CHROMOSOMAL DISORDER

any syndrome characterized by malformations or malfunctions in any of the body's systems, and caused by abnormal chromosome number or constitution. Normally, humans have 46 chromosomes arranged in 23 pairs; the pairs vary in size and shape and are numbered by convention. Twenty-two of the pairs are autosomes, and one pair, number 23, is the sex chromosomes. Any variation from this pattern causes abnormalities. A chromosome from any of the pairs may be duplicated (trisomy) or absent (monosomy); an entire set of 23 chromosome pairs can be duplicated three (triploidy) or more (polyploidy) times; or one arm or part of one arm of a single chromosome may be missing (deletion). Part of one chromosome may be transferred to another (translocation), which has no effect on the person in which it occurs but generally causes a deletion or duplication syndrome in his or her children. Changes in chromosome number occur during sperm or egg formation or in the early development of the embryo. In the latter case, a mixture of cells, some normal (euploid) and some containing abnormal chromosome complements, may occur, a condition known as mosaicism. In either case, abnormalities of development occur because of the unusual genetic signals transmitted by the chromosomes. Some one of these chromosome imbalances occurs in 0.5 percent of all births. Down syndrome (mongolism), trisomy of chromosome 21, was the first chromosomal disorder identified (in 1959); it is the most common trisomy and the most common cause of mental retardation. Mental retardation is perhaps the most common manifestation of chromosomal abnormalities, occurring to some extent in all major autosomal abnormalities. Several chromosomal abnormalities, including Down syndrome, have also been related to heart disease or malformations. Other evidence of chromosome abnormalities include abnormal sexual development, behavioral disturbances, malignancy (e.g., the Philadelphia chromosome in chronic myelocytic leukemia), and spontaneous abortion. Sex chromosome abnormalities are more common and tend to have less-drastic effects than autosomal abnormalities. Normal females have two X chromosomes and males have an X and a Y; abnormalities in sex chromosome distribution produce Turner's syndrome (XO), Klinefelter's syndrome (XXY), and the so-called "supermale" (XYY). Turner's and Klinefelter's individuals have female and male genitalia, respectively, with retarded development of sexual characteristics. Supermales tend to be taller than average and to have learning disabilities. In addition, some studies have revealed that the percentage of XYY individuals in institutions for the criminally insane exceeds the incidence of supermaleness in newborn male populations. However, many XYY are socially well adjusted, and the link between supermaleness and criminal behaviour is highly debatable. Several chromosomal disorders can now be diagnosed before birth by examining cells obtained from the amniotic fluid. See also Down syndrome; cri-du-chat syndrome; trisomy 13; trisomy 18; Turner's syndrome; X-trisomy; Klinefelter's syndrome; XYY-trisomy.