5. ABO Blood Types

ABO blood type system is controlled by multiple alleles, ie more than 2 alleles are present in the human population that controls the inheritance of the blood type.

There are a total of three alleles that determine blood type, namely I^A, I^B and I^O. The two dominant alleles are I^A and I^B which means that when both alleles are present in the genotype, both alleles are expressed equally. This phenomenon is known as co-dominance.

The table below shows the genotypes and phenotypes of different individuals.

Case Study: With the aid of a genetic diagram, suggest how a couple with the husband having Blood type A and the wife having Blood type B can conceive a child with Blood type O.

The child with blood type O has genotype I^OI^O. By Mendel's Law of Segregation, the child would have inherited one I^O allele from each parent. Thus the genotype of the child's father is I^AI^O while the genotype of the child's mother is I^BI^O_.