Advanced Placement (AP) Biology Practice Exam

Which of the following best defines phenotype?

1. The genetic structure of an organism

2. Physical characteristics of an organism

3. The possible gene combinations

4. The ability to express dominant traits

The correct answer is: Physical characteristics of an organism

The concept of phenotype refers specifically to the observable physical characteristics of an organism, which arise from the interaction between its genotype (the genetic makeup) and the environment. This includes traits like height, color, and shape, as well as more complex features like behavior or physiological responses. In contrast, the genetic structure of an organism pertains to the genotype, which encompasses all the alleles that an organism carries. The possible gene combinations relate to the different genotypes that can exist, while the ability to express dominant traits speaks to the genetic mechanisms underlying trait expression rather than the traits themselves. Thus, the physical characteristics define what we recognize and can measure or observe, making the definition of phenotype best captured by the options available.