Final answer:
Alleles are the term given to multiple forms of a gene that are produced by mutation. Alleles determine an organism's genotype while the observable traits expressed by an organism are described as its phenotype.
Step-by-step explanation:
Different versions of genes are called alleles. Diploid organisms that have two identical alleles of a gene on their two homologous chromosomes are homozygous for that trait. Diploid organisms that have two different alleles of a gene on their two homologous chromosomes are heterozygous for that trait.
Different alleles for a given gene in a diploid organism interact to express physical characteristics such as pea color in plants or hairline appearance in humans. The observable traits of an organism are referred to as its phenotype. The organism's underlying genetic makeup, i.e., the combination of alleles, is called its genotype.