An allele is an alternative version of a gene. Many times, an allele may be dominant over another allele for that trait. Sometimes, two alleles have incomplete dominance where the effects of both genes appear in the organism's phenotype like when a red and white allele are both present in four o'clock flowers and this produces a pink offspring. The genes in this case blend together. Sometimes, both alleles are expressed at the same time like in cattle-the red allele and the white allele both present in an organism produce a roan colored cow with a combination of both hair colors in their fur.
In multiple allele inheritance, rather than two versions of the gene, there can exist several different alleles that produce the phenotype. However, an individual still only inherits one copy of the allele from each parent. An example is the ABO blood type in humans. For this example, the alleles for type A and type B are co-dominant and the allele for O is recessive. The A allele codes for the A carbohydrate on the surface of red blood cells and the B allele codes for the B carbohydrate on the red blood cells. Individuals with the O allele who are homozygous have neither the A nor the B carbohydrate on their blood cells. Therefore, three possible alleles can code for blood type in humans. For example, a person who inherits an A and an O allele with have type A blood because allele A is dominant to O. If an individual inherits an A and a B allele, because they are co-dominant, they will have both carbohydrates present on the red blood cells.