In humans, normal vision dominates color-deficiency, and both are X-linked. (The genes are located on the X chromosome.) A color-deficient male marries a woman who is heterozygous for normal vision. If they have a boy, the chance that he will be color-deficient is

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Since the gene for color vision is X-linked, a male child inherits his X chromosome from his mother and his Y chromosome from his father. The mother is heterozygous, meaning she has one allele for normal vision and one for color deficiency. Therefore, there is a 50% chance she will pass on the allele for color deficiency to a male child, making the probability that he will be color-deficient 50%.

In Humans, Normal Vision Dominates Color-Deficiency, and Both Are X-Linked