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 IA, IB and i. The alleles IA and IB 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. IA and IB are completely dominant over i, in other words when IA and i are present only IA expresses (because i does not produce any sugar), and when IB and i are present IB expresses. But when IA and IB 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?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Six
Each person has two of the three alleles IA, IB and i. The possible pairs are three homozygous ones (IAIA, IBIB, ii) and three heterozygous ones (IAIB, IAi, IBi). That makes six genotypes, which give only four blood groups.
Option (1) Four — Incorrect. Four is the number of phenotypes (A, B, AB and O), not of genotypes.
Option (2) Six — Correct. Three homozygous plus three heterozygous combinations give six genotypes (Table 4.2).
Option (3) Three — Incorrect. Three is the number of alleles of the gene I, not of the genotypes they form.
Option (4) Five — Incorrect. Five leaves out one combination. All three homozygotes and all three heterozygotes are possible.
NCERT Extract:
Co-dominance – “… Hence red blood cells have both A and B types of sugars. Since there are three different alleles, there are six different combinations of these three alleles that are possible, and therefore, a total of six different genotypes of the human ABO blood types (Table 4.2).”
Table 4.2: Table Showing the Genetic Basis of Blood Groups in Human Population
| Allele from Parent 1 | Allele from Parent 2 | Genotype of offspring | Blood types of offspring |
|---|---|---|---|
| IA | IA | IAIA | A |
| IA | IB | IAIB | AB |
| IA | i | IAi | A |
| IB | IA | IAIB | AB |
| IB | IB | IBIB | B |
| IB | i | IBi | B |
| i | i | ii | O |