Final answer:
The physical expressions of a gene are known as an organism's phenotype, which includes all the observable traits such as eye color, height, or, in the case of pea plants, pod color. It differs from genotype, which is the organism's genetic makeup, including alleles that may not be expressed visibly.
Step-by-step explanation:
The physical expressions of a gene are known as an organism's phenotype. This concept is fundamental in the field of genetics and implies the external, observable characteristics of an organism, such as height, eye color, and flower color in plants. In contrast, the term genotype refers to the genetic constitution of an organism—the actual alleles present at a specific locus on the DNA. Mendelian genetics provides a clear distinction between the two, with the classic example of pea plants, where the phenotype is the observable trait—yellow or green pods—while the genotype is the pair of alleles that determine this trait, such as YY or Yy.
An example to further understand phenotype and genotype can be seen in Gregor Mendel's experiments with pea plants. When he cross-pollinated plants with yellow pods and green pods, the F1 generation all exhibited yellow pods, which was their phenotype. However, the green pod allele reemerged in the F2 generation, confirming that the F1 plants had a different genotype from the true-breeding parent with yellow pods.