In Mendelian genetics, only one allele is discussed at a time (i.e. one trait); however there are often more than two alleles for a single gene in most traits. This is known as having multiple alleles. When there are more than two types of allele for a gene, a multiple allele system is present. The most infamous example of this is the ABO blood grouping system.

Case Study: Blood Type The notation for blood type often uses the letter I, which stands for immunoglobulin. There are three alleles possible for one gene in the ABO blood grouping system. On the image below, the lefthand side shows the phenotype and the righthand side shows the genotype. This demonstrates all potential blood types.

Transmission still occurs in a normal Mendelian fashion, despite the fact that there are more than two alleles responsible for determining blood group. Consider the cross shown below.

In this situation, the parental cross is between someone with blood type AB and blood type B. The resulting phenotype ratio in this instance is 1 AB : 2 B : 1 A In the population, there are four possible phenotypes present, and no one shows variation that is in between each variation. As a result of this, this produces discontinuous variation as only one set of alleles for one gene determines the phenotype.

Discontinuous variation is a set of discrete phenotypic categories controlled by a single gene and its set of alleles. Discrete variation can be shown using a bar graph.