Sex chromosomes

Sex chromosome abnormalities are described here although they are not always associated with LD.

GENE ABNORMALITIES

Male phenotype

Fragile X (Martin-Bell syndrome) is the next most common genetic cause of LD after Down's syndrome. The long arm of the X chromosome appears to have a fragile tip when grown in special media, due to a gene with multiple abnormal DNA triplicate repeat sequences (of CGG). Fragile X syndrome is a common cause of severe LD in males, and causes lesser degrees of disability in some carrier females. Affected males have large testes (macroorchidism), 'bat ears', and long faces. Psychotic symptoms, aggressive or socially impaired behaviour associated with social anxiety, and language impairments are common. Screening in pregnancy for women from affected families offers the possibility of aborting foetuses with Fragile X.

XYY syndrome is associated with low IQ, infertility, neurological disorders, tallness, skeletal disorders, and myopia, but some cases are phenotypically normal.

XXY syndrome (Klinefelter's syndrome) comprises hypogonadism, infertility, tall-ness, effeminate body shape, and usually low IQ. Testosterone therapy may render the appearance more masculine.

Female phenotype

XO syndrome (Turner's syndrome) comprises ovarian dysgenesis, short stature, webbed neck, and sometimes coarctation of the aorta. IQ may be normal or low, and visuo-spatial disorders are often present. Patients are infertile, but oestrogen therapy may render their appearance more feminine.

XXX syndrome (triple X, 'superfemale') is sometimes associated with low IQ and physical abnormalities, but other cases are phenotypically normal. Patients are not infertile, but their children may have various chromosome abnormalities. Women with more than three X chromosomes have LD and numerous physical defects.

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