A man was born with six fingers on each and six toes on each foot.His wife and their son have a normal number of digits.Having extra digits is a dominant trait.The couple's second child has extra digits.What is the probability that their next child will have extra digits,assuming that the trait exhibits complete penetrance?
A) 10%
B) 25%
C) 50%
D) 100%
E) 0%

Question

Quizplus · Accepted Answer

Since having extra digits is a dominant trait and the man has extra digits, he must have at least one dominant allele (let's denote it as "A"). Given that his wife and their first son have a normal number of digits, the wife must have two recessive alleles (aa), and their son must have inherited one recessive allele from each parent, indicating the man is heterozygous (Aa). For their second child to have extra digits, they must have inherited the dominant allele from the father and a recessive allele from the mother, confirming the father's heterozygous state. Therefore, for each child, there is a 50% chance of inheriting the dominant allele from the father (Aa x aa cross results in 50% Aa offspring with extra digits and 50% aa offspring with a normal number of digits).

A Man Was Born with Six Fingers on Each and Six