Question

Red/green color blindness in humans is caused by an X-linked recessive allele. If a male who is affected by the condition mates with a female who is not affected, and is also not a carrier, what percentage of their sons will have the disorder?Red/green color blindness in humans is caused by an X-linked recessive allele. If a male who is affected by the condition mates with a female who is not affected, and is also not a carrier, what percentage of their sons will have the disorder?

Answer 1

50% of the sons have the disorder.

When this couple has children, one half of their sons will have the disorder. When a male has an X-linked, recessive disorder, his genome will be YXₙ, where Xₙ represents his affected X-chromosome. Since boys can only have one X-chromosome, they cannot be carriers. Having one affected X-chromosome makes them have the condition. His wife, who is not a carrier and does not have the condition will have the genome XX. As you can see in the punnet square, this cross will lead to two sons, one of which has the color-blindness gene. So, 50% of the sons will have the disorder.