomach does not exist

Etiology
the cause or causes of a disease or disorder

Euploid
the normal number of chromosomes for a cell or organism, 46 in the case of humans

Exencephaly
disorder wherein the brain is located outside of the skull, causing the neural tissue to gradually degenerate; usually found in embryos as an early stage of anencephaly

Exome
entire portion of the protein-coding sequences of the genome, omitting introns and noncoding DNA

Exon
a segment of a DNA or RNA molecule containing information coding for a protein or peptide sequence, usually flanked by introns

Exophthalmos
abnormal protrusion of the eyeballs

Exostose
benign outgrowth of cartilaginous tissue on a bone

Expressivity
the degree to which a genotype is expressed as a phenotype

Exstrophy
Eversion of a hollow organ at birth, eg, exstrophic bladder is turned inside out like a rubber glove

Exudative enteropathy
diarrhea that occurs in diseases characterized by inflammation or destruction of intestinal mucosa

Eyelid, cleft
short discontinuity of the margin of the lower or upper eyelid

Face, coarse features
absence of fine and sharp appearance of brows, nose, lips, mouth and chin, usually because of rounded and heavy features or thickened skin with or without thickening of subcutaneous and bony tissues

Face, flat
absence of concavity or convexity of the face when viewed in profile

Failure to thrive
term used to describe a delay in a child's growth or development; usually applied to children up to two years of age whose growth is below the 3rd or 5th percentiles for their age

Faulty gene
a gene that does not function properly

Feingold syndrome
autosomal dominant disorder characterized by microcephaly, distal limb malformations (eg, short middle phalanges of the 2nd and 5th fingers), and esophageal/duodenal atresia; usually associated with learning disabilities are common

Finger, tapered
gradual reduction in girth of the digit from proximal to distal

Fingers, cutaneous syndactyly
soft tissue continuity in the A/P axis between two fingers that lies significantly distal to the flexion crease that overlies the metacarpophalangeal joint of the adjacent fingers

Fistula
abnormal or surgically made passage between a hollow or tubular organ and the body surface, or between two hollow or tubular organs

Fluorescence in situ hybridization (FISH)
a form of molecular cytogenetics; an extremely sensitive assay for determining the presence of deletions on chromosomes, which uses a fluorescence-tagged segment of DNA that binds to the DNA region being studied

Focal cortical dysplasia
congenital abnormality where the neurons in an area of the brain failed to migrate in the proper formation in utero; malformation of cortical development, which is the most common cause of medically refractory epilepsy in the pediatric population

Fontanel
“soft spot” on the skull that remains open for the brain to develop and closes during infancy

Founder effect
reduced genetic diversity that results when a population is descended from a small number of colonizing ancestors

Fragile site
small break or constriction of a chromosome that can be visualized after special treatment of the chromosomes (in individuals affected with Fragile X syndrome, a fragile site can often be seen on their X chromosome)

Fragile X syndrome
chromosomal disorder caused by mutations in the FMR1 gene on the X chromosome, resulting in an increased number of copies of a portion of the gene called CGG repeats; the greater the number of copies of CGG, the more likely there will be increased severity of the disorder; disorder occurs mostly in males and is characterized by moderate intellectual disability, motor and language delays, and autistic behaviors; facial features include large head, long face, prominent forehead and chin, and protruding ears

Frame-shift mutation
a DNA mutation involving the insertion or deletion of one of several nucleotides that are not in multiples of three, resulting in a shift of the codon reading frame; usually produces nonfunctional proteins. Normally, the cell reads a gene in groups of three bases. Each group of three bases corresponds to one of 20 different amino acids used to build a protein. If a mutation disrupts this reading frame, then the entire DNA sequence following the mutation will be read incorrectly

Frontal bossing
bilateral bulging of the lateral frontal bone prominences with relative sparing of the midline

G
the abbreviation for guanine, a purine nitrogenous base found in the structure of both DNA and RNA

G-banding
series of dark and light stripes across a chromosome which are produced by treating (staining) the chromosomes with different chemicals in a laboratory

Gain-of-function
mutations that change the gene product such that it gains a new and abnormal function and usually have dominant phenotypes

Gardner syndrome
disorder is characterized by adenomatous polyposis, an association of numerous polyps of the colon with soft tissue tumors, mostly in the brain and thyroid; also present are osteomas, causing susceptibility to colon cancer; congenital hypertrophy of the retinal pigment epithelium, jaw and/or sebaceous cysts

Gastro–esophageal reflux (GERD)
condition in which the stomach contents leak backwards from the stomach into the esophagus (the tube from the mouth to the stomach)

Gene
a portion of a DNA molecule containing the genetic information necessary to produce a molecule of messenger RNA (via transcription) that can then be used to produce a protein (via translation)

Gene expression
the combined biochemical processes (transcription and translation) that convert the linearly encoded information in the bases of DNA into the three-dimensional structures of proteins

Gene families
multiple copies of the same or similar genes in the same genome; copies can be identical and tandemly repeated, or may differ slightly and be scattered on the same or different chromosomes

Gene mapping
determining the relative locations of different genes on chromosomes

Gene pool
the complete assortment of genes present in the gametes of the members of a population that are eligible to reproduce

Gene silencing
any form of genetic regulation in which the expression of a gene is completely repressed, either by preventing transcription or after a messenger RNA (mRNA) has been transcribed

Gene therapy
any procedure that genetically alters the cells of the person to alleviate or treat the symptoms of a disease or condition

Genetic code
information contained in the DNA which is ‘interpreted’ by the cells to produce proteins; nucleotides which make up DNA are described by the letters A (adenine), T (thymine), C (cytosine) and G (guanine); written in a series of these letters in triplicate

Genetic counseling
a discipline concerned with analyzing the inheritance patterns of a particular genetic defect within a given family, including the determination of the risk associated with the presence of a genetic defect in future generations and options for treatment of the existing genetic defect

Genetic screening
the testing of individuals for disease-causing genes or genetic disease

Genetic testing
the use of techniques of genetics research to determine a person’s risk of developing, or status as a carrier of, a disease or other disorder

Genetics
a branch of biology involving the study of heredity

Genome
complete set of genes carried by an individual or all the DNA in the cell or in one of the organelles (eg, mitochondrion)

Genomic imprinting
a genetic phenomenon in which the phenotype associated with a particular allele depends on which parent donated the allele

Genomics
branch of genetics dealing with the study of genetic sequences, including their structure and arrangement

Genotype
the genetic characteristics of a cell or organism, expressed as a set of symbols representing the alleles present at one or more loci (sites)

Genotype-phenotype
phenotype is the constellation of observable traits; genotype is the genetic endowment of the individual. Genotype is the set of genes responsible for a particular trait, phenotype is the physical expression of that trait

Germline mosaicism
when germ cells (sperm or egg cells) have a different genetic makeup from cells in the rest of the body

Germline mutation
heritable change in the genes of an individual’s reproductive cells, often linked to hereditary diseases

Glabella
smooth part of the forehead above and between the eyebrows

Glaucoma
condition of increased pressure within the eyeball, causing gradual loss of sight

Glomerular fibrosis
formation of fibrous tissue in the renal glomerulus as a result of repair or a reactive process

Glossoptosis
posterior displacement of the tongue into the pharynx

Goldenhar syndrome
disorder characterized by hemifacial microsomia (hypoplasia of one side of the face), epibulbar dermoid (benign tumors over the cornea and sclera), and absence or severe hypoplasia of one ear

Gonadal dysgenesis
congenital developmental disorder of the reproductive organs

Gonadogenesis
generation and development of the gonads (organ that produces testes or ovaries)

Gorlin syndrome
autosomal dominant genetic disorder characterized by a broad face, rib malformations, and high susceptibility to basal cell carcinoma, a type of skin cancer

