Narrative:
Achondroplasia, a hereditary form of dwarfism, is an autosomal dominant disorder; however homozygous dominant individuals either die before birth or very shortly thereafter. The homozygous recessive condition is normal. A dwarf couple decide to have children. Use this information to answer the following question:
Of the children of this couple who survive, what is probability that they will be affected by achondroplasia?
A) 0
B) ¼
C) 1/3
D) 2/3
E) ½

Question

Quizplus · Accepted Answer

Since achondroplasia is autosomal dominant, the parents are both heterozygous (Aa). The possible genotypes for their children are AA, Aa, and aa. The AA genotype is lethal, so only Aa (affected) and aa (normal) survive. Out of the surviving children, 2/3 will be Aa (affected) and 1/3 will be aa (normal).

Narrative: Achondroplasia, a Hereditary Form of Dwarfism, Is an Autosomal Dominant