A normal-visioned man whose father was colour blind, marries a woman whose father was also colour bl

Question

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

Accepted Answer

Sol.(b) [Key Concept: X-linked Colour Blindness — Daughter's Probability] Father's father was colour blind → father must have received X^c from his mother (but father is normal-visioned, so father is X^C Y — he has normal X^C from his mother, Y from his colour blind grandfather). Wait — actually the normal-visioned man's father was colour blind: he received Y from father, X^C from mother → he is X^C Y (normal, can't be carrier as males are hemizygous). Mother's father was colour blind → mother is carrier X^C X^c. Cross: X^C Y × X^C X^c → daughters: X^C X^C (normal) or X^C X^c (carrier normal) — NO daughter can be colour blind (0%). Answer: zero percent (b).

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