sex-linked disorder, any disease or abnormal condition that is determined by the sex chromosomes or by a defective gene on a sex chromosome. Sex-linked disorders may involve a deviation in the number of either the X or Y chromosomes, as occurs in Turner’s syndrome and Klinefelter’s syndrome, most occurrences of which are a result of nondisjunction during meiosis. Such aberrations in the number of sex chromosomes do not produce the severe clinical effects that are associated with autosomal aberrations, although they usually cause some degree of mental deficiency. Other sex-linked disorders are transmitted by single-gene defects carried on the X chromosome. X-linked dominant conditions, such as hypophosphatemic vitamin D–resistant rickets, are rare, and males are more seriously affected than females. In inheritance patterns, X-linked dominant conditions are transmitted by affected males to all of their daughters but none of their sons, by affected heterozygous females to one half of their children regardless of sex, and by affected homozygous females to all of their children. More common are X-linked recessive conditions, such as color blindness, ocular albinism, the Xg blood types, hemophilia, Duchenne muscular dystrophy, and inborn errors of metabolism. Such conditions are always transmitted by females. Those predominantly affected are males because they have only one X chromosome, and all of its genes, whether recessive or dominant, are expressed. Occasionally, females heterozygous for X-linked recessive disorders show varying degrees of expression, but never as severe as those of affected males. There are no known clinically significant traits or conditions associated with the genes on the Y chromosome; their only known function is to trigger the development of male characteristics.
