Question

A normal visioned man whose father was colourblind, marries a woman whose father was also colourblind. They have their first child as a daughter. What are the chances that this child would be colourblind?

• A. 50%
• B. 100%
• C. 0%
• D. 25%

Answer 1

The correct option is C 0%

Colourblindness is X-linked recessive disorder. Two copies of defective gene is required in females to cause colourblindness. Since father is colourblind, he will have genotype X${}^C$Y. A normal visioned man will have genotype XY. A woman will have genotype either X${}^C$X${}^C$ or X${}^C$X as she received X${}^C$ chromosome from father. If man with genotype XY marries woman with genotype either X${}^C$X${}^C$ or X${}^C$X, none of their daughter will be colourblind as she will receive one normal X chromosome from father.

Thus, the correct answer is option C.