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← Theoretical 1

Q37 — ABO Blood Groups — H Antigen and the Bombay Phenotype

Theoretical 1 Real exam question — full text reproduced under IBO's CC BY-NC-SA 4.0 license
Pedigree Analysis & Human Genetic Disorders

There are glycoproteins on the surface of human red blood cells with antigenic properties called H-antigen. The A allele encodes an enzyme converting part of the H-antigen into A antigen (blood type A); the B allele converts it to B antigen (blood type B); the O allele encodes an inactive enzyme that doesn’t modify H-antigen. In rare cases (Bombay phenotype), the initial H antigen doesn’t appear on erythrocytes at all — even with A and B alleles present, no A or B antigens are produced. Synthesis of the H antigen is encoded by the H gene, whose recessive allele is h.

Using the information, determine whether the following statements are true or false:

A. Parents with genotype ABHH and OOhh may have a baby with blood type B with a 25% of probability.
B. If both parents have normal phenotypes (absence of Bombay phenotype), their child cannot have a B blood type.
C. If a child with AB blood type was born in the family, it is possible that his/her parents' genotypes were AAHH and BBhh.
D. If the parents have the AAHh and BBHh genotypes, then their children will all have blood type AB.
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Question reproduced from IBO 2022, Theoretical Paper 1, licensed under CC BY-NC-SA 4.0 — attributed to the International Biology Olympiad. Open the full exam PDF · Community solutions (unofficial)