Variations from normal in the number or structure of chromosomes contained in a person’s cells. The cause is generally a fault in the process of chromosome division, either during the formation of an egg or sperm, or during the first few divisions of a fertilized egg. Chromosomal abnormalities are classified according to whether they involve the 44 autosomes or the 2 X and Y sex chromosomes. A complete extra set of chromosomes per cell is called polyploidy and is lethal. Autosomal abnormalities cause physical and mental defects of varying severity. Some types of autosomal abnormality, known as trisomy, consist of an extra chromosome on 1 of the 22 pairs of autosomes. The most common trisomy is Down’s syndrome. Sometimes, part of a chromosome is missing, as in cri du chat syndrome. In translocation, a part of a chromosome is joined to another, causing no ill effects in the person but a risk of abnormality in his or her children. Sex chromosome abnormalities include Turner’s syndrome, in which a girl is born with a single X chromosome in her cells instead of 2, causing physical abnormalities, defective sexual development, and infertility. A boy with 1 or more extra X chromosomes has Klinefelter’s syndrome, which causes defective sexual development and infertility. The presence of an extra X chromosome in women or an extra Y chromosome in men normally has no physical effect but increases the risk of mild mental handicap. Chromosomal abnormalities are diagnosed by chromosome analysis in early pregnancy, using amniocentesis or chorionic villus sampling.
Chromosomal abnormalities |
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