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.
What type of dominance is illustrated by the ABO blood group system?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Co-dominance
Three patterns are seen in the heterozygote. In dominance the F1 looks like one parent; in incomplete dominance it is in between the two; in co-dominance it resembles both parents. In the ABO system, a person with IA and IB has both A and B types of sugar, so both alleles are fully expressed together.
Option (1) Incomplete dominance — Incorrect. In incomplete dominance the heterozygote is a blend, like the pink snapdragon. An IAIB person does not have a blended sugar; both sugars are present.
Option (2) Complete dominance — Incorrect. Complete dominance does hold between IA (or IB) and i, but it does not describe IAIB, where neither allele hides the other.
Option (3) Co-dominance — Correct. When IA and IB are present together they both express their own types of sugars, and NCERT names this co-dominance.
Option (4) Epistasis — Incorrect. Epistasis is an interaction between different genes, where one gene masks the effect of another. The ABO groups depend on alleles of a single gene, I.
NCERT Extract:
Co-dominance – “Till now we were discussing crosses where the F1 resembled either of the two parents (dominance) or was in-between (incomplete dominance). But, in the case of co-dominance the F1 generation resembles both parents. A good example is different types of red blood cells that determine ABO blood grouping in human beings.”