A color-blind man and a woman with normal vision whose father was color blind have a son.Color blindness, in this case, is caused by an X-linked recessive gene.If only the male offspring are considered, the probability that their son is color blind is
A)0.25 (or 25%).
B)0.50 (or 50%).
C)0.75 (or 75%).
D)1.00 (or 100%).
E)none of these.

Question

Quizplus · Accepted Answer

Since the woman has normal vision, she must have inherited one normal X chromosome from her father. However, since her father was color blind, we know that he must have passed on his recessive color-blindness allele to his daughter. Therefore, the woman is a carrier of color blindness.

When the woman has a son, she can only pass on one of her X chromosomes - either the normal one or the color-blindness carrier one. The father, being color blind, can only pass on his Y chromosome to his son. Therefore, the son has a 50% chance of inheriting the mother's normal X chromosome and a 50% chance of inheriting her color-blindness carrier X chromosome.

If the son inherits the normal X chromosome, he will not be color blind. If he inherits the color-blindness carrier X chromosome from his mother, he will not be color blind but will be a carrier like his mother. If he inherits the color-blindness X chromosome from his mother and the Y chromosome from his father, he will be color blind.

Thus, the probability that their son is color blind is 0.50 (or 50%).

A Color-Blind Man and a Woman with Normal Vision Whose