Question

Multiple genes on the X chromosome and autosomal chromosome are needed for full colour-vision. Mutations in these will make them colour-blind.

1.Consider that a colour-blind woman and a colour-blind man have a daughter with normal vision. Explain how this could occur.

2.Suppose they have multiple children and all the males are colour blind but none of the females are. Explain how this could occur (With symbols and cross).

The answers for questions 1 and 2 don't have to be necessarily the same

Answer 1

1. Since the genes for normal color vision are present on both X and autosomal chromosomes, we get many combinations where affected parents can have normal child. Since the child will get alleles from both the parents, the complementation of normal alleles will compensate the mutations and the daughter can have normal vision.

What if the mother has mutation in autosomal gene and father has it in X chromosome. Then the normal alleles from the parents will make the daughter normal.

Mother genotype = aa XX; Father genotype = Aa X^cY.

Here A and a refer to autosomal alleles and a is mutated allele. X^c is mutated X chromosome. From the cross between the above example, we can get normal daughter as Aa XX^c.

2. In this case where all sons are affected, we need to focus on X chromsome. As autosomal is not gender oriented. Here the case is possible if mother has both mutated X^c alleles as AA X^cX^c. But here father has to be aa XY, in order to make all females normal. Cross between AA X^cX^c and aa XY would be-

Aa XX^c and Aa X^cY.

This means all sons affected and daughter have normal vision.