Genetic and Medical Glossary

Glossary

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There are 69 names in this directory beginning with the letter A.
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

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