Final answer:
The expression of alleles refers to the phenotype of an organism, while its genotype represents the underlying genetic makeup. Phenotype describes the observable traits, while genotype includes both expressed and non-expressed alleles.
Step-by-step explanation:
The expression of alleles refers to the observable traits expressed by an organism, which is known as its phenotype. An organism's underlying genetic makeup, consisting of both physically visible and non-expressed alleles, is called its genotype. For example, in Mendel's experiments with pea plants, the yellow-seed allele is dominant, and the green-seed allele is recessive. When true-breeding plants with yellow and green seeds were cross-fertilized, the hybrid offspring had yellow seeds, indicating a different genotype from the parent with green seeds.