Question

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

• A. 25%
• B. 50%
• C. 100%
• D. 0%

Answer 1

The correct option is D 0%

Colourblindness is a X-linked recessive disorder. Father transmit its X chromosome to the daughters so the colourblind father will inherit its affected X chromosome to the daughter. Hence, the genotype of carrier female will be (X${}^c$X). The father of man was colourblind but that as son don't receive X-chromosome of father, the man will be normal (XY). The carrier female and normal man will inherit the disease to 1 son and no daughter. They have their first child a daughter and there are 0% chances for her to have disease. Correct answer is option D.

Parent generation : "X${}^c$X" x "XY"

Gametes : X${}^c$X --> XY	X${}^c$	X
X	X${}^c$X normal carrier girl	XX normal girl
Y	X${}^c$Y colourblind boy	XY normal boy