Genetic and Medical Glossary

Glossary

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There are 46 names in this directory beginning with the letter M.
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

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