Question

ABO blood groups in humans is controlled by the gene I. It has three alleles: I${}^A$, I${}^B$ and i. Since, there are three different alleles, six different genotypes are possible. How many phenotypes can occur?

• A. Four
• B. Two
• C. Three
• D. One

Answer 1

Answer: (A)

Four

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. I${}^A$ and I${}^B$ show codominance while allele "i" is recessive to both I${}^A$ and I${}^B$. 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${}^O$.

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${}^O$ 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 respectively. Thus, the six genotypes give total four phenotypes. Hence, parents with A and B blood group can have child with A, B, AB and O blood groups. Correct option is "A".