Answer:
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.
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</em><u><em>Blood type A</em></u><em> corresponds to the presence of gene A, and its genotypic expression can be A/A or A/O.
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- <u><em>Type B blood</em></u><em>, whose genotype is B/B or B/O, is due to the presence of a gene containing the B antigen.
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- <u><em>AB blood</em></u><em> —due to codominance— has one gene for A and another for B, with genotype A/B.
</em>
- <em>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.
</em>
By making a hypothetical crossing between mother and man, one can observe why he can be exonerated.
<u>Man with blood type O
</u>
♂ 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.
<u>Male with blood type A
</u>
♂ 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.
<em>So, a </em><em>man with blood types A (any genotype) and O, can be exonerated from paternity</em><em>, since it is </em><em>impossible for him to have a child of blood type B, when the mother is of blood type O</em>.