Greig cephalopolysyndactyly syndrome
genetic disorder characterized by physical abnormalities affecting the fingers and toes and the head and facial (craniofacial) area; extra (supernumerary) fingers and/or toes (polydactyly), webbing and/or fusion of the fingers and/or toes (syndactyly) may be present; craniofacial malformations may include a large and/or unusually shaped skull, high, prominent forehead (frontal bossing), abnormally broad nasal bridge, and widely spaced eyes (ocular hypertelorism)

Guanine (G)
a purine nitrogenous base found in the structure of both DNA and RNA

Gyri
convolutions on the surface of a cerebral hemisphere produced by the infolding of the cerebral cortex

Hair whorl, abnormal number
more than two clockwise hair whorls

Hair whorl, abnormal position
hair growth from a single point on the scalp in any location other than lateral to the midline and close to the vertex of the skull

Hair, frontal upsweep
upward and/or sideward growth of anterior hair (cowlick)

Hallucination
Subjectively experienced sensations or false sensory perception in the absence of an appropriate stimulus, but which are regarded by the individual as real

Hamartoma
benign tumor-like malformation resulting from faulty development in an organ and composed of an abnormal mixture of tissue elements that develop and grow at the same rate as normal elements but are not likely to compress adjacent tissue

Hand–foot–genital syndrome
condition that affects the development of the hands and feet, the urinary tract, and the reproductive system; caused by mutations in the HOXA13 gene in chromosome 7 at 7p15.2

Haploid
refers to a cell or an organism with one set of chromosomes

Haploinsufficiency
refers to a protein produced by a single gene copy which is then insufficient for normal functioning, leading to an abnormal or diseased state; responsible for some but not all autosomal dominant disorders

Hemifacial microstomia
congenital disorder that affects the development of the lower half of the face, most commonly the ears, the mouth and the mandible

Hemihypertrophy
condition in which one side of the body or a part of one side is larger than the other

Hemivertebrae
congenital anomaly of the spine that is characterized by the formation of half of a vertebral body, a corresponding pedicle, and a corresponding hemilamina

Heredity
the overall mechanism by which characteristics or traits are passed from one generation of organisms to the next; genetics is the scientific study of heredity

Heterochromia irides
difference in color between the irises of the two eyes

Heterogeneous
not uniform in structure or composition; phenomenon in which a single phenotype or genetic disorder may be caused by any one of a multiple number of alleles or non-allele (locus) mutations

Heterotaxy
abnormality where the internal thoraco-abdominal organs demonstrate abnormal arrangement across the left-right axis of the body

Heterotopia
clusters of neurons placed in unusual places along the lateral ventricles

Heterozygote
an individual with two different alleles at a gene locus (site)

Heterozygous
composed of two alleles that are different

Hexadactyly
presence of an extra digit (ie, a sixth finger or toe)

Hippocampus
elongated ridges on the floor of each lateral ventricle of the brain, thought to be the center of emotion, memory, and the autonomic nervous system

Hirschsprung’s disease
characterized by the absence of particular nerve cells (ganglions) in a segment of the bowel, causing muscles to lose their ability to move the stool through the intestine (peristalsis) and resulting in constipation and obstruction of the colon

Hirsutism
excessive growth of facial or body hair in women

Histone
specialized protein in eukaryotic cells that binds to DNA molecules and causes them to become more compact; also thought to be involved in regulation of gene expression

Holoprosencephaly (HPE)
failure of the forebrain (prosencephalon) to develop normally; instead of the normal complete separation of the left and right halves of the forebrain, there is an abnormal continuity between the two sides; alobar HPE is the most severe form

Homologous
refers to chromosomes that are identical in terms of types of genes present and the location of the centromere

Homozygote
an individual with two identical alleles at a gene locus (site)

Homozygous
characterized by a genotype composed of two alleles that are the same and at the same locus

Horseshoe kidney
congenital partial fusion of the kidneys resulting in a horseshoe shape

Human Genome Project
multiyear genetic research endeavor that sequenced the entire human genome; officially completed in 2003

Hunter–McAlpine syndrome
dominantly inherited syndrome characterized mainly by premature fusion of skull bones, mental retardation, small mouth, short stature and finger and toe anomalies; thought to be caused by duplication of the region 5q35-qter on chromosome 5

Hybridization
a process of base pairing involving two single-stranded nucleic acid molecules with complementary sequences; the extent to which two unrelated nucleic acid molecules will hybridize is often used as a way to determine the amount of similarity between the sequences of the two molecules

Hydrocephalus
condition in which abnormally widened (dilated) cerebral spaces in the brain (ventricles) inhibit the normal flow of cerebrospinal fluid (CSF), resulting in accumulation of cerebrospinal fluid (CSF) in the skull which puts pressure on the brain tissue; treatment involves insertion of a shunt to redirect the CSF for elimination, usually through the stomach

Hydrocolpos
accumulation of mucus or other non- sanguineous fluid in the vagina

Hydronephrosis
distention of the kidney with urine; caused by obstruction of urine outflow

Hygroma
sac, cyst, or bursa distended with a fluid

Hyperacusia
over-sensitivity to loud noises

Hypercalcemia
abnormally high level of calcium in the blood, usually more than 10.5 milligrams per deciliter of blood

Hyperopia
condition of the eye where incoming rays of light reach the retina before they converge into a focused image

Hyperphagia
over–eating, sometimes with insatiable appetite

Hyperpigmentation
darkening of an area of skin or nails caused by increased melanin

Hypertelorism
abnormally increased distance between two organs or parts; usually refers to the eyes

Hyperthyroidism
over active thyroid gland

Hypertrichosis
abnormal amount of hair growth over the body

Hypodontia
congenital condition marked by a less than normal number of teeth

Hypogonadotropic-hypogonadism
pubertal failure similar to Kallman syndrome, but with a normal sense of smell; due to deficiency of the reproductive hormone gonadotropin-releasing hormone

Hypomelanosis of Ito
not a specific entity but rather represents features of many different forms of mosaicism; characterized by unilateral or bilateral hypopigmented macules in whorls, streaks, and patches in a “marble-cake” pattern, variably associated with epidermal nevi, alopecia, and ocular, skeletal, and neural abnormalities

Hypoparathyroidism
Underfunctioning of the parathyroid glands with deficient production of the hormone parathormone which leads to low blood calcium

Hypoparathyroidism, deafness, renal syndrome
autosomal dominant disorder characterized by hypoparathyroidism, sensorineural deafness, and renal disease caused by mutation of the GATA3 gene located on chromosome 10 at 10p15

Hypopigmentation
diminished pigmentation in a bodily part or tissue

Hypoplasia
incomplete development or underdevelopment of an organ or tissue

Hypoplastic
abnormal deficiency of cells or structural elements

Hypoplastic left heart syndrome
form of congenital heart disease in which the whole left half of the heart (including the aorta, aortic valve, left ventricle and mitral valve) is underdeveloped (hypoplastic)

Hypoplastic supraorbital ridges
Flatness of the supraorbital portion of the frontal bones (brow area)

Hypospadias
developmental anomaly of the urethra in which a part of the urethral canal is open on the undersurface of the penis or on the perineum

Hypotelorism
abnormally decreased distance between two organs or parts. Usually refers to the eyes

Hypothyroidism
underactive or inactive thyroid gland

Hypotonia
floppiness, low or underdeveloped muscle tone

Ichthyosis
general term for a family of genetic skin diseases characterized by dry, thickened, and scaling skin; various forms are distinguished from one another by extent of scaling and how widely and where the scaling is over the body, by presence or absence and intensity of reddening of the skin (erythroderma), by the mode of inheritance, and by the character of associated abnormalities

