The autosomal dominant trait may en defined as the trait that can express itself even in the heterozygous condition and in the homozygous dominant condition as well.
Let the B allele is responisble for the roll tongue and b is recessive allele. The woman can roll the tongue and she must have the genotype Bb because her one parents is normal. The woman married with a man with non roller ( bb). The Bb × bb cross results in the following progeny Dd, Dd ( roller), dd and dd ( non roller). The probability of the child being a roller is 1/2,
The expression would be $0.20 x f. Once you figure out what the 'f' variable stands for, you have to multiply it by $0.20, and you will have your answer.
