sex-linked genes cannot be masked in men, recessive sex-linked phenotypes occur more often in men. Results of crosses involving sex-linked genes depend on which parent has the gene for the trait. In humans, for example, color blindness and hemophilia result from recessive sex-linked genes. Even if neither parent expresses such traits, if the mother is heterozygous for such traits, the chance exists that half the male offspring will express the traits. Females, in contrast, can only get such traits if both the father expresses one of them and the mother is homozygous recessive or heterozygous for one of them.
In females, only one of the X chromosomes is active in somatic cells. The other condenses into a small dark body called a Barr body. The genes on the Barr body are inactive. In women who are heterozygous for traits coded by genes on the sex chromosomes, half of the dominant alleles and half of the recessive alleles are condensed into inactive Barr bodies, but, as long as some dominant alleles function in some cells, the recessive alleles are not expressed.