Immunoglobulin A (IgA)
antibody that plays a critical role in mucosal immunity

Imperforate anus
inborn abnormality characterized by the absence (anal atresia) or abnormal localization of the anus; rectum or colon may be connected to the vagina or the bladder by a fistula

Imprinting
‘memory’ held by a chromosome as to whether it was inherited from the mother or the father and chemically ‘stamped’ into the DNA; results in chromosomes or genes located on the chromosomes that behave differently, depending on the parent of origin; faulty genes on chromosome 15 cause both Angelman (from the mother) and Prader-Willi syndromes (from the father)

Incidence
the frequency in which a disorder appears in a given population

Infantile spasms
seizure disorder of infancy and early childhood with the onset predominantly in the first year of life characterized by myoclonic seizures, hypsarrhythmia (abnormal, chaotic electroencephalogram), and mental retardation

Informed consent
the right of patients to know the risks of medical treatment and to determine what is done to their bodies, including the right to accept or refuse treatment based on this information; usually refers to participation in a clinical trial

Infra-orbital folds
Elevated ridge(s) of skin starting well below the medial aspect of the lower lid and curves gradually upward toward and/or across the nasal bridge

Inherited
genetic transfer from parents to children

Initiation codon
also called the “start codon,” a codon, composed of the nucleotides AUG, that signals the beginning of a protein-coding sequence in a messenger RNA (mRNA) molecule; in the genetic code, AUG always represents the amino acid methionine

Insertion
type of mutation whereby a DNA sequence is abnormally inserted into a gene, disrupting the normal function and structure of the gene and leading to a chromosomal abnormality

Interrupted aortic arch
congenital heart malformation in which a small section of the aorta is missing, causing a loss of luminal continuity between the ascending and descending portions of the aorta

Interstitial deletion
deletion that does not involve the terminal parts (ends) of a chromosome

Intestinal atresia
broad term used to describe a complete blockage or obstruction anywhere in the intestine

Intestinal malrotation
congenital anomaly of rotation of the midgut in which the gut undergoes a complex rotation outside the abdomen during development

Intrachromosomal duplication
translocation that occurs within a chromosome

Intragenic
being or occurring within a gene

Intron
a segment of a DNA or RNA molecule that does not code for proteins and interrupts a gene-coding sequence or nontranslated sequence; segment of a gene situated between exons that is removed before translation of messenger RNA and does not function in coding for protein synthesis

Inversion
a chromosomal abnormality resulting in a region of the chromosome where the normal order of genes is reversed or the segment of the chromosome has two breaks and “flips over”; both instances can cause the genetic code to be translated (read) wrong

Isochromosome
chromosome that has lost one of its arms and replaced it with an exact copy of the other arm

Isodicentric
extra piece of chromosome that has been duplicated end-to-end like a mirror image

Isodisomy
two identical copies of one of the parental alleles

Jacobsen syndrome
11q deletion syndrome in which the deletion occurs at the end (terminus) of the long (q) arm of chromosome 11; characterized by delayed development in motor and speech, cognitive impairment, ADHD-like behavior, distinctive facial features, and the bleeding disorder Paris-Trousseau syndrome

Kallman syndrome
genetic disorder defined by delay/absence of signs of puberty along with an absent/impaired sense of smell; caused by isolated deficiency of a key reproductive hormone called gonadotropin-releasing hormone

Karyotype
the complete set of chromosomes possessed by an individual, usually isolated during metaphase and arranged by size and type as a method of detecting chromosomal abnormalities

Keratoconus
non-inflammatory eye (ocular) condition characterized by progressive changes in the shape of the cornea

Kyphoscoliosis
combination of outward curvature (kyphosis) and lateral curvature (scoliosis) of the spine

Lacrimal punctum, absence
No identifiable superior and/or inferior lacrimal punctum (tear ducts)

Langer–Giedion syndrome
genetic condition that causes bone abnormalities (exostoses, short stature and cone-shaped ends of the long bones) and distinctive facial features (sparse scalp hair, rounded nose, long flat philtrum, and thin upper lip); caused by at least two genes on chromosome 8

Leukocoria
white pupillary reflex in which the pupil looks white instead of red when a bright light is shone it; occurs with congenital cataracts

Leukopenia
abnormally low number of white blood cells (leukocytes) in the blood circulation

Library
a cloned collection of DNA created from a specific organism

Lionization
see X-inactivation

Lipoma
common benign tumor usually found in the fatty layer of the skin

Lissencephaly
smooth brain; brain malformation characterized by absence (agyria) or incomplete development (pachygyria) of the ridges or convolutions (gyri) of the outer region of the brain (cerebral cortex); occurs as an isolated abnormality or in association with certain underlying syndromes (eg, Miller-Dieker syndrome)

Locus (pl. loci)
the specific location of a particular gene on a chromosome

Lordosis
increased inward curving of the lumbar spine

Loss of function
gene product having less or no function. When the allele has a complete loss of function (null allele) it is often called an amorphic mutation. Phenotypes associated with such mutations are most often recessive

Low set ears
upper insertion of the ear to the scalp below an imaginary horizontal line passing through the inner canthi and extending that line posteriorly to the ear

Lymphedema
condition in which excess fluid collects in tissue and causes swelling

Macrocephaly
occipitofrontal (head) circumference greater than 97th centile compared to appropriate, age matched, sex-matched normal standards

Macular retinal dystrophy
group of disorders involving predominantly the posterior portion of the ocular fundus; due to degeneration in the sensory layer of the retina, retinal pigment epithelium, Bruch membrane, choroid, or a combination of these tissues

Malformation
an inherited organ defect that occurs during fetal development

Mandible, cleft
midline deficiency of the mandible and some or all overlying tissues

Marker chromosome
structurally abnormal chromosome in which no part can be identified

Medial
term meaning toward the median plane (point in the centre of the organism (where the left-right axis intersects the midsagittal plane)

Medulla
innermost part

Megalocornea
largeness of the corneas

Melatonin
hormone that is produced by the pineal gland and is intimately involved in regulating the sleeping and waking cycles

Meningocele
protrusion of the membranes that cover the spine and part of the spinal cord through a bone defect in the vertebral column

Mesencephalon
part of the brain developed from the middle of the three primary vesicles of the embryonic neural tube, comprising the tectum and the cerebral peduncles

Mesomelic dysplasia
deformity of the middle parts of a limb; disorder of bone growth in which the long bones in the arms and legs are extremely shortened

Messenger RNA (mRNA)
a type of RNA molecule containing the genetic information necessary to produce a protein through the process of translation; produced from the DNA sequence of a gene in the process of transcription

Metacentric chromosome
a chromosome with the centromere located at or near the middle of the chromosome

Metatarsus adductus
metatarsals of the foot are deviated medially (tibially)

Metencephalon
anterior segment of the developing vertebrate hindbrain

Metopic suture
persistent frontal suture, sometimes discernible a short distance above sutura frontonasalis

Microarray
flat surface on which 10,000 to 100,000 tiny spots of short DNA molecules (oligonucleotides) are fixed and are used to detect the presence of DNA or RNA molecules that are homologous to the oligonucleotides

Microcephaly
occipito-frontal (head) circumference (OFC) less than 3rd centile compared to appropriate, age matched, normal standards; decreased size of the cranium and marked posterior sloping of the forehead

Microdeletion
loss of a tiny piece from a chromosome that is too small to be seen readily through a microscope

Microdontia
condition in which a single tooth, pairs of teeth, or the whole dentition, are disproportionately small

Microduplication
gain (repeat) of a tiny piece from a chromosome that is too small to be seen readily through a microscope

Micrognathia
small chin; reduced length and width of the mandible when viewed from the front but not from the side; shortening and narrowing of the mandible and chin

