What term describes the observable expression of a genotype?

The observable expression of a genotype is referred to as a phenotype. This term encompasses all the physical and behavioral traits of an organism that result from the interaction of its genotype with the environment. The genotype represents the genetic makeup of an organism—essentially the information carried within its DNA—while the phenotype is the manifestation of that genetic information, which can include observable characteristics like height, eye color, or the presence of certain behaviors.

Understanding the distinction between genotype and phenotype is crucial in genetics because it highlights how the underlying genetic code does not always directly translate into physical traits due to the influence of environmental factors and complex interactions among genes. In terms of the other options, an allele refers to different versions of a gene, while a chromosome is a structure that contains DNA. The term genotype itself describes the genetic constitution of an organism, not its expression or observable traits. Hence, the concept of phenotype precisely captures the idea of traits that can be seen and measured.