Alleles are alternative genes for a specific trait. Alleles can be thought of as variations of a particular gene. When an organism has two identical genes (alleles) for a trait, it is said to be homozygous and when an organism has two different genes (alleles) for a trait, it is said to be heterozygous for that trait. An example is a person who has two alleles for brown eyes and is homozygous for the trait of eye color.
All of the alleles in an organism make up its genome. An organism's genotype is its specific set of genes while its phenotype are all of its observable traits. Due to mutation and natural selection, many loci along the DNA have various alleles. More alleles may lead to a greater variety of traits in offspring. This is particularly important because habitats on Earth are constantly changing and if a species' genes remained the same, they may not provide a selective advantage in a new environment. Thus, the species would die out.