Now let’s look at the genetics behind the evolutionary process. We will consider a single gene locus with two possible alleles. The terms “locus” and “allele” are more specific than the term “gene,” which has multiple meanings. A locus refers to a specific location on the chromosome that contains one of the possible alleles. Alleles are alternate forms of a gene.
For example, in the peppered moth, there is a gene locus that determines the color of the wings. The locus can have either allele A or allele a. A codes for dark-colored wings and a codes for light-colored wings.
Like humans, the peppered moth is a diploid (2n) organism, meaning there are two copies of each gene locus. So an individual in a population can have a genotype of AA, Aa, or aa. The A allele for dark-colored wings is dominant over the a allele for light-colored wings. Moths with a genotype of AA or Aa will have a dark wing phenotype, while moths with a genotype of aa will have a light wing phenotype. In this example, 3 different genotypes (AA, Aa and aa) result in 2 different phenotypes: dark-colored and light-colored wings.
A population can have any number of individuals with different genotypes, and consequently with different phenotypes.