Q.ABO blood group in man is controlled by

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.