ABO blood groups in human beings are controlled by the gene I. The gene I has three alleles – IA, IB and i. Since there are three different alleles, six different genotypes are possible. How many phenotypes can occur?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → 4
The six genotypes do not give six phenotypes, because IA and IB are completely dominant over i. IAIA and IAi both give blood group A; IBIB and IBi both give B. IAIB gives AB, since the two alleles are co-dominant, and ii gives O. So there are four phenotypes: A, B, AB and O.
Option (1) 6 — Incorrect. Six is the number of genotypes. Pairs of genotypes such as IAIA and IAi look the same, so there are fewer phenotypes.
Option (2) 2 — Incorrect. Two would leave out blood groups AB and O, which arise from IAIB and ii.
Option (3) 3 — Incorrect. Three would miss AB, the group in which both IA and IB express because of co-dominance.
NCERT Extract:
Co-dominance – “… 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). How many phenotypes are possible?”
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 |