Micromelia
Condition of having disproportionately short or small limbs

Microphthalmia
congenital malformation of the globe, a birth defect of the eye

Microstomia
abnormal smallness of the mouth

Microtia
small ear; some ear components may be missing or disfigured; median longitudinal length of the ear is more than 2 standard deviations below the mean median longitudinal length;

Midface retrusion (midface hypoplasia)
posterior positioning and/or vertical shortening of the infraorbital and perialar regions, or increased concavity of the face and/or reduced nasolabial angle; underdevelopment of the maxilla (upper jaw) in length (decreased midface height) or depth

Miller–Dieker syndrome
condition characterized by a pattern of abnormal brain development (lissencephaly), causing severe intellectual disability, developmental delay, seizures, abnormal muscle stiffness (spasticity), weak muscle tone (hypotonia), feeding difficulties, and distinctive facial features; caused by caused by a deletion of genetic material near the end of the short (p) arm of chromosome 17

Missense mutation
single base pair substitution (point mutation) that results in the translation of a different amino acid at that position, usually rendering the resultant protein nonfunctional because the base change or substitution results in a codon that causes insertion of a different amino acid

Molecular genetics
the branch of genetics concerned with the central role that molecules, particularly the nucleic acids DNA and RNA, play in heredity

Monosomy
a genetic condition in which one chromosome from a homologous chromosome pair is missing, producing a 2n-1 genotype; usually causes significant problems in phenotype, when a particular chromosome appears only once instead of twice in cells

Morphology
science of structure and form of organisms without regard to function

Mosaicism
a condition in which an individual has two or more cell populations derived from the same fertilized ovum, or zygote; some cells have an abnormal or unusual genetic or chromosomal makeup while the rest of the cells in the body have the usual genetic or chromosomal constitution

Mouth, downturned corners (carp mouth)
oral commissures positioned inferior to the midline labial fissure

Mowat–Wilson Syndrome
genetic disorder characterized by intellectual disability, distinctive facial features and seizures; other congenital anomalies occur in some individuals, including Hirschsprung’s disease (40-50% of individuals), heart (cardiac) defects, kidney (renal) abnormalities, male genital abnormalities and short stature; caused by an abnormality in the ZEB2 gene located on chromosome 2 at 2q22.3 and is usually the result of a new genetic change (mutation) in the affected person

MRI
Magnetic resonance imaging

Müllerian ducts
pair of ducts parallel to the Wolffian ducts and giving rise to the fallopian tubes, uterus, cervix, and upper portion of the vagina

Mutant
a trait or organism different from the normal, or wild-type, trait or organism seen commonly in nature; mutants can arise either through expression of particular alleles in the organism or through spontaneous or intentional mutations in the genome

Mutation
a change in the genetic sequence of an organism, usually leading to an altered phenotype

Myelin
soft white material of lipid and protein that is secreted by oligodendrocytes and Schwann cells and forms a thick sheath about axons

Myelin sheath
The cover that surrounds many nerve cells and helps to increase the speed by which information travels along the nerve

Myelination
development, or formation of a myelin sheath around a nerve fiber

Myoclonic seizure
seizure characterized by jerking (myoclonic) movements of a muscle or muscle group, without loss of consciousness

Myoclonus dystonia
movement disorder that typically affects the upper half of the body such that quick, involuntary muscle jerking or twitching (myoclonus) occurs, usually restricted to the arms, neck, and trunk

Myopia
nearsightedness

Nail, bifid
digit with two nails, with at least some soft tissue between them

Nares, anteverted
anteriorly-facing nostrils viewed with the head in the Frankfurt horizontal and the eyes of the observer level with the eyes of the subject; tip of the nose is upturned and is positioned superiorly to the nasal base, allowing the nares to be easily visualized from the front

Naris, single
one external opening of the nose; may occur in the midline or on one side and be symmetrical or asymmetrical

Naris, supernumerary
more than two nares

Nasal bridge, depressed
posterior positioning of the nasal root in relation to the overall facial profile for age

Nasal bridge, wide
increased breadth of the nasal bridge

Nasal cartilage, absent
lack of a palpable nasal cartilage; there may also be deficiency of the nasal bone, often related to holoprosencephaly

Nasal tip, overhanging
positioning of the nasal tip inferior to the nasal base

Neck webbing
paravertically oriented fold of skin on the postero-lateral aspect of the neck, usually extending from the mastoid region of the skull to the acromion, most noticeable in frontal or posterior view

Nephroblastomatosis
clusters of microscopic blastema cells, tubules, and stromal cells at the periphery of the renal lobes in an infant; believed to be a precursor of Wilms’ tumor

Neural tube
the embryonic precursor to the spinal cord and brain, which normally closes at small openings, or neuropores, by the twenty-eighth day of gestation in humans

Neuroblastoma
childhood form of cancer that arises in the adrenal gland or in tissue in the nervous system that is related to the adrenal gland

Neurofibromatosis I (NF1)
genetic disorder characterized by the development of multiple noncancerous (benign) tumors of nerves and skin (neurofibromas) and areas of abnormally decreased or increased coloration (hypo- or hyperpigmentation) of the skin; areas of abnormal pigmentation typically include pale tan or light brown discolorations (cafe-au-lait spots) on the skin of the trunk and other regions as well as freckling, particularly under the arms and in the groin; caused by mutations of gene on the long arm (q) of chromosome 17 at 17q11.2

Neurofibromatosis type II
characterized by benign (noncancerous) tumors of the nerves that transmit sound impulses from the inner ears to the brain; associated symptoms may include disturbances of balance and walking (gait), dizziness, headache, facial weakness or numbness, ringing in the ears (tinnitus), and/or progressive hearing loss; caused by mutations of a gene on the long arm (q) of chromosome 22 at 22q12.2

Noncoding DNA
a strand of DNA that is unable to make protein because it lacks the necessary information

Nondisjunction
the improper division of chromosomes during anaphase of mitosis or meiosis, resulting in cells with abnormal numbers of chromosomes and sometimes seriously altered phenotypes

Nonhistone proteins
a heterogeneous group of acidic or neutral proteins found in chromatin that may be involved with chromosome structure, chromatin packaging, or the control of gene expression

Nonsense codon
another term for a termination or stop codon (UAA, UAG, or UGA)

Nonsense mutation
a DNA mutation that changes an existing amino acid codon in a message to one of the three termination, or stop, codons; this results in an abnormally short or truncated protein that is usually nonfunctional

Nose, absent
complete absence of all nasal structures

Nose, bifid
visually assessable vertical indentation, cleft, or depression of the nasal bridge, ridge and tip

Nuchal skin, Redundant
excess skin around the neck, often lying in horizontal folds

Nucleic acid
the genetic material of cells, found in two forms: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA); composed of repeating subunits called nucleotides

Nucleoside
a building block of nucleic acids, composed of a sugar (deoxyribose or ribose) and one of the nitrogenous bases: adenine (A), cytosine (C), guanine (G), thymine (T), or uracil (U)

Nucleotide
a building block of nucleic acids, composed of a sugar (deoxyribose or ribose), one of the nitrogenous bases (adenine, cytosine, guanine, thymine, or uracil) and one or more phosphate groups

Nullisomy
having two less than the diploid number of chromosomes due to loss of one chromosome pair

Nystagmus
Rapid, rhythmic, repetitious, and involuntary eye movements

Occiput, flat
reduced convexity of the occiput (posterior part of skull), giving the appearance of flattening of the back of the skull

Occiput, prominent
increased convexity of the occiput (posterior part of the skull), giving the appearance of prominence at the back of the skull

Okihiro syndrome
association of Duane syndrome (eye retraction) with forearm malformation and deafness; autosomal dominant disorder due to mutation of gene located in chromosome region 20q13.13-q13.2

