A woman with normal vision, but whose father was colour-blind, marries a colour-blind man. Suppose that the fourth child of this couple was a boy. This boy-
maybe colour blind or may be of normal vision
The correct option is B.
Explanation of correct option:
Table 1: Carrier woman (XXc) X Colour-blind man (XYXXS)
Gametes | Xc | Y |
X | XXc (Carrier daughter) | XY (Normal son) |
Xc | XcXc (Colour-blind daughter) | XcY (Colour-blind son) |
Explanation of incorrect option:
Option A:
When a carrier woman marries a color-blind man, their fourth son can be color blind or have normal vision.
Option C:
Male receives one X chromosome from the mother, hence they will be color blind even if they have one affected allele. They are never heterozygous for the color-blind mutant allele.
Option D:
The fourth son of a carrier mother and color-blind father may be color-blind or have normal vision.
Final answer: A normal-visioned woman whose father was color-blind marries a color-blind man. Their fourth son may be color blind or may be of normal vision.