NEET 2020 Botany Question 6 Solution

The human ABO blood group system is governed by a single polymorphic gene, gene $I$, located on chromosome 9: - The plasma membrane of red blood cells contains protruding glycoprotein/glycolipid polymers based on a core H-antigen. - Three Alleles: Gene $I$ exists as three distinct alleles in human populations: $I^A$, $I^B$, and $i$. - Allele $I^A$: Codes for $\alpha$-1,3-N-acetylgalactosaminyltransferase, which adds N-acetylgalactosamine (A-sugar antigen). - Allele $I^B$: Codes for $\alpha$-1,3-galactosyltransferase, which adds galactose (B-sugar antigen, a distinctly different sugar). - Allele $i$: Contains a single base deletion causing a frameshift, producing a truncated non-functional enzyme that does not produce any sugar.

Statement (3) asserts that "When $I^A$ and $I^B$ are present together, they express same type of sugar." In reality, they express different types of sugars because of codominance. Therefore, statement (3) is false and represents the correct choice.