Question

In humans, one form of polydactyly (extra fingers and toes) is controlled by a single autosomal gene with two possible alleles; the dominant allele leads to polydactyly, the recessive is for the normal number of fingers and toes. A woman with this form of polydactyly whose extra digits were removed when she was a baby mates with a man who also has this form of polydactyly and who also had his extra digits removed when he was a baby. Their first child is born with the normal number of fingers and toes. What the odds that their second child will also be born with the normal number of fingers and toes?

Answer 1

25% chance / 0.25 chance the second baby will be born with this polydactyl. If we make the allele for polydactyl p, we know both the parents have this allele as Pp because the first child was able to get pp. So if you do a punnette square, the 3 possible options are PP (25% chance) Pp (50% chance) and pp (25% chance).