Every species of living organism has a unique gene pool, which is the collection of all the possible genes in the species population. Each individual gene will have one or more versions, called alleles. In diploid organisms -- ones with sets of genes from each parent -- heterozygosity is the condition in which the two alleles of a particular gene location, called a locus, in an individual have different genetic codes, or genotypes. An individual with two identical alleles is said to be homozygous for that gene.
Number of Possible Genotypes
To calculate heterozygosity for a particular gene locus, you must first know how many different alleles for the gene exist in the population. For example, the flower-color gene for a particular plant species might specify red, yellow or white petals. The number of alleles, or "k," for this locus is three. To calculate the number of possible genotypes at the gene locus, use the formula k(k+1)/2, which in this example is 3(4)/2, or 6. For the flower, the possibilities are red-red, red-yellow, red-white, yellow-yellow, yellow-white and white-white.
Heterozygosity and Homozygosity
You can calculate the heterozygosity of a gene locus -- that is, the number of nonidentical allele combinations -- by using the formula k[(k-1)/2]. For the example flower, this is 3[(2/2)], or 3: red-white, red-yellow and yellow-white. The same formula gives the number of homozygous genotypes: red-red, yellow-yellow and white-white. Even when an individual is homozygous for a given gene, the resulting physical trait, or phenotype, may not be certain. This can occur when two or more gene loci cooperate to express the trait. For instance, suppose the example flower has a separate gene that turns color expression on or off. In this case, any homozygous or heterozygous combination of color alleles will produce white flowers if the color expression gene is turned off.
Dominant and Recessive Alleles
In classical genetics, the phenotype of a heterozygous individual depends in part on the dominant-recessive relationships between the two different alleles at the gene locus. For example, suppose the flower's red allele dominates the yellow, which in turn dominates the white. An individual flower's petals will be red if it has at least one red allele. Lacking a red, the individual will be yellow if it has at least one yellow allele. Only a recessive white-white pair of alleles will produce a white flower, unless other genes, such as the one for color expression, also play a role.
Not all alleles display the classical dominant-recessive relationships. A codominant allele pair produces a mixture of two phenotypes. For example, if the flower's red and yellow alleles were codominant, the red-yellow genotype would produce a flower with red and yellow spots. If the two alleles were semidominant, the resulting flower would be orange. Complete and incomplete dominance can coexist in the same species. For example, a flower codominant for red and yellow might still be recessive for white.