What are alleles? Each cell actually contains two copies of each gene. The copies could be identical or they could be different versions of the same gene, called alleles. For example, everyone has a pair of genes that code for blood type. A person with Type AB blood has one allele that codes for A-type blood cell proteins and another allele that codes for B-type blood cell proteins, while a person with Type O blood has two alleles that don’t code for proteins at all, so the proteins are missing.

Alleles are