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 the probability that they will be normal?
A) 0
B) ¼
C) 1/3
D) 2/3
E) ½

Question

Quizplus · Accepted Answer

The couple are both heterozygous (Aa) for achondroplasia. The possible genotypes for their children are AA, Aa, Aa, and aa. The AA genotype is lethal, so only Aa and aa survive. Out of the surviving children, 1 out of 3 (aa) will be normal, giving a probability of 1/3.

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