Haemophilia
Haemophilia is the standard example of a sex-linked recessive condition, and working one cross through carefully makes every other sex-linked question straightforward.
Haemophilia
A condition in which the blood does not clot properly, so bleeding continues much longer than usual. It is caused by a recessive allele on the X chromosome, written Xh, with the normal allele written XH.
A carrier mother and an unaffected father
A woman with the genotype XHXh does not have haemophilia — the dominant allele on her other X masks it — but she can pass the recessive allele on. She is called a carrier. Cross her with an unaffected man, XHY.
| X HX h × X HY | XH | Y |
| XH | XHXH | XHY |
| Xh | XHXh | XhY |
The mother's two kinds of ovum down the side, the father's two kinds of sperm along the top.
The four outcomes, each one chance in four
- XHXH — a daughter who neither has the condition nor carries it.
- XHXh — a daughter who is a carrier, like her mother.
- XHY — an unaffected son.
- XhY — a son with haemophilia. He has no second X, so the recessive allele is unopposed.
Half the sons, none of the daughters
Of the two sons, one has the condition; of the two daughters, neither does. For a daughter to have haemophilia she would need Xh from both parents, which means her father would have to have the condition himself. That is possible but far less common.
It passes from mother to son
A father gives his son a Y, never his X — so a man cannot pass haemophilia to his son at all. An affected boy got the allele from his mother, who very often has no idea she carries it. That is why the condition appears to skip generations and to run down the female line while showing in males.