Question

The inheritance pattern for an autosomal dominant trait is shown in the pedigree Shaded symbols represent individuals that express the dominant trait Pedigree.Shaded symbols represent individuals that represents that express the dominant trait.
Based on this pedigree, what are the most likely genotypes of individuals I1 and I2
A.
I1 - aa
I2 - Aa

B
I1 - AA
I2 - Aa

C
I1 - Aa
I2 - aa

D
I1 - aa
I2 - AA

( please explain, this is a test prep question and I really need help with it. )

Answer 1

Answer:

C. Aa and aa

Explanation:

The question tell us that shaded symbols represent individuals who express a dominant trait (or phenotype). This means that symbols that are not shaded represent individuals who express a recessive trait (or phenotype).

A dominant allele (A in this case) will always mask a recessive allele (a in this case) when present and be shown in the organism's phenotype. Therefore genotypes AA and Aa, which both contain the dominant allele A, will both result in dominant phenotypes, while the genotype aa will result in a recessive one due to the absence of a dominant A allele.

Looking at the pedigree chart we can see that individual 1 has a dominant phenotype, and individual 2, a recessive phenotype. Since the only way for an organism to have a recessive phenotype is to only have recessive alleles, organism 2 must have genotype aa. Similarly organism 1 must have at least one dominant A allele, therefore C is the correct answer.

Hope this helped!