Polydactyly is a rare autosomal dominant trait that is 80% penetrant. What is the chance that a child born to an affected father and a normal mother will have the disorder

Polydactyly is a rare autosomal dominant trait that is 80% penetrant. What is the chance that a child born to an affected father and a normal mother will have the disorder?

Assume the paternal grandfather is normal.

I will use P to encode polydactyl as it is dominant and use p allele to encode normal phenotype

P= Polydactyl

p= Normal

From what is given in the question,the paternal grandfather is normal and that means that the father of the child receives one wild type allele from the paternal grandfather, however,the father is affected with the disease,hence the genotype is A a but the mother being normal will have the genotype of aa.

That is Parents;

Father Aa × Mother aa

Gametes a a A Aa ( P ) Aa ( P )

a aa ( p ) aa ( p )

So if it is 100% penetrant, that implies that the chances of the affected child is;

(2/4 ) ×100=50%

However, only 80% penetrant is given here,so that is why the chances of the affected is then ; (50/100) ×(80/100) ×100= 40%

That means the affected child chances are 40%.