Hemophilia is an X-linked recessive trait. If a non-hemophiliac man mates with a non-hemophiliac woman and they have a hemophiliac son, (a) what is the probability that their next son will have hemophilia? (b) What phenotypes and genotypes are possible for their daughters?

(a) 1/2

(b) Possible Genotype =

    Possible phenotype = non-haemophilic

Explanation:

Let the allele for haemophilia be h, the alternate form would be H.

Genotype of non-haemophilic man would be

Genotype of non haemophilic woman could be either

If they have an haemophilic son, it means the genotype of the woman is

Crossing the two:

  x    =

(a) One out of the two sons is affected. Hence the probability of their next son having haemophilia is 1/2.

(b) the possible genotypes for their daughters are and both of them have no haemophilia.

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