trisomy

trisomy /tri·so·my/ (tri´so-me) the presence of an additional (third) chromosome of one type in an otherwise diploid cell (2n + 1). See also entries under syndrome. triso´mic

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tri·so·my (tr-sm, trs-)

n.

The condition of having three copies of a given chromosome or chromosome segment in each somatic cell rather than the normal number of two.

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tri·somic adj.

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Trisomy

An abnormality in chromosomal development. Chromosomes are the structures within a cell that carry its genetic information. They are organized in pairs. Humans have 23 pairs of chromosomes. In a trisomy syndrome, an extra chromosome is present so that the individual has three of a particular chromosome instead of the normal pair. An extra chromosome 18 (trisomy 18) causes mental retardation.

Mentioned in: Down Syndrome, Edwards' Syndrome, Mental Retardation, Patau Syndrome, Polydactyly and Syndactyly

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trisomy

[trī′səmē]

Etymology: Gk, treis + soma, body

a chromosomal aberration characterized by the presence of one more than the normal number of chromosomes in a diploid complement. In humans the trisomic cell contains 47 chromosomes and is designated 2n + 1. The additional member can join any of the normal homologous pairs, although most human trisomies involve the small chromosomes, such as those in the E or G group or the sex chromosomes. Partial trisomy occurs when only a part of a chromosome attaches to another. In genetic nomenclature, trisomies are indicated by the exact chromosome or karyotypic group in which the addition is made, such as trisomy 13 or trisomy D. Also called trisomia. Compare monosomy. See also aneuploidy, multipolar mitosis, trisomy syndrome. trisomic, adj.

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trisomy (trī´smē),

n an additional chromosome in the normal complement, so that in each nucleus a chromosome is represented three times rather than twice.

trisomy B,

n (trisomy 13-15, Patau's syndrome) Clinical syndrome associated with an autosomal abnormality in which the extra chromosome occurs in the 13 to 15 group. Numerous anatomic defects are present, including hemangiomas, hernia, arrhinencephaly, eye anomalies, cleft lip and palate, and characteristic changes in the footprint and palm print.

trisomy syndrome,

n a congenital condition caused by the addition of an extra member to a normal pair of homologous autosomes or to the sex chromosome or by the translocation of a portion of one chromosome to another. Trisomy 21 results in Down syndrome.

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trisomy

the presence of an additional (third) chromosome of one type in an otherwise diploid cell (2n + 1); associated with many congenital deformities in humans.

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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.