Read the given passage and answer the question.
ABO blood groups in human beings 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 \(i\).
Select the correct option for number of alleles present in ABO blood groups:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Three different alleles
Option (2) Three different alleles — Correct. Of the options, only three fits: the gene I has the alleles IA, IB and i. Six is the number of genotypes, not alleles.
Note: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
NCERT Extract:
Co-dominance – “… A good example is different types of red blood cells that determine ABO blood grouping in human beings. 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.”