Question

A woman has just had twins, a boy and a girl. The woman’s father has two X-linked recessive disorders, haemophilia and colour-blindness, but the woman is unaffected. The father of the children has neither disorder. Both disorders are completely penetrant. The loci for the two traits are known to be 5cM apart. a) What is the probability that the boy will be colour-blind but not haemophilic? Explain your answer, including showing the genotypes and phenotypes of the family. [10 marks] b) What is the probability that the girl will be unaffected (i.e. not haemophilic and not colour-blind)? Explain your answer.

Answer 1

a.

Both disorders are X-linked recessive disorders, thus the boy will be either colour-blind and haemophilia or unaffected.

0.5 probability of colour-blind and haemophilia. If genes are 5 cM apart that means 5% chances it has for recombination.

so probability of colour-blind but not haemophilic is 0.5 * 0.05 = 0.025

b.

The girl will be unaffected (i.e. not haemophilic and not colour-blind) because it receives unaffected dominant genes from his father.

so the probability of the girl being unaffected is 1.