Q.If the childs blood group is 'O' and fathers blood group is 'A' and mother's blood group is 'B' the genotype of the parents will be

Step 1. Recall that a type-O child has genotype IOIO, meaning the child received one IO allele from the father and one IO allele from the mother — so both parents must carry (and be able to pass on) an IO allele, even though neither parent is phenotypically type O themselves.

Step 2. Since the father is phenotype A but must carry an IO allele, his genotype must be the heterozygote IAIo, not the homozygote IAIA (which carries no IO to pass on). Likewise the mother, phenotype B but carrying IO, must be IBIo, not IBIB. …

Work backward from the type-O child's IOIO genotype to require an IO allele from each stated-phenoty …

• Assuming a type-A or type-B parent is automatically homozygous. …