trisomy
[tri´so-me] the presence of an additional (third) chromosome of one type in an otherwise diploid cell (2n +1). adj., adj triso´mic.
trisomy 8 syndrome a syndrome associated with an extra chromosome 8, usually mosaic (trisomy 8/normal), characterized by mild to severe mental retardation, prominent forehead, deep-set eyes, thick lips, prominent ears, and
camptodactyly (abnormally flexed fingers).
trisomy 13 syndrome holoprosencephaly due to an extra chromosome 13, in which central nervous system defects are associated with mental retardation, cleft lip and palate,
polydactyly (extra fingers or toes), and dermal pattern anomalies, as well as abnormalities of the heart, viscera, and genitalia. Called also
Patau's syndrome. Information for families affected by this disorder can be obtained from the Support Organization for Trisomy 18, 13, and Related Disorders (S.O.F.T.), 2982 S. Union St., Rochester, NY 14624.
trisomy 18 syndrome a condition due to the presence of an extra chromosome 18, characterized by neonatal hepatitis, mental retardation, scaphocephaly or other skull abnormality, small receding mandible,
blepharoptosis (drooping eyelids), low-set ears, corneal opacities,
deafness,
webbed neck, short digits, ventricular septal defects,
Meckel's diverticulum, and other deformities. Called also
Edwards' syndrome. Information for families affected by this disorder and professionals caring for affected individuals can be obtained from the Support Organization for Trisomy 18, 13, and Related Disorders (S.O.F.T.), 2982 S. Union St., Rochester, NY 14624.
trisomy 22 syndrome a syndrome due to an extra chromosome 22, characterized typically by mental and growth retardation, undersized head, low-set or malformed ears, small receding mandible, long philtrum on the upper lip, preauricular skin tag or sinus, and congenital heart disease. In males, there is a small penis or undescended testes.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.
chromosome
One of the thread-like structures located within the cell nucleus composed of an extremely long, double-stranded DNA (deoxyribonucleic acid) helix tightly folded around proteins called histones. Each chromosome carries genes that contain the hereditary material that controls the growth and characteristics of the body. There are 46 chromosomes in each human somatic cell organized in 23 pairs, of which 22 pairs are similar in appearance but differ at the molecular level. They are called
autosomal chromosomes or autosomes and are designated by a number (with chromosome 1 being the longest, followed by chromosome 2, etc.). The other pair, the
sex chromosomes determines the sex of the individual. In mammals the two sex chromosomes of females are alike (homologous) and are referred to as X chromosomes. Males carry one X chromosome along with a much shorter chromosome, the Y chromosome. Each chromosome has a centromere that divides it into two arms, the short arm 'p' and the long arm 'q'. Disorders of chromosome number in which the number of chromosomes is above or below the normal (46) are called
aneuploidy. Common forms of aneuploidy are
trisomy in which there is one extra chromosome and
monosomy in which there is one less, than the normal 46. They rarely cause specific eye diseases but affected individuals present ocular manifestations.
Examples: Down's syndrome (trisomy of chromosome 21), Edwards' syndrome (trisomy 18), Turner's syndrome (monosomy 45 XO). There are other chromosome abnormalities such as
translocation (one segment of a chromosome is transferred to another chromosome) as may occur in congenital anterior polar cataract,
deletion (a loss of a piece of chromosome) as in aniridia, choroideremia, retinoblastoma, etc. Other cases involve damage of a chromosome (e.g. fragile X syndrome).
See defective colour vision;
gene;
mitosis;
mutation.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann