Advanced Placement (AP) Biology Practice Exam

Which of the following represents a heterozygous condition?

1. bb

2. BB

3. Bb

4. AA

The correct answer is: Bb

A heterozygous condition occurs when an individual has two different alleles for a particular gene. In this case, the choice that represents a heterozygous condition is "Bb," where one allele is dominant (B) and the other is recessive (b). This combination indicates that the individual has inherited different versions of the gene from each parent, which is characteristic of heterozygosity. In contrast, the other options represent homozygous conditions, where the individual carries two identical alleles—either both dominant (BB) or both recessive (bb or AA). Thus, "Bb" is the only option that shows the presence of different alleles at the same locus, confirming its status as a heterozygous condition.