Q.Which of the following phenotypes is not possible in the progeny of the parental genotypic combination IAIO X IAIB?

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). …