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.
Which of the following is the blood group phenotype of the genotypes IBIB and IBi?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → B phenotype
The passage states that “IA and IB are completely dominant over i” and that “when IB and i are present IB expresses.” In IBIB only IB is present, and in IBi the allele i makes no sugar, so both genotypes give only the B type of sugar: blood group B.
Option (1) AB phenotype — Incorrect. The AB phenotype needs both IA and IB together (IAIB). Neither genotype here carries IA.
Option (2) A phenotype — Incorrect. The A phenotype comes from IAIA or IAi. There is no IA in these genotypes.
Option (3) B phenotype — Correct. Both IBIB and IBi express only IB, so the phenotype is B.
Option (4) O phenotype — Incorrect. The O phenotype comes from ii, where no sugar is made. Here IB is present and makes its sugar.