Some genes on sex chromosomes play a role in sex determination, but these chromosomes also contain genes for other traits. Most X-linked genes have no homologous loci on the Y chromosome. 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.