Some human traits are controlled by single genes with two alleles, and others by single genes with multiple alleles. Still other traits are controlled by many genes that act together.
Example of Single Genes with Two Alleles:
Example of Single Genes with Two Alleles:
Example of Single Genes with Multiple Alleles:
Human blood type is controlled by a gene with multiple alleles. There are FOUR main blood types - A, B, AB, O. This is where understanding CODOMINANCE applies. The allele for blood type A and blood type B are codominant. This means that neither one is masked. Instead they both show through. Below in the picture are all if the different blood types and their possible genotypes.
A person who inherits an IA allele from one parent and an IB allele from the other parents will have type AB blood. Notice that the only people who inherit two I alleles have type O blood.
Watch this clip below for a better understanding of Codominance.