Hemophilia is a sex-linked trait in humans, inherited in the same way white eye color in Drosophilia.

Males who receive their only X chromosome from their mother are hemizygous. Females who inherit X chromosomes from both parents are heterozygous.

If the only X chromosome of the male is mutated, a X linked disease will affect him because the other Y chromosome he has will not protect him. The normal gene on the second X chromosome of a female will protect her from the effects of a defective gene on her other X chromosome.

A son will not be affected by hemophilia even if the father has the defective gene and has the disease if his mother has two normal alleles. The daughter of the same parents will be a heterozygous carrier.

A normal father and a heterozygous carrier mother pass the gene for hemophilia on to possibly one- half of their children. Half the sons will be hemophilic and half the daughters will be carriers. . Since the normal gene on the second X chromosome counteracts the defect, the daughters can only be carriers as long as one parent is genotypically normal. Since the Y chromosome of the son cannot offset the defective gene from his X chromosome, a son will be hemophilic if he receives the defective gene from his mother as the Y chromosome cannot offset the defective gene from his X chromosome.