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A man who is an achondroplastic dwarf with normal vision marries a color-blind woman of normal height. The man's father was 6 feet tall, and both the woman's parents were of average height. Achondroplastic dwarfism is autosomal dominant, and red-green color blindness is X-linked recessive.
-What proportion of their sons would be color-blind and of normal height?
A) none
B) half
C) one out of four
D) three out of four
E) all

Question

Quizplus · Accepted Answer

Since red-green color blindness is X-linked recessive, all sons will inherit their X chromosome from their mother. If the mother is color-blind, she must have two copies of the color-blindness allele (since it's recessive and she expresses the trait), meaning all sons will inherit one copy and thus be color-blind. Achondroplastic dwarfism is autosomal dominant, so a child needs to inherit only one copy of the allele to express the trait. The father, being a dwarf, has one copy of the dwarfism allele. Since dwarfism is not linked to the X or Y chromosome, each child has a 50% chance of inheriting the allele for dwarfism from their father. Therefore, half of the sons will inherit the normal height allele from their father and be of normal height while also being color-blind, as they inherit the X-linked recessive allele for color blindness from their mother.

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