Final answer:
The observable traits expressed by an organism are described as its phenotype, which results from the expression of an organism's unique set of genes known as the genotype.
Step-by-step explanation:
The observable traits expressed by an organism, such as eye color, hair color, height, and other characteristics, are described as its phenotype. The phenotype results from the expression of an organism's genotype, which is the unique set of genes the organism carries. While the genotype provides the instructions on what traits may be expressed, the phenotype is the actual physical and biochemical characteristics that are visible or measurable.
The genotype consists of all the alleles (versions of genes) an organism has inherited. The alleles interact with each other and with the organism's environment to produce the phenotype. For example, two plants with different genotypes may have the same phenotype due to similar environmental conditions. On the other hand, two genetically identical twins (same genotype) might exhibit different phenotypes if they are raised in significantly different environments.
In contrast, a zygote is the initial cell formed when two gamete cells are joined by means of sexual reproduction. Although the zygote's genetic material will determine the genotype and ultimately influence the phenotype, it is not itself a description of observable traits.