Human abo blood type is determined by a single gene that comes in 3 distinct alleles: ia , ib , and i. the ia and ib alleles are each dominant to the i allele, but are codominant with each other, meaning that both phenotypes are expressed in the heterozygote. the punnett square below shows a cross between two parents with different blood types.
given, mother and father are heterozygotes.
let, father has B blood group and mother has A blood group.
then their heterozygous genotypes are --
father = iB, iO and mother = iA, iO
punnett square will be,
iA iA iB iA iO
iO iB iO iO iO
the genotypes of their progeny will be, iAiB , iAiO ,iBiO and iOiO
and phenotypes will be 3 are heterozygous and 1 is homozygous.