An allele is a variant form of a gene that is found at a specific location on a chromosome. Genes are responsible for determining traits, such as eye color or height, and each gene can have multiple alleles. For example, the gene for eye color may have alleles for blue, brown, or green eyes. Alleles can be dominant or recessive. A dominant allele will express its trait even if only one copy is present, while a recessive allele requires two copies to show its effect. This combination of alleles inherited from parents contributes to the genetic diversity seen in organisms.