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The majority of the genes located on the X chromosome do not have a complementary gene on the Y chromosome. Therefore, males are “hemizygous” at these loci because they have only a single copy of the gene. X‐linked conditions were historically described as “X‐linked dominant” or “X‐linked recessive.” In an X‐linked recessive condition, a single abnormal allele was sufficient to cause disease in a hemizygous male, while females needed two. On the other hand, in “X‐linked dominant” conditions, a single abnormal allele was sufficient to cause a condition in both males and females. Although these patterns of inheritance hold true for many conditions, female carriers may exhibit features of an X‐linked recessive condition, due to skewed X‐inactivation. For example, in Duchenne muscular dystrophy, approximately 25% of female carriers will exhibit some symptoms of the condition, ranging from muscle weakness to features as severe as seen in males (Hoogerwaard et al. 1999). This has led many researchers to argue that conditions with loci on the X‐chromosome should simply be termed, “X‐linked.”