If the mother's blood type is O, and the child's blood type is B, a man would be exonerated from paternity if he has blood type A or O.
Explanation:
In the above case, if a mother has blood type O and her child has blood type B, a man can be exonerated from paternity only if his blood type is A or O, and responsible if he is type B or AB.
The blood type is defined by the presence of surface antigens in the red blood cell, called A and B. The presence of each antigen is determined by a gene present on the parental chromosomes.
Blood type A corresponds to the presence of gene A, and its genotypic expression can be A/A or A/O.
Type B blood, whose genotype is B/B or B/O, is due to the presence of a gene containing the B antigen.
AB blood —due to codominance— has one gene for A and another for B, with genotype A/B.
Blood type O, characterised by the absence of surface antigens, behaves like a recessive trait, which only manifests itself in the absence of surface antigens A and B. The genotype is O/O.
By making a hypothetical crossing between mother and man, one can observe why he can be exonerated.
Man with blood type O
♂ O|O
♀ O|O
Alleles O O
O O|O O|O
O O|O O|O
The only possibility, in this case, is to have children with type O blood. No chance of children B.
Male with blood type A
♂ A|O
♀ O|O
Alleles A O
O A|O O|O
O A|O O|O
In this case there is a 50% chance that the children will be blood type A and 50% chance that they will be O. No chance of children B.
In the case of a man A|A the result is 100% probability of children A|O, and no probability of children B.
So, a man with blood types A (any genotype) and O, can be exonerated from paternity, since it is impossible for him to have a child of blood type B, when the mother is of blood type O.
