Question

The number of phenotypes in ABO blood groups is

• A. 1
• B. 4
• C. 6
• D. 8

Answer 1

Answer: (B)

4

Human blood group inheritance is the example of codominance and multiple alleles. It is governed by three alleles namely I${}^A$, I${}^B$ and I${}^O$. I${}^A$ and I${}^B$ show codominance while allele I${}^O$ is recessive to both, I${}^A$ and I${}^B$. Since, each allele can combine with any of three alleles, this gives total six genotypes I${}^A$I${}^A$, I${}^A$I${}^O$, I${}^B$I${}^B$, I${}^B$I${}^O$, I${}^A$I${}^B$ and I${}^O$I${}^O$. Thus, option A is wrong. Since, I${}^A$ and I${}^B$ alleles are dominant over I${}^O$ allele; the genotypes, I${}^A$ I${}^O$ and I${}^B$ I${}^O$ gives A and B blood group respectively. Genotypes I${}^A$I${}^A$, I${}^B$I${}^B$, I${}^A$I${}^B$ and I${}^O$I${}^O$ show A, B and O blood groups respectively. Due to codominance of I${}^A$ and I${}^B$ alleles, the genotype I${}^A$ and I${}^B$ produce “AB” blood group that has both A and B antigen. Thus, the six genotypes give total four phenotypes, this makes options C and D wrong.

Thus, the correct answer is option B.