X-linked Recessive Inheritance - Example

Pedigree

The X-linked recessive trait is not expressed in female carriers, who have at least some cells with an unaffected, active X chromosome. However, female carriers can pass the trait to their sons, who will be affected, and to half their daughters. Affected males cannot pass the trait to their sons, but will pass it to all daughters. The trait appears to skip a generation when female carriers only are present. A female can be affected (homozygous) if an affected father marries a carrier.

Hemophilia A

An example of an X-linked recessive condition is the blood clotting disorder known as hemophilia A. In this condition, there is a mutation in the gene coding for the production of blood clotting factor VIII. This disease illustrates the fact that the amount of product made can vary somewhat, and factor VIII activity determines the severity of the disease, so the phenotype varies from mild to severe, a phenomenon called variable expression. Females are typically carriers. Fathers with the disease cannot transmit the disease to sons, but may give the trait to daughters who become carriers. Thus, the disease often shows a pattern of skipping a generation. In females, one X chromosome is typically inactivated, and this is random, so female carriers are usually left with adequate factor VIII activity.