A woman is heterozygous for a recessive allele that causes X-linked hemophilia. This recessive allele has a mutation that makes a gene for a blood-clotting factor nonfunctional. Considering the implications of X-inactivation, the level of clotting factor in her blood is likely to be approximately _____ that of a homozygous nonmutant woman.

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In females, X-inactivation randomly inactivates one of the two X chromosomes in each cell. Therefore, in a heterozygous woman for X-linked hemophilia, approximately half of the cells will express the nonmutant clotting factor gene (from the active X chromosome) and half of the cells will express the nonfunctional clotting factor gene (from the inactive X chromosome). As a result, the heterozygous woman will produce approximately half the amount of clotting factor as a homozygous nonmutant woman.

A Woman Is Heterozygous for a Recessive Allele That Causes