Question

A woman has normal vision although her maternal grandfather (her mother's father) had red-green colorblindness, a sex-linked recessive trait. Her maternal grandmother and the woman's own father are assumed to not possess a copy of the mutant allele. The woman marries a man with normal vision although his father was colorblind. What is the probability that the first child of this couple will be colorblind?

Answer 1

Red-green colorblindness is a sex-linked recessive trait. A woman who is a carrier of this trait and a man with normal vision have a 50% chance of having a colorblind son. Daughters will not be colorblind but may be carriers.

Step-by-step explanation:

Red-green colorblindness is a sex-linked recessive trait, which means that it is carried on the X chromosome. Females have two X chromosomes while males have one X and one Y chromosome. Because it is recessive, a female would need to inherit two copies of the allele to exhibit colorblindness, while a male would only need one copy because they do not have a second X chromosome to mask the trait.

Given that the woman's maternal grandfather had red-green colorblindness and her mother did not, the woman is a carrier of the colorblindness allele. Her husband has normal vision, but his father was colorblind, which does not affect the probabilities of their children being colorblind since men pass the Y chromosome, not the X, to their sons. Therefore, the probabilities for their children would be as follows:

• If they have a son, there is a 50% chance the son will inherit the colorblindness allele from his mother and thus be colorblind because men only have one X chromosome.
• If they have a daughter, she must inherit the X chromosome carrying the colorblindness trait from her mother and an affected X from her father, which is impossible in this scenario since the father has normal vision. Therefore, the daughters will not be colorblind but could be carriers if they inherit the affected X from their mother.