1. Multiple Alleles
An individual has two copies of each gene, so can only have two alleles of any gene, but there can be more than two alleles of a gene in a population.
An example of this is blood group in humans. The red blood cell antigen is coded for by the gene I (for isohaemaglutinogen), which has three alleles IA, IB and Io. (They are written this way to show that they are alleles of the same gene.)
IA and IB are co-dominant, while Io is recessive. The six possible genotypes and four phenotypes are: