Advanced Placement (AP) Biology Practice Exam

What is the definition of homozygous in genetics?

1. Having two identical alleles for a trait

2. Having two different alleles for a trait

3. Having one dominant and one recessive allele

4. Having neither dominant nor recessive alleles

The correct answer is: Having two identical alleles for a trait

The term "homozygous" in genetics refers to an organism that has two identical alleles for a specific trait. This means that both alleles, which are variations of a gene, are the same, whether they are both dominant or both recessive. For example, in terms of flower color, a plant might have two alleles for red flowers (RR), making it homozygous dominant, or two alleles for white flowers (rr), making it homozygous recessive. This genetic uniformity can affect the expression of traits, as an individual with homozygous alleles will consistently express that trait. The other choices describe different genetic situations. Having two different alleles for a trait is referred to as heterozygous. The presence of one dominant and one recessive allele still describes a heterozygous condition rather than homozygous. Lastly, the concept of having neither dominant nor recessive alleles does not pertain to the classification of alleles as homozygous, as all alleles are categorized based on their dominance or recessiveness in relation to one another.