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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 chances that this child would be colour-blind?
A
50%
B
100%
C
Zero percent
D
25%
Detailed Solution
Colour blindness is X-linked recessive. The man is normal-visioned, so his genotype is $XY$ (he received Y, not X, from his colour-blind father).
The woman received $X^c$ from her colour-blind father, so she is a carrier: $XX^c$.
A daughter gets the normal X from her father and either $X$ or $X^c$ from her mother: $XX$ (normal) or $XX^c$ (carrier).
No daughter can be colour-blind, so the chance is zero percent.
The woman received $X^c$ from her colour-blind father, so she is a carrier: $XX^c$.
A daughter gets the normal X from her father and either $X$ or $X^c$ from her mother: $XX$ (normal) or $XX^c$ (carrier).
No daughter can be colour-blind, so the chance is zero percent.