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Klinefelter'S Syndrome

Klinefelter's syndrome (genotype 47, XXY) is a chromosomal anomaly in which affected males have an extra X chromosome. It occurs in 1:1,000 to 1:2,000 newborn males and has been detected in .003 percent of spontaneous abortions. This condition generally arises from failure of chromosomes to separate properly during meiosis. Specifically, an egg cell bearing an additional X chromosome (or a sperm bearing both an X and a Y chromosome) unites with a normal sex cell. Approximately one-third of affected individuals show multiple Xs (48, XXXY or 49, XXXXY) or both normal and abnormal cell lines (46, XY/47, XXY).

Klinefelter's syndrome is generally not apparent until puberty, at which time secondary sexual development does not proceed. Clinical features include small testes, tall stature, breast development, reduced hormonal levels, and sterility. Mosaics, or individuals with mixed cell lines, may be fertile. Behavioral features include depressed verbal intelligence, reduced activity, increased stress, and feelings of reduced masculinity. Families may be advised to seek hormonal therapy and counseling for these children.


"Klinefelter Syndrome." In the On-line Medical Dictionary, 1995-1998. Available from http://www.graylab.ac.uk/omd/index.html; INTERNET.

Plomin, Robert, John DeFries, Gerald McClearn, and Peter Mc-Guffin. Behavioral Genetics, 4th edition. New York: Worth Publishers, 2001.

Nancy L. Segal

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