If a colour blind female marries a man whose mother was also colour blind, what are the chances of her progeny having colour blindness?

Solution & Explanation

Answer: (D) 100% Solution: Colour blindness is X-linked recessive. A colour-blind female is X(c)X(c). A man whose mother was colour blind, if he is unaffected, is X(C)Y but his single X carries the dominant normal allele; however every son receives the mother's X(c) and is colour blind, and every daughter receives X(c) from mother and X(C) from father, making them carriers (not colour blind). For ALL progeny to be colour blind, the father must himself be colour blind X(c)Y (inheriting his colour-blind mother's X(c)); then sons X(c)Y are colour blind and daughters X(c)X(c) are colour blind too, giving 100%. The intended answer treats the man as colour blind, so all offspring are affected. NCERT Reference: NCERT Class XII, Ch 4, p.73, lines 30-33: "Colour Blindness : It is a sex-linked recessive disorder ... This defect is due to mutation in certain genes present in the X chromosome."