X-linked Inheritance: Hemophilia
Hemophilia is a sex linked trait in humans, inherited in the same way white
eye color in Drosophila.
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.
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.