special subjects
Single Gene Defects
X Linked Dominant
X-linked dominant traits are carried on the X chromosome. Most are rare. Usually, males are more severely affected; some X-linked dominant disorders are often lethal in males. Females who carry only one abnormal allele are affected but less severely. A typical pedigree is shown in figure. In general, the following rules of inheritance apply: Affected males transmit the trait to all of their daughters but to none of their sons. Heterozygous females transmit the trait to half of their children, regardless of sex. Homozygous females transmit the trait to all of their children. Heterozygous females are also affected, albeit less severely. Because females can be heterozygous or homozygous, more females have the trait than males. The difference between the sexes is even larger if the disorder is lethal in males. X-linked dominant inheritance may be difficult to differentiate from autosomal dominant inheritance by studying only inheritance patterns. Large pedigrees are required, with particular attention to children of affected males because male-to-male transmission rules out X-linkage (males pass only their Y chromosomes to their sons). 