A baby boy is born with six toes (polydactyly) on his right foot, a trait that neither parent has but one that the paternal grandfather has. What is the bestexplanation for this occurrence?
A)The baby's grandfather is really his father.
B)The baby inherited a recessive allele for this trait from each parent.
C)The father's phenotype results from incomplete penetrance of a dominant allele.
D)The baby's phenotype demonstrates a higher level of expressivity than his father's phenotype.

Question

Quizplus · Accepted Answer

Polydactyly is an autosomal-dominant single-gene trait. Thus, this baby boy did not inherit a recessive allele from each parent to express the trait. It is possible that the baby's grandfather could be his father, but this is not the best explanation. In expressivity, the trait is always expressed, but the degree to which it is expressed can vary. This means that the baby's father might have more than one extra digit, but if expressivity were the actual problem, the father would definitely also have an extra digit. With variation in penetrance, the gene is present but not always expressed. For polydactyly, the penetrance is about 80%. So, the father most likely does have the allele, but it is not expressed in him. However, because the allele is present, he can transmit it (with a probability of 50% for each pregnancy) to his children.

A Baby Boy Is Born with Six Toes (Polydactyly) on His