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X-linked recessive traits appear more frequently in males.

I 6.7 X-linked recessive traits appear more often in males and are not passed from father to son.

the bone. Fortunately, bleeding in people with hemophilia A can be now controlled by administering concentrated doses of factor VIII.

X-Linked Dominant Traits

X-linked dominant traits appear in males and females, although they often affect more females than males. As with X-linked recessive traits, a male inherits an X-linked dominant trait only from his mother—the trait is not passed from father to son. A female, on the other hand, inherits an X chromosome from both her mother and father; so females can receive an X-linked trait from either parent. Each child with an X-linked dominant trait must have an affected parent (unless the child possesses a new mutation or the trait has reduced penetrance). X-linked dominant traits do not skip generations ( FIGURE 6.9); affected men pass the trait on to all their daughters and none of their sons, as is seen in the children of I-1 in Figure 6.9. In contrast, affected women (if heterozygous) pass the trait on to 1/2 of their sons and 1/2 of their daughters, as seen in the children of II-5 in the pedigree.

Concepts]"

X-linked dominant traits affect both males and females. Affected males must have affected mothers (unless they possess a new mutation), and they pass the trait on to all their daughters.

An example of an X-linked dominant trait in humans is hypophosphatemia, also called familial vitamin D-resis-tant rickets. People with this trait have features that superficially resemble those produced by rickets: bone deformi-

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