Oligodactyly
presence of fewer than five fingers or toes on a hand or foot

Oligohydramnios
deficiency of amniotic fluid which compresses the fetus

Oligonucleotide
a short molecule of DNA, generally fewer than 20 bases long and usually synthesized artificially; an important tool for numerous molecular biology procedures, including site-directed mutagenesis

Omphalocele
birth defect in which an infant's intestine or other abdominal organs are outside of the body because of a hole in the navel

Open reading frame (ORF)
a putative protein coding DNA sequence, marked by a start codon at one end and a stop codon at the other end

Orbeli syndrome
distal 13q deletion characterized by retinoblastoma, mental and growth retardation, brain malformations, heart defects, distal limb deformities, and digestive, urogenital abnormalities, especially when band q32 is involved

Osseous Syndactyly of the foot
Lateral (A/P) fusion of the digits (phalanges and/or metatarsals) by hard tissue (cartilage and/or bone)

Ossification
process of bone formation

Osteopoikilosis
inherited condition characterized by bones that are mottled because of widespread small foci of compact bone in the spongy bone

p arm
short region of a chromosome attached to the centromere at the top end and the telomere at the other; contains bands of genes numbered from the centromere out

Pachygiria
unusually thick convolutions of the cerebral cortex

Palate, Hard, Short
distance between the labial point of the incisive papilla to the midline junction of the hard and soft palate more than 2 SD below the mean; decreased length of the hard palate

Palate, high
height of the palate more than 2 SD above the mean; palatal height at the level of the first permanent molar more than twice the height of the teeth

Palate, submucous cleft
soft palatal defect with intact overlying mucosa comprising two of the following three findings: (1) notching of the posterior border of the hard palate, (2) bifid uvula, or (3) muscular diastasis leading to a midline translucent zone or furrow in the soft palate

Pallister-Killian syndrome (Tetrasomy 12p)
chromosomal disorder caused by the presence of four copies of the short arm of chromosome 12; symptoms may include a coarse face with a high forehead, sparse hair on the scalp, an abnormally wide space between the eyes, a fold of the skin over the inner corner of the eyes, a broad nasal bridge with a highly arched palate, and streaks of skin lacking color; intellectual impairment and loss of muscle tone are common

Palmar crease, absent
absence of a major crease of the palm (distal transverse crease, proximal transverse crease, or thenar crease)

Palpebral fissure
scaphoid space or outline formed by eyelid margins when the eye is open

Palpebral fissure, down-slanted
inclination is more than 2 SD below the mean for age

Palpebral fissure, up-slanted
inclination is more than 2 SD above the mean for age; may be associated with microcephaly

Pancytopenia
abnormal reduction in the number of red blood cells, white blood cells, and blood platelets in the blood

Paracentric inversion
an inversion of a chromosome’s sequence that does not involve the centromere, taking place on a single arm of the chromosome

Paranoia
unfounded or exaggerated distrust of others, sometimes reaching delusional proportions

Parenchyma
essential or functional elements of an organ

Parietal foramina
inconstant foramen in the parietal bone occasionally found bilaterally near the sagittal margin posteriorly; when present it transmits an emissary vein to the superior sagittal sinus

Paroxysm
sudden attack or increase of symptoms of a disease

Partial trisomy
occurs when only a part of a chromosome attaches to another

Patau syndrome (trisomy 13)
genetic disorder in which there is an extra copy of chromosome 13; generally causes malformed, low set ears, severe intellectual impairment, kidney and heart defects, and microcephaly. Mortality is high

Patent ductus arteriosus
congenital heart in which the ductus arteriosus does not close

Patent foramen ovale
hole between the left and right atria (upper chambers) of the heart that did not close before birth

Pathologic
altered or caused by disease; indicative of disease

Pectus carinatum
protrusion of the sternum

Pectus excavatum
inward malformation of the sternum

Pedigree
diagrammatic representation of a family health history or family health tree

Pena-Shokeir syndrome
autosomal recessive non-aneuploidic condition with some clinical features similar to that of trisomy 18; most often it is lethal

Pericentric inversion
an inversion of a chromosome’s sequence involving the centromere

Periventricular heterotopia
brain malformation, due to abnormal neuronal migration, in which a subset of neurons fails to migrate into the developing cerebral cortex and remains as nodules that line the ventricular surface; X-linked dominant disorder mapped to FLN1gene; females display borderline mental retardation, epilepsy of variable severity, and extra-central nervous system signs, especially cardiovascular defects or coagulopathy

Pes cavus
presence of an unusually high plantar arch

Pes Planus (flat feet)
arch of the foot is in contact with the ground or floor when the individual is standing

Peters’ anomaly
congenital eye disorder in which development of the anterior segment is abnormal, leading to incomplete separation of the cornea from the iris or the lens

Peutz–Jeghers syndrome
autosomal dominant genetic condition characterized by multiple benign polyps called hamartomas on the mucous lining in the gastrointestinal system and dark skin discolorations, especially around the eyes, nostrils, mucous membranes of the mouth, perianal area, and inside the mouth

Phalange
bones of the fingers and toes

Phelan–McDermid syndrome
chromosomal disorder in which a portion of the long arm (q) of chromosome 22 is missing; characterized by low muscle tone, normal to accelerated growth, absent to severely delayed speech, moderate to profound intellectual impairment, and minor dysmorphic features

Phenotype
the physical appearance or biochemical and physiological characteristics of an individual, which is determined by both heredity and environment

Philtral ridges, malaligned
absence of the usual parallel position of philtral ridges

Philtrum
vertical groove in the midline portion of the upper lip bordered by two lateral ridges or pillars

Philtrum, deep
accentuated, prominent philtral ridges giving rise to an exaggerated groove in the midline between the nasal base and upper vermillion border

Philtrum, long
distance between nasal base and midline upper lip vermilion border more than 2 SD above the mean; increased distance between nasal base and midline upper lip vermilion border; length of philtrum usually shows accompanied by a short nose

Phonophobia
persistent, abnormal, and unwarranted fear of sound

Piebaldism
Patchy absence of the pigment of scalp hair, giving a streaked appearance

Pierre Robin complex
congenital anomaly characterized by an unusually small lower jaw (micrognathia), downward displacement of the tongue (glossoptosis), and occasionally, cleft palate

Pinna
part of the ear that projects like a little wing from the head

Pitt–Hopkins syndrome
characterized by severe intellectual disability and developmental delay, breathing problems, recurrent seizures (epilepsy), and distinctive facial features and caused by mutations in the TCF4 gene on chromosome 18 at 18q21.1

Plagiocephaly
asymmetric head shape, which is usually a combination of unilateral occipital flattening with ipsilateral frontal prominence, leading to a rhomboid cranial shape

Politelia
presence of supernumerary (extra) nipples

Polycystic Kidney Disease
autosomal recessive genetic disorder caused by mutations of the PKHD1 gene located on chromosome 16 at 16p13.3 and characterized by the formation of fluid-filled sacs (cysts) in the kidneys; additional organ systems may be affected, especially the liver; high blood pressure (hypertension), excessive thirst, frequent urination and feeding difficulties may also occur

Polydactyly
a genetic disorder where the embryo develops extra fingers or toes

Polydactyly, mesoaxial
presence of a supernumerary finger or toe (not a thumb or hallux) involving the third or fourth metacarpal/tarsal with associated osseous syndactyly

Polydactyly, Mirror Image
hand or foot with more than five digits that has a recognizable A/P axis of symmetry

Polymicrogyria
developmental anomaly of the brain marked by development of numerous small convolutions (microgyri), causing cognitive impairment

