Haemophilia

What haemophilia is

Haemophilia is a disorder in which the blood does not clot normally, so even minor injuries can lead to prolonged bleeding. It is caused by a recessive allele on the X chromosome.

The dominant allele (X^H) allows normal blood clotting; the recessive allele (X^h) does not.

Worked example: carrier mother x father with haemophilia

A mother who is a carrier (X^H X^h) has children with a father who has haemophilia (X^h Y).

Parental genotypes: X^H X^h x X^h Y.

Gametes: mother produces X^H and X^h; father produces X^h and Y.

Reading the result

Among daughters: 1 X^H X^h (carrier, unaffected) : 1 X^h X^h (has haemophilia), half the daughters are affected, unlike in most carrier-mother crosses, because here the father also contributes a recessive allele.

Among sons: 1 X^H Y (unaffected) : 1 X^h Y (has haemophilia), half the sons are affected.

Overall, the phenotype ratio among all children is 1 affected : 1 unaffected.

How it is examined

SPM questions typically state the haemophilia status of both parents, ask for their genotypes, require a completed Punnett square, and ask for the probability that a son or daughter is affected.

Worked problem 1 (easy): carrier mother x normal father

Question. A carrier mother (X^H X^h) has children with a father who has normal blood clotting (X^H Y). Determine the genotype and phenotype ratio of their children.

Gametes. Mother: X^H and X^h. Father: X^H and Y.

Worked problem 2 (medium): normal mother x father with haemophilia

Question. A homozygous normal mother (X^H X^H) has children with a father who has haemophilia (X^h Y). Determine the phenotypes of the children and explain the risk carried into the next generation.

Gametes. Mother: X^H only. Father: X^h and Y.

Worked problem 3 (SPM-level): deducing a hidden genotype

Question. A woman whose father had haemophilia marries a man with normal blood clotting. Neither the woman nor her mother shows haemophilia.

Determine the probability that a son of this couple has haemophilia.

Reasoning. The woman's father was X^h Y and passed his only X chromosome (X^h) to every daughter, so the woman must carry X^h. She does not have haemophilia, so her other allele is X^H; her genotype is X^H X^h (carrier).

The husband is X^H Y. Gametes: X^H and X^h from the woman; X^H and Y from the man.

Practice problems

1. A carrier mother (X^H X^h) and a father with normal clotting (X^H Y) have a daughter. State the probability that she is a carrier.
2. A man has haemophilia (X^h Y). Explain, using genotypes, whether he inherited the recessive allele from his mother or his father.
3. State the genotypes of a couple who could have a daughter with haemophilia, and give the phenotype ratio of their daughters.

Answers to the practice problems

Common mistakes

Source:SRC-DSKP-EN

Frequently asked questions