Hemophilia is a sex linked trait in humans, inherited in the same way white eye color in Drosophilia. Males are hemizygous, receiving their only X chromosome from their mother.
Females are heterozygous, inheriting X chromosomes from both parents.
If a female has a defective gene on one of her two X chromosomes, she will be protected from its effects by the normal gene on her second X chromosome. If a male has a mutant X and a normal Y chromosome, he will be affected by a X-linked disease. A son, whose mother has two normal alleles, will not be affected by hemophilia even if the father has the disease and the defective gene. A daughter of the same parents will be a heterozygous carrier. Half the daughters will be carriers and half the sons will be hemophilic.