Polymorphism
the presence of many different alleles for a particular locus in individuals of the same species

Polyploid
a cell or organism that possesses multiple sets of chromosomes

Polyposis
condition characterized by the presence of numerous polyps

Postaxial
pertaining to or situated behind the axis of the body, especially the posterior side

Postaxial polydactyly of the foot
presence of a supernumerary digit that is not a hallux (big toe)

Posterior
toward the back or denoting the back surface of the body

Posterior embryotoxon
ring-like opacity at the margin of the cornea in the eye

Potocki-Lupski syndrome
caused by duplication on chromosome 17p11.2; wide range of developmental and motor issues; strong association with Charcot-Marie-Tooth disease

Potocki–Shaffer syndrome
disorder that affecting development of the bones, brain, and other organs, characterized by the growth of exostoses, enlarged parietal foramina, intellectual disability, delayed development of speech, motor and social skills, and distinctive facial features; caused by deletions on the short (p) arm of chromosome 11 at 11p11.2

Potter sequence
kidneys fail to develop properly, causing lack of amniotic fluid (oligohydramnios) that, in turn causes pressure of the uterine wall which leads to an unusual facial appearance and stops development of the lungs; can also lead to abnormal limbs

Prader–Willi syndrome
imprinting chromosomal disorder caused by mutation of chromosome 15 in the region of 15q11.2-q13 on the paternal side (mutation of this region on the maternal side causes Angelman syndrome); characterized by low muscle tone and poor feeding in infancy and an insatiable appetite as the child grows; varying degrees of cognitive impairment are common

Preauricular pits
Small indentation in front of the ear opening

Preaxial
Situated in front of or superior to the median axis of the body or a body part

Preaxial polydactyly of the hand
Duplication of all or part of the first ray

Prenatal hypoplasia
low birth weight when born at term

Prenatal testing
testing that is done during pregnancy to examine the chromosomes or genes of a fetus to detect the presence or absence of a genetic disorder

Prevalence
the percentage of people in a given population that are affected by a disorder at that particular time

Proband
affected person whose disorder, or concern about the disorder, brings a family or pedigree to be genetically evaluated

Proboscis
fleshy, tube-like structure usually located in the midline of the face or just to one side of the midline; has a single cavity and may be found in association with a malformed nose and absence of one or both nares

Propositus
the individual in a human pedigree who is the focus of the pedigree, usually by being the first person who came to the attention of the geneticist

Prosencephalon
another term for forebrain

Protein
a biological molecule composed of amino acids linked together by peptide bonds; used as structural components of the cell or as enzymes; the term “protein” can refer to a single chain of amino acids or to multiple chains of amino acids functioning in a concerted way

Proximal
situated nearer to the center of the body or the point of attachment

Psycho-motor development
Of or relating to movement or muscular activity associated with mental processes

Pterygia
wing–like triangular membrane between the torso and upper arm in the armpit area

Pterygium colli
redundant neck skin

Ptosis
upper eyelid lid margin is positioned 3mm or more lower than usual and covers the superior portion of the iris; upper lid margin obscures at least part of the pupil; reduction in eyelid opening that is not fixed but can be increased actively or passively

Pubertas praecox
Sexual development before the age of eight in girls, and age 10 in boys

Pulmonary hypoplasia
incomplete development of the lungs, resulting in an abnormally low number or size of bronchopulmonary segments or alveoli

Pulmonary stenosis
condition in which the pulmonary valve is too tight, so that the flow of blood from the right ventricle of the heart into the pulmonary artery is impeded

Purine
either of the nitrogenous bases adenine or guanine; used in the structure of nucleic acids

Pyelocalyceal
Relating to the renal pelvis and calices

Pyeloectasis
dilation of the renal pelvis

Pyloric stenosis
abnormal narrowing of the valve at the outlet from the stomach, preventing normal passage of food into the small intestine

Pyrimidine
any of the nitrogenous bases cytosine, thymine, or uracil; used in the structure of nucleic acids

q arm
long region of a chromosome attached to the centromere at the bottom and the telomere at the other end contains bands of genes numbered from the centromere out

qter
meaning terminal end of a chromosome arm; to the end

Radio-ulnar synostosis
abnormal connection between the radius and ulna, the two bones of the forearm

Ray, absent
absence of all phalanges of a digit and the associated metacarpal/metatarsal

Reading frame
refers to the manner in which a messenger RNA (mRNA) sequence is interpreted as a series of amino acid codons by the ribosome; because of the triplet nature of the genetic code, a typical messenger RNA (mRNA) molecule has three possible reading frames, although usually only one of these will actually code for a functional protein

Recessive
a term referring to an allele or trait that will only be expressed if another, dominant, trait or allele is not also present

Reciprocal deletion
recombination products of crossing-over are a duplication at the site of the exchange and a reciprocal deletion

Reciprocal translocation
a two-way exchange of genetic material between two nonhomologous chromosomes, resulting in a wide variety of genetic problems depending on which chromosomes are involved in the translocation

Recombination
formation of new set of alleles on a single chromosome that is not the same as either parent owing to a crossover during meiosis

Renal
of the kidneys

Renal agenesis
absence of one or both kidneys

Rethore syndrome (trisomy 9p)
characterized by cognitive impairment, distinctive malformations of the skull and facial (craniofacial) region, skeletal defects, and structural malformations of the heart; caused by duplication of the short arm (p) of chromosome 9

Retinitis pigmentosa
group of inherited vision disorders that cause progressive degeneration of the retina, the light sensitive membrane that coats the inside of the eyes; may be associated with deafness, obesity, kidney disease, central nervous system and metabolic disorders, and occasionally chromosomal abnormalities

Retinoblastoma
malignant tumor that develops in the nerve-rich layers that line the back of the eyes (retina)

Retrognathia
posteriorly positioned lower jaw, which is set back from the plane of the face when viewed from the side but not from the front; may be accompanied by micrognathia

Reverse transcription polymerase chain reaction (RT-PCR)
a technique, requiring isolated RNA, for quickly determining if a gene or a small set of genes are transcribed in a population of cell

Rhizomelic
disproportion in the length of the most proximal segment of the limbs (upper arms and thighs)

Ribonucleic acid (RNA)
a form of nucleic acid in the cell used primarily for genetic expression through transcription and translation; in structure, it is virtually identical to DNA, except that ribose is used as the sugar in each nucleotide and the nitrogenous base thymine is replaced by uracil; present in three major forms in the cell: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA)

Rieger syndrome
genetic disorder characterized by absent or underdeveloped (hypodontia or partial adontia) teeth, mild craniofacial abnormalities, and various abnormalities of the eye, especially glaucoma

Ring chromosome
occurs as a result of the fusion of the two ends of the same chromosome; there is a consequent loss of genetic material

Ritscher–Schinzel syndrome (craniocerebellocardiac dysplasia)
characterized by heart defects, cerebellar hypoplasia, and cranial dysmorphism; autosomal recessive disease caused by mutation on the long arm (q) of chromosome 8 at 8q24

Robertsonian translocation
type of translocation exclusive to the acrocentric chromosomes (13, 14, 15, 21 and 22) in which two of these chromosomes join at or near their centromeres, constituting a fusion between two whole chromosomes

Rocker bottom feet
presence of both a ‘prominent heel’ and a ‘convex contour of the sole’

Rubinstein–Taybi syndrome
multisystem disorder characterized by growth retardation and delayed bone age, developmental and psycho-motor delays, cognitive impairment, distinctive abnormalities of the head and face (craniofacial dysmorphism), and/or breathing and swallowing difficulties; many may have malformations of the heart, kidneys, urogenital system, and/or skeletal system; gene responsible is located on the short arm (p) of chromosome 16 at 16p13.3

