X-linked recessive disorders affect males, whereas female carriers are generally spared. This is due in part to the random inactivation in females of one of the two X chromosomes in all somatic cells. Normal females are thus a mosaic of two cell populations, each expressing the alleles from one X chromosome or the other. Thus, in female carriers of an X-linked mutation, approximately 50 percent of cells on average have the normal allele on the active X chromosome (with the mutant allele being on the inactive X chromosome), and these functionally normal cells are generally sufficient to spare females from . . .