A man who is a dwarf with achondroplasia and normal vision marries a colour-blind woman of normal height. The man's father was six feet tall, and both the woman's parents were of average height. Dwarfism caused by achondroplasia is autosomal dominant, and red-green colour blindness is X-linked recessive. How many of their daughters might be expected to be colour-blind with achondroplasia?
A) none
B) half
C) one out of four
D) three out of four

Question

Quizplus · Accepted Answer

Since achondroplasia is autosomal dominant, the man must be heterozygous (Aa) because his father was of normal height (aa). The woman is homozygous recessive for height (aa) and is colour-blind, meaning she has two X-linked recessive alleles (XbXb). Their daughters will inherit one X chromosome from each parent, so they will all be carriers for colour blindness (XBXb) but not colour-blind themselves. For achondroplasia, each daughter has a 50% chance of inheriting the dominant allele (A) from their father, so none of the daughters will be colour-blind with achondroplasia.

A Man Who Is a Dwarf with Achondroplasia and Normal