Genes that occur at the same place on the chromosome but occur in different forms are called alleles. Alleles are essentially different versions of a gene, and they can cause dissimilarities in an organism's function or structure. An example is the gene responsible for melanin production in most animals. One version of this gene causes the organism to produce adequate melanin, while the other allele prevents melanin from being produced.
Alleles are expressed differently depending on whether they are dominant or recessive. If an organism has a copy of each allele, the dominant one becomes active and expressed in the organism’s construction. In contrast, recessive genes are only expressed when they occur in pairs. For an organism to have two copies of a recessive gene, both of its parent organisms had to have had them and passed them on to the offspring.
Some genes have a greater effect on the function of an organism than on its structure. For example, in humans, some alleles can cause a person to have metabolic problems, while others may cause mental illness. Genes can have multiple different alleles; they are not limited to two different versions of a gene. For example, a single litter of animals may include a number of different appearances.