Advanced Placement (AP) Biology Practice Exam

What is an example of homozygous alleles?

1. BB or bb

2. Bb

3. AB

4. AA

The correct answer is: BB or bb

Homozygous alleles refer to a pair of identical alleles for a particular gene. For example, when both alleles are dominant, represented as 'BB,' or both are recessive, represented as 'bb,' this indicates a homozygous condition. In both cases, the organism carries two copies of the same allele, whether that allele is dominant or recessive, which plays a crucial role in determining the phenotype of the organism. The other combinations described in the options do not reflect homozygous alleles. For instance, 'Bb' represents a heterozygous condition where there is one dominant (B) and one recessive (b) allele. Similarly, 'AB' refers to different alleles for a single gene, indicating heterozygosity, and 'AA' is indeed a homozygous condition but does not encompass the breadth shown in the first choice, which includes both possible homozygous scenarios. Therefore, the answer correctly identifies a clear example of homozygous alleles.