Inheritance



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.
 * Hemophilia** is a sex linked trait in humans, inherited in the same way white eye color in males are hemizygous, receiving their only X chromosome from their mother. Females are heterozygous, inheriting X chromosomes from both parents.

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.

A heterozygous carrier mother and a normal father pass the gene for hemophilia on to possibly one- half of their children. Half the daughters will be carriers and half the sons will be hemophilic. The rest of the siblings will be normal. Daughters, as long as one parent is genotypically normal, can only be carriers. The normal gene on the second X chromosome counteracts the defect and the daughters do not suffer from the trait. If a son receives the defective gene from his mother, he will be hemophilic because the Y chromosome can not counteract the defective gene located on his X chromosome.