Examples of sex-linked traits in humans are color blindness and hemophilia. Fathers pass X-linked alleles to only and all of their daughters. Males receive their X chromosome only from their mothers. Fathers cannot, therefore, pass sex-linked traits to their sons. Mothers can pass sex-linked alleles to both sons and daughters.
Females receive two X-chromosomes, one from each parent.
If a sex-linked trait is due to a recessive allele, a female will express the trait only if she is homozygous. Females have two X chromosomes, therefore they can be either homozygous or heterozygous for sex-linked alleles. There are fewer females with sex-linked disorders than males, because even if they have one recessive allele, the other dominant allele is the one that is expressed. A female that is heterozygous for the trait can be a carrier , but not show the recessive trait herself. A carrier that mates with a normal male will pass the mutation to half her sons and half her daughters.