A man is heterozygous for a rare trait that is assumed to be caused by a dominant autosomal mutant allele. He marries a woman who is phenotypically normal. They have a phenotypically normal daughter who marries a phenotypically normal man. Unexpectedly their first child, a girl, has the same abnormal trait as her grandfather. Which of the following BEST explains this situation?
A) The trait is really due a mutation in a cytoplasmic gene and can be only inherited from the mother.
B) This is a sex-influenced trait and is expressed more often in females than in males.
C) This is an example of genomic imprinting and the mutant allele is only expressed if it is inherited from the father.
D) The mutant allele shows incomplete dominance when heterozygous with the normal allele.
E) The mutant allele shows reduced penetrance and the normal daughter carries the mutant allele.

Question

Quizplus · Accepted Answer

The mutant allele shows reduced penetrance, meaning that the daughter could carry the allele without expressing the trait, and then pass it on to her child, who does express the trait.

A Man Is Heterozygous for a Rare Trait That Is