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.
In the ABO blood group system, which genotype results in blood type O?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → ii
The passage states that “allele i does not produce any sugar”, while IA and IB each produce a form of the sugar. A person with blood group O has neither A nor B sugar on the red blood cells, so both alleles must be i: the genotype is ii.
Option (1) IAIA — Incorrect. IAIA makes the A type of sugar and gives blood group A.
Option (2) IBIB — Incorrect. IBIB makes the B type of sugar and gives blood group B.
Option (3) ii — Correct. With ii no sugar is made, so neither A nor B is present and the blood group is O.
Option (4) IAIB — Incorrect. In IAIB both alleles express their own sugars, giving blood group AB.