A restriction-fragment length polymorphism at a site in the X chromosome has three alleles, A₁, A₂, and A₃. The gel below is the DNA fingerprint for two girls who have the same parents. What is the genotype of the mother? Of the father?

Question

Quizplus · Accepted Answer

From the gel, it can be observed that one sister has the A₁/A₃ genotype and the other has the A₂/A₃ genotype. This means that the mother has to be heterozygous at the site, carrying both the A₃ allele and one of either A₁ or A₂ allele. The father must also be heterozygous at the site, carrying the other allele that the mother does not have, i.e., if the mother is A₃/A₁, then the father is A₂/A₃ and if the mother is A₃/A₂, then the father is A₁/A₃. Therefore, the correct answer is A.

A Restriction-Fragment Length Polymorphism at a Site in the X