Is it possible for parents carrying blood group O for both of them to have one of their children to have a blood group AB? If no, why? Is it possible for parents carrying blood group O and A to have children who have blood groups A and O? If YES, why?
Blood types are determined by multiple alleles, `I^A, I^B, I^O` . Alleles `I^A, I^B` are co-dominant and are both dominant over `I^O` . The blood groups arise from the combination of these alleles from the parents. We all have two of the alleles (just like other genes). Blood type O, for example which is from the recessive O-allele, only arises if the two alleles are both O. A may arise from two A-alleles (AA) or from one A and one O-allele (AO). The same is true for B which is either (BB) or (BO). AB, on the other hand, only arises when both A and B are present - (AB). The offspring gets its alleles from the parents.
Is it possible for parents carrying blood group O for both of them to have one of their children to have a blood group AB?
No. If both parents have blood type O, that means that the mother has genotype OO, and father has genotype OO. That means that the offspring can only have a blood group of O, genotype OO (since s/he can only get O-alleles from both parents.
Is it possible for parents carrying blood group O and A to have children who have blood groups A and O?
Yes. The parent with a blood group O definitely has genotype OO (since O is recessive), while the parent with blood group A may have a genotype AA (homozygous) or AO (heterozygous).
If the parent is homozygous AA, all the children will have blood group A (all of them will be heterozygous AO). If the parent is heterozygous AO, then children can either be A or O, since if they get the A-allele from the heterozygous A parent, they would have blood type A, but if they get the O-allele from the heterozygous A parent, they will be blood group O (since the other parent is of bloog group O).