X-linked Inheritance: Hemophilia
Legend:
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.
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