In a genetic cross, what percentage of the offspring will express the dominant trait if the parents are heterozygous?

In a genetic cross involving two heterozygous parents (often represented as Aa x Aa), the offspring can inherit combinations of the alleles from each parent. The dominant trait is represented by the uppercase letter (A), while the recessive trait is represented by the lowercase letter (a).

When you set up a Punnett square for this cross, you would outline all possible allele combinations that can occur in the offspring. The combinations produced will be:

1. AA (homozygous dominant)

2. Aa (heterozygous)

3. Aa (heterozygous)

4. aa (homozygous recessive)

From this Punnett square, you can count the outcomes:

• 1 offspring with AA (which expresses the dominant trait)

• 2 offspring with Aa (which also express the dominant trait)

• 1 offspring with aa (which does not express the dominant trait)

Adding these, you find that 3 out of 4 combinations express the dominant trait (AA and Aa), which translates to 75% of the offspring. Therefore, when both parents are heterozygous, the percentage of offspring that will express the dominant trait is indeed 75%.