Red-Green color blindness is inherited as an X-linked recessive trait in humans. A woman, whose mother had red-green color blindness, marries a colorblind man. What are the chances that their child will be a colorblind female? Answer Here: colorblind female? Answer Here: normal male? Answer Here: Explain why males cannot “carry” the colorblindness recessive allele.
The Correct Answer and Explanation is:
Colorblind female? 50% Normal male? 0%
Explanation:
Red-green color blindness is caused by a mutation in a gene located on the X chromosome. Since it is a recessive trait, a person must inherit two copies of the defective gene (one from each parent) to express the condition if they are female (XX), or just one copy if they are male (XY), because males have only one X chromosome.
Let’s analyze the genetic background of the couple:
- The woman’s mother was colorblind, meaning the mother had two defective X chromosomes (XᶜXᶜ).
- Since the woman is not colorblind but her mother was, she must be a carrier. This means she has one normal X and one defective X (XᶜX).
- The man is colorblind, so his genotype is XᶜY.
Now we consider the possible combinations of their children:
- Female children inherit one X from each parent. The mother can pass on either X or Xᶜ, and the father can only pass on Xᶜ.
- If the mother passes Xᶜ and the father passes Xᶜ, the daughter will be XᶜXᶜ — colorblind.
- If the mother passes X and the father passes Xᶜ, the daughter will be XᶜX — a carrier, but not colorblind.
- Therefore, there is a 50% chance of a colorblind daughter.
- Male children inherit the X chromosome from their mother and the Y chromosome from their father.
- If the mother passes Xᶜ, the son will be XᶜY — colorblind.
- If the mother passes X, the son will be XY — normal.
- So, there is a 50% chance of a colorblind son and 50% chance of a normal son.
Males cannot “carry” the colorblindness allele without expressing it because they have only one X chromosome. If that X carries the defective gene, they will be colorblind. There is no second X to mask the effect, unlike in females.