Russell -Silver syndrome
disorder characterized by prenatal and postnatal growth retardation, large forehead that protrudes out from the plane of the face, triangular-shaped face, pinky that is fixed or "locked" in a bent position (clinodactyly), lack of appetite/low BMI, and undergrowth of one side or limb(s) of the body (hemihypotrophy), resulting in unequal (asymmetric) growth; heterogeneous, but believed to be caused by abnormalities certain genes on chromosomes 7 or 11

Saethre–Chotzen syndrome
"acrocephalosyndactyly" disorder characterized by anomalies of the craniofacial area and hands and feet including craniofacial asymmetry, widely spaced eyes (ocular hypertelorism) with unusually shallow eye cavities (orbits), drooping of the upper eyelids (ptosis), and strabismus; occasional "beaked" nose, deviated nasal septum, small, low-set ears; or underdeveloped upper jaw (hypoplastic maxilla), partial fusion of soft tissues (cutaneous syndactyly) of certain fingers and toes (digits); unusually short digits (brachydactyly); and broad great toes; caused by mutations in the TWIST1 gene located on the short arm (p) of chromosome 7 at 7p21; autosomal dominant inheritance

San Luis Valley Syndrome
recombinant 8 syndrome; characterized by heart and urinary tract abnormalities, moderate to severe intellectual disability, and a distinctive facial appearance and caused rearrangement of chromosome 8 that results in a deletion of a piece of the short (p) arm and a duplication of a piece of the long (q) arm

Sandal Gap
widely spaced gap between the first toe (the great toe) and the second toe

Sanger sequencing
method of DNA sequencing based on the selective incorporation of chain-terminating dideoxynucleotides by DNA polymerase during in vitro DNA replication

Schizophrenia
long-term mental disorder of a type involving a breakdown in the relation between thought, emotion, and behavior, leading to faulty perception, inappropriate actions and feelings, a sense of mental fragmentation, and withdrawal from reality and personal relationships into fantasy and delusion

Sclerocornea
scarring of the cornea of the eye

Scoliosis
side-to-side curvature of the spine

Septate
having or partitioned by a septum or septa

Septum pellucidum
thin, triangular, vertical membrane separating the anterior horns of the left and right lateral ventricles of the brain

Sex chromosomes
chromosomes carrying the genes responsible for determining the sex of offspring, in humans the sex chromosomes are designated X and Y; females have 2 X chromosomes and males have one X and one Y chromosome

Sex-linked
disorders that are inherited through one of the sex chromosomes, X or Y

Single nucleotide polymorphism (SNP)
differences at the individual nucleotide level among individuals

Sinus
cavity in the body

Situs inversus
condition in which the heart and other organs of the body are transposed through the sagittal plane to lie on the opposite side from the usual

Skin tag
benign growths thought to be caused by skin friction

Smith–Magenis syndrome
complex developmental disorder characterized by distinctive facial features, skeletal malformations, varying degrees of intellectual impairment, speech and motor delays, sleep disturbances, and self-injurious or attention-seeking behaviors; caused by deletion of the RAI1 gene located on the short arm of chromosome 17 within the 17p11.2 region

Somatic mutation
change in the genetic structure that is neither inherited nor passed to offspring; can occur in any of the cells of the body except the germ cells (sperm and egg)

Sotos Syndrome
overgrowth disorder, characterized by excessive growth prenatally and postnatally, a large elongated head, distinctive facial configuration, and a non-progressive neurological disorder with intellectual disability; may show advanced bone age; caused by mutation in the NSD1 gene located on the long arm of chromosome 5 at 5q35.2-q35.3

Spasticity
state of increased tone of a muscle (and an increase in the deep tendon reflexes

Spherocytosis
disorder of the red blood cell membrane clinically characterized by anemia, jaundice (yellowing) and splenomegaly (enlargement of the spleen)

Spina bifida
major birth defect and a type of neural tube defect that involves an opening in the vertebral column caused by the failure of the neural tube to close properly during embryonic development

Split foot (ectrodactyly)
longitudinal deficiency of a digital ray of the foot except rays 1 or 5

Split hand (ectrodactyly)
longitudinal deficiency of a digital ray of the hand except rays 1 or 5

Stem cell
an undifferentiated cell that retains the ability to give rise to other, more specialized cells

Stenosis
abnormal narrowing or contraction of a duct or canal

Sternal tag
pedunculated skin and subcutaneous tissue originating from the sternal area

Stop codon
Any of three codons, UAA, UAG, or UGA, that signal the termination of the synthesis of a protein

Strabismus
disorder of vision due to a deviation from normal orientation of one or both eyes so that both cannot be directed at the same object at the same time; squint; crossed eyes

Subluxation
incomplete or partial dislocation

Subtelomeric
below the telomere, the end of a chromosome

Sulci
shallow furrows on the surface of the brain separating adjacent convolutions

Supernumerary
present in excess of the normal or requisite number, in particular

Supravalvular aortic stenosis
fixed form of congenital left ventricular outflow tract (LVOT) obstruction that occurs as a localized or diffuse narrowing of the ascending aorta beyond the superior margin of the sinuses of Valsalva

Symblepharon
adhesion between an eyelid and the eyeball

Syndactyly
condition in which fingers or toes are joined together; can involve the bones or just the skin

Syndactyly, cutaneous fingers
soft tissue continuity in the A/P axis between two fingers that extends distally to at least the level of the proximal interphalangeal joints

Syndactyly, cutaneous toes
soft tissue continuity in the A/P axis between adjacent foot digits that involves at least half of the P/D length of one of the two involved digits

Syndrome
group of three or more characteristics and/or symptoms that occur together in a recognizable pattern and cause a particular disease state or condition

Synophrys
meeting of the medial eyebrows in the midline

Synostosis
union or fusion of adjacent bones by the growth of bony substance

T
the abbreviation for thymine, a pyrimidine nitrogenous base found in the structure of DNA; in RNA, thymine is replaced by uracil

Tandem repeats
sequence of two or more DNA base pairs that is repeated in such a way that the repeats lie adjacent to each other on the chromosome

Telecanthus
abnormally increased distance between the medial canthi (corners of the eye) of the eyelids

Telocentric chromosome
a chromosome with a centromere at the end; also called an acrocentric chromosome

Telomere
the ends of a eukaryotic chromosome

Teratoma
type of germ cell tumor that may contain several different types of tissue and sometimes mature elements such as hair, muscle, and bone

Terminal deletion
deletion that occurs towards the end of a chromosome

Tetralogy of Fallot
the combination of four different heart defects: ventricular septal defect (VSD), pulmonary stenosis (obstructed outflow of blood from the right ventricle to the lungs), dextroposition or overriding aorta (displaced aorta, causing blood to flow into the aorta from both right and left ventricles); and right ventricular hypertrophy (abnormal enlargement of the right ventricle)

Tetrasomy
four copies of a particular chromosome present in a cell, resulting in 48 chromosomes in the cell instead of the usual 46

Thenar region
fleshy mass of the outer side of the palm of the hand

Thrombocytopenia
low blood platelet count

Thumb, absent
absence of both phalanges of a thumb and the associated soft tissues

Thumb, adducted
in the resting position, the tip of the thumb is on, or near, the palm, close to the base of the fourth or fifth finger

Thumb, triphalangeal
thumb with three phalanges in a single, proximodistal axis

Thymine (T)
pyrimidine nitrogenous base found in the structure of DNA; in RNA, thymine is replaced by uracil

Tongue, bifid
tongue with a median apical indentation or fork; may be associated with ankyloglossia

Tongue, large
increased length and width of the tongue; leads to protrusion of the tongue

