Q.Three children of a family have blood groups A, AB and B. What could be the genotypes of their parents?

ABO blood typing is the textbook case of multiple allelism in humans. Karl Landsteiner identified two red-cell surface antigens, A and B, and on that basis recognised blood types A, B and O; the rarer AB type was found shortly after by two of his students. Bernstein (1925) showed the whole system is controlled by one gene (historically called I or L) on chromosome 9, existing in three allelic forms: IA (produces an enzyme that adds N-acetyl galactosamine to a precursor molecule, giving the A antigen), IB (produces a different enzyme that instead adds galactose to the same precursor, giving the B antigen), and IO (a functionally 'null' allele that adds neither).

IA and IB are each dominant over IO, but co-dominant with each other — a person with both alleles (IAIB) displays both antigens rather than one masking the other, giving the AB phenotype. This IA = IB > IO hierarchy generates six genotypes but only four phenotypes: IAIA/IAIO → type A; IBIB/IBIO → type B; IAIB → type AB; IOIO → type O. Antibodies always form in plasma against whichever antigen a person's own cells do not carry (anti-B in type A, anti-A in type B, both in type O, neither in type AB), which is why mismatched transfusions cause agglutination, and why type O is the 'universal donor' (no antigens to react against) while type AB is the 'universal recipient' (no antibodies against either antigen). Working an ABO cross is just standard multi-allele Mendelian genetics once the genotype-to-phenotype mapping above is fixed — for example, a type-A parent (IAIA or IAIO) and a type-B parent (IBIB or IBIO) can, depending on their exact heterozygosity, produce children of type A, B, AB, or even O.