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.
Given below are two statements:
Statement I: Children of a person of blood group AB and a person of blood group O can have blood group A or blood group B.
Statement II: Multiple alleles of a gene can be found when a single individual is studied.
In the light of the above statements, choose the most appropriate answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → Statement I is true but Statement II is false
Statement I: Children of a person of blood group AB and a person of blood group O can have blood group A or blood group B. — True. The AB parent is IAIB and gives either IA or IB; the O parent is ii and gives only i. The children are IAi (group A) or IBi (group B), as in Table 4.2.
Statement II: Multiple alleles of a gene can be found when a single individual is studied. — False. A diploid individual carries only two alleles of a gene. The three alleles of the gene I can be seen only when a population is studied.
NCERT Extract:
Co-dominance – “… Here you can see that there are more than two, i.e., three alleles, governing the same character. Since in an individual only two alleles can be present, multiple alleles can be found only when population studies are made.”
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 |