Tooth, natal
erupted tooth or teeth at birth

Tooth, supernumerary
extra tooth or teeth

Townes–Brocks syndrome
autosomal dominant genetic disorder characterized by absence of the anal opening (imperforate anus), abnormal ears associated with hearing impairment and thumb malformations; abnormalities in the feet, heart and kidneys also occur frequently. Townes-Brocks syndrome is associated with a mutation in the SALL1 gene located on the long arm of chromosome 16 at 16q12.1

Tracheaesophageal fistula
birth defect in which the trachea is connected to the esophagus

Trait
phenotypic characteristic that can be inherited

Transcription
cellular process by which genetic information in the form of a gene in a DNA molecule is converted into the form of a messenger RNA (mRNA) molecule

Transfer RNA (tRNA)
type of RNA molecule necessary for translation to occur properly; provides the basis of the genetic code, in which codons in a messenger RNA (mRNA) molecule are used to direct the sequence of amino acids in a polypeptide; contains a binding site for a particular amino acid and a region complementary to a messenger RNA (mRNA) codon (an anticodon)

Transition mutation
DNA mutation in which one pyrimidine (cytosine or thymine) takes the place of another, or a purine (adenine or guanine) takes the place of another

Translation
cellular process by which genetic information in the form of a messenger RNA (mRNA) molecule is converted into the amino acid sequence of a protein, using ribosomes and RNA molecules as accessory molecules

Translocation
movement of a chromosome segment to a nonhomologous chromosome as a result of an error in recombination

Transposition of great arteries
birth defect in which the two major vessels that carry blood away from the heart the aorta and the pulmonary artery are switched (transposed)

Transverse limb reduction
partial or complete absence of a part of one or more limbs in the absence of other abnormalities

Tricho–rhino–phalangeal syndrome
genetic disorder consisting of fine and sparse scalp hair, thin nails, pear-shaped broad nose, and cone-shaped epiphyses of the middle phalanges of some fingers and toes

Trigonocephaly
Wedge-shaped or triangular head, with the apex of the triangle at the midline of the forehead and the base of the triangle at the occiput, assessed from above

Triploid
possessing three complete sets of chromosomes

Trisomy
genetic condition involving one chromosome of a homologous chromosome pair that has been duplicated in some way, giving rise to a 2N + 1 genotype and causing serious phenotypic abnormalities; well-known trisomies are Trisomy 13, 18, and 21

Trisomy 13
see Patau syndrome

Trisomy 18
see Edwards syndrome

Trisomy 21
see Down syndrome

Truncus arteriosus
congenital heart defect in which there is a single main blood vessel carrying blood away from the heart which then branches into other blood vessels; with no pulmonary artery to carry blood to the lungs, and aorta blood from both ventricles of the heart is mixed, resulting in some oxygen-rich blood travelling back to the lungs and some oxygen-poor blood travelling to the rest of the body

Tuberous sclerosis (TS)
genetic multisystem disorder characterized by episodes of uncontrolled electrical activity in the brain (seizures), intellectual impairment, distinctive skin abnormalities (lesions), and benign (noncancerous), tumor-like nodules (hamartomas) that appear in the brain, certain regions of the eyes (e.g., retinas), the heart, the kidneys, the lungs, or other tissues or organs; caused by mutations of either the TSC1 gene located on the long arm (q) of chromosome 9 at 9q34 or the TSC2 gene located on the short arm (p) of chromosome 16 at 16p13

Turner syndrome
a human genetic defect in which a female has only forty-five chromosomes and is lacking a second X sex chromosome; abnormalities such as sterility and anatomical defects are common

Turricephaly
tall head relative to width and length; head length and width are reduced compared to normal age-related standards

Unbalanced translocation
translocation of chromosomes resulting in the gain or loss of genetic material that can cause problems in health, growth or development

Uniparental disomy
both chromosomes coming from one parent rather than one from each; may be maternal or paternal in origin

Univentricular heart
anomaly in which all blood flows through one ventricle or in which the arterioventricular valves are committed to empty into only one chamber in the ventricular mass

Uracil (U)
pyrimidine nitrogenous base found in the structure of RNA; in DNA, uracil is replaced by thymine

VACTERL–association
acronym for association of seven birth defects—vertebral abnormalities (V), anal atresia (A), cardiac (heart) defects (C), tracheal-esophageal abnormalities (TE), renal/kidney and radial abnormalities (R), limb abnormalities (L) and single umbilical artery (S); cause unknown

Variant
something that differs in some characteristic from the class to which it belongs

Velopharyngeal insufficiency
improper closing of the velopharyngeal sphincter (soft palate muscle) during speech characterized by an acute nasal quality of the voice

Ventricular septal defect
hole or defect in the muscular wall of the heart that splits the lower chamber into two ventricles; small holes can close spontaneously; large holes can lead to progressive heart failure and require surgery

Vermilion, lower lip, everted
inner aspect of the lower lip vermilion (normally apposing the teeth) visible in a frontal view

Vermilion, upper lip, tented
triangular appearance of the oral aperture with the apex in the midpoint of the upper vermilion and the lower vermilion forming the base

Vesico–ureteral reflux
abnormal flow of urine from the bladder to the upper urinary tract

Visceral
referring to the viscera, the internal organs of the body, specifically those within the chest (as the heart or lungs) or abdomen (as the liver, pancreas or intestines)

Waardenburg syndrome
genetic disorder with evident genetic anomolies, including distinctive facial abnormalities; unusually diminished coloration (pigmentation) of the hair, the skin, and/or the iris of both eyes (irides); and/or congenital deafness; four types are known and thought to be caused by mutations in EDN3, EDNRB, MITF, PAX3 and SOX10 genes; gene responsible for WS1 is the "PAX3" gene located on the long arm (q) of chromosome 2 at 2q35

WAGR syndrome
11p deletion syndrome; acronym for Wilms' Tumor (W), aniridia (A), genitourinary abnormalities (G), mental retardation/intellectual disability (R)

Webbed neck
neck that appears to be unusually broad because of bilateral folds of skin extending from the clavicles to the head but containing no muscles, bones, or other structures

Whole exome sequencing
mode of identifying those variants found in the coding region of genes which affect protein function

Whole-genome sequencing
laboratory process that determines the complete DNA sequence of an organism's genome at a single time

Wild-type
interacting with more than one messenger RNA (mRNA) codon by virtue of the inherent flexibility present in the third base of the anticodon; refers to the phenotype of the typical form of a species as it occurs in nature

Williams syndrome
autosomal dominant chromosomal disorder characterized by intellectual disability, unique personality characteristics, unusual facial features, cardiovascular disease .and high level of calcium in blood (hypercalcemia) and urine (hypercalciuria); caused by deletion on chromosome 7 at band 7q11 and includes the ELN (elastic) gene

Wilms’ tumor
a type of malignant tumor of the kidney that occurs in young children

Wolf–Hirschhorn syndrome (4p-syndrome)
chromosomal disorder characterized by distinctive facial appearance, delayed growth and development, intellectual disability, and seizures and caused by deletions near the end of the short (p) arm of chromosome 4 which include the genes WHSC1, LETM1, and MSX1

Wolff–Parkinson–White syndrome
abnormality in the electrical functioning of the heart which may cause rapid heart rates; affects the electrical signal between the atria and ventricles

X linkage
a genetic phenomenon involving a gene located on the X chromosome; the typical pattern of X linkage involves recessive alleles, such as that for hemophilia, which exert their effects when passed from mother to son and are more likely to be exhibited by males than females

X-inactivation
phenomenon in a female by which one X chromosome (either the maternally or paternally derived X) is randomly inactivated in an early embryonic cell, with fixed inactivation of that same X in all cells descended from that cell

Y linkage
a genetic phenomenon involving a gene located on the Y chromosome; as a result, such a condition can only be passed from father to son

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