Question

the inheritance pattern for an autosomal dominant trait is shown in the pedigree. shaded symbols represent individuals that express the dominant trait.
based on this pedigree, what are the most likely genotypes of individuals i - 1 and i - 2?
a i - 1: aa
i - 2: aa
b i - 1: aa
i - 2: aa
c i - 1: aa
i - 2: aa
d i - 1: aa
i - 2: aa

Explanation:

• Since it is an autosomal dominant trait, individuals expressing the trait have at least one dominant allele (\(A\)).
• Individual \(I - 2\) does not express the trait, so her genotype must be \(aa\) (recessive homozygous).
• Looking at their offspring: some offspring express the trait (have \(A\) allele) and some do not. If \(I - 1\) was \(AA\), all offspring would inherit an \(A\) allele and express the trait. But since there are non - expressing offspring (e.g., \(II - 3\), \(II - 6\)), \(I - 1\) must be \(Aa\) (heterozygous). When \(I - 1\) (\(Aa\)) and \(I - 2\) (\(aa\)) have children, the possible genotypes are \(Aa\) (express the trait) and \(aa\) (do not express the trait) in a \(1:1\) ratio, which is consistent with the pedigree.

Answer:

B. \(I - 1: Aa\), \(I - 2: aa\)