A woman has just had twins, a boy and a girl. The woman\'s father has two X-link

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. 12 marks]

Explanation / Answer

Please find the answers below:

Answer a) According to the information, the woman's father has two alleles for hemophilia and color blindness but the woman herself is unaffected. This means that the woman is heterozygous for both these alleles. Secondarily, the father has neither of the disorders making his genotype devoid of these alleles. Thus, the genotypes for mother and father are XabX and XY, respectively.

Here, it is important to note that the loci are located only 5cM apart, thus demonstrating that they will be inherited as single unit due to complete linkage. Thus, all the offsprings, either male of female, will carry both the alleles together or no allele at all. Thus, there is 0% probability that a boy would be color-blind but not haemophilic in nature.

Answer b) The Punnett square can be formulatd as below:

Thus, this Punnett square clearly shows that all girls would be unaffected for both the diseases since they are either heterozygous for both alleles or do not carry them at all.

Xab X X XXab XX Y XabY XY

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