Read the passage and answer the following questions :
ABO blood groups are controlled by the gene I. The plasma membrane of the red blood cells has sugar polymers that protrude from its surface and the kind of sugar is controlled by the gene. The gene (I) has three alleles I^A, I^B and i. The alleles I^A and I^B produce a slightly different form of the sugar while allele i does not produce any sugar. Because humans are diploid organisms, each person possesses any two of the three I gene alleles. I^A and I^B are completely dominant over i, in other words when I^A and i are present only I^A expresses (because i does not produce any sugar), and when I^B and i are present I^B expresses. But when I^A and I^B are present together they both express their own types of sugars. Hence red blood cells have both A and B types of sugars.

How many different genotypes are possible for the human ABO blood groups?

Six

The correct answer is Option (2) –Six

The ABO blood group system in humans is controlled by a single gene (I) with three alleles: I^A,I^Band i. Because humans are diploid organisms, each person has two alleles for this gene, leading to six possible genotype combinations.

Here are the genotypes and their corresponding phenotypes:

1. I^A I^A - Blood type A (homozygous)
2. I^A i - Blood type A (heterozygous)
3. I^BI^B - Blood type B (homozygous)
4. I^B i - Blood type B (heterozygous)
5. I^A I^B- Blood type AB (co-dominant, both sugars expressed)
6. ii - Blood type O (no sugar produced)

Each of these genotypes results in a different combination of the alleles, leading to the expression of different blood types (phenotypes), confirming that there are six different genotypes possible for the human ABO blood groups.