Question

A recessive allele on the X chromosome is responsible for red-green color blindness in humans. A woman with normal vision whose father is color-blind marries a color-blind male. What is the probability that a son of this couple will be color-blind?

1) 0
2) 1/4
3) 1/2
4) 3/4
5) 1

Answer 1

The probability that a son of the couple will be color-blind is 1/2 or 50%, considering the mother is a carrier and the father is color-blind. This is due to X-linked recessive inheritance where the son has an equal chance of inheriting either the normal X or the colorblind Xc from his mother.

Step-by-step explanation:

Probability of Colorblindness in a Son

To determine the probability that a son of a couple will be color-blind when the mother has normal vision but is a carrier of the colorblindness trait, and the father is color-blind, we must understand X-linked recessive inheritance. In this scenario, the woman has a normal vision allele (X) and a colorblindness allele (Xc) because her father was colorblind, indicating she is a carrier. The color-blind male will contribute a Y chromosome to his sons and his only X chromosome, which carries the colorblindness allele (Xc), to his daughters.

Here's a breakdown of the inheritance:

• The sons will inherit their mother's normal X or colorblind Xc and their father's Y chromosome. They have a 50% chance of inheriting the Xc (and being color-blind) or a normal X (and having normal vision).
• The daughters will always inherit the father's colorblind Xc along with either the mother's normal X or colorblind Xc. If they inherit the mother's Xc, they will be colorblind, but if they inherit her normal X, they will only be carriers.

Therefore, the probability that a son will be color-blind is 1/2 or 50%, as there is an equal chance of him inheriting either the mother's normal X chromosome or the colorblind Xc chromosome.