The pedigree diagram shows how a dominant trait is inherited in a family. The red circle and squares show family members with the trait.

Question

A pedigree diagram of 4 family members, where 3 of them have similar traits.
Based on the pedigree diagram, who must be heterozygous for the trait?

A.
The child without the trait
B.
Only one parent
C.
The child with the trait
D.
Both parents

Answer 1

In a pedigree diagram illustrating a dominant trait, individuals with the trait can either be homozygous dominant (AA) or heterozygous (Aa).

If we have a parent with the trait (who could be homozygous or heterozygous) and a child without the trait (who must be homozygous recessive, aa), we can deduce the genotypes.

For a child without the trait to be born to at least one parent with the trait, that parent must be heterozygous (Aa) because if the parent were homozygous dominant (AA), they could only pass on the dominant allele, resulting in all offspring having the trait.

Therefore, the correct answer is:

B. Only one parent

This parent is heterozygous for the trait. The other parent could be homozygous recessive (aa).