Question

A man and woman, both having colour blind fathers got married. Probability of their first daughter being colour blind is

• A. 100%
• B. 50%
• C. 25%
• D. 0%

Answer 1

The correct option is D 0%

Colour blindness is X-linked recessive disorder. Father transmits its X chromosome to the daughters so the colour blind father has passed his affected X chromosome to daughter, the woman. A woman with two copies of affected allele does not survive which means that genotype of the woman is X${}^c$X. Since a son gets his X-chromosome from her mother, not from the father, the genotype of man is “XY”. The carrier woman will transmit the disease to 50% sons; as father lack the affected allele, the daughter will be normal. The correct answer is D.

Parent generation : “X${}^c$X" x “XY”

Gametes X${}^c$X ---> XY	X${}^c$	X
X	X${}^c$X (normal carrier daughter)	XX (normal daughter)
Y	X${}^c$Y (colour blind son)	XY (normal son)