Question

17. Two adults with a completely penetrant version of piebald spotting (a mendelian trait, where the phenotype is patches of skin that lack pigment) have one child who has this trait and a second who does not:

    1. How is it inherited

       1. Autosomal dominant

          1. I got this question wrong on one of my exams and my professor said it was autosomal dominant, but I don't really understand why.

    2. What information led you to this answer? ​​​​​​​What are the genotypes of the parents?​​​​​​​

       1. ​​​​​​​So it's hard for me to answer this section.

Answer 1

both parents are affected with the trait, and they have one child with the trait and a normal child.

first let`s check whether this is possible if it was a recessive trait.

since the sex of the individuals are not mentioned, the trait is autosomal, both males and females are affected equally.

for an individual to be affected with the recessive trait the genotype of the individual has to be homozygous recessive.

let the alleles be D- dominant normal and d-recessive

dd * dd

dd ( all the progenies will be affected)

so it is not possible to have a child with normal phenotype for two parents affected with a recessive trait.

means the trait is dominant.

let`s check whether that is possible.

an individual will be affected with a completely penetrant dominant trait even when one dominant allele is present. ( completely penetrant means all individuals having the dominant trait will show the dominant phenotype).

Let the alleles be D- dominant, d-recessive normal

if the genotype of the parents are heterozygous.

Dd * Dd

	D	d
D	DD (piebald)	Dd(piebald)
d	Dd(piebald)	dd ( normal)

so it is possible to have a normal phenotype for the progeny. so this is an autosomal dominant trait.

the information led you to this answer: two affected parents having a normal child.

genotypes of the parents: Dd ( both parents are heterozygous)