Question

A child with a mother of blood group A and father of blood group AB will not have which of the following blood groups?
(a)A
(b)B
(c)AB
(d)O

Answer 1

Karl Landsteiner has discovered the ABO blood group system. The blood group inheritance depends upon dominance, recessive, and co-dominance phenomenon. The alleles for both the A and B blood group is dominant and the allele for the O blood group is recessive.

Complete answer:
In this case, the progeny cannot be O. It is because of the phenomenon called dominance.
-According to the law of dominance, one allele in a gene pair will be dominant over the other allele and it is called the dominant allele. The other allele is called the recessive allele.
-If a gene pair is homozygous for the dominant allele, then the dominant character will get expressed.
-If the gene pair is homozygous for the recessive alleles, then the recessive character will get expressed.
-On the contrary, if both dominant and recessive alleles are present, then the person will be heterozygous for that gene pair. In this case, only the dominant trait will get expressed.
-Another interesting phenomenon is observed when the alleles in a gene pair are heterozygous and both the alleles get fully expressed. This phenomenon is called codominance.
Example: Blood group inheritance. In a gene pair, one allele will be inherited from the mother and the other allele will be inherited from the father.
The blood group A, B, O are regulated by the gene I. A and B blood groups are dominant over the I gene whereas the O blood group is recessive.
In the given case, the father has an AB blood group and the mother has an A blood group. For the mother to have A blood group, one allele can be ${ I }^{ A }$ and the other allele can be ${ I }^{ O }$ (${ I }^{ A }{ I }^{ O }$). On the contrary, the wife can have both alleles of a gene pair to code for A group (${ I }^{ A }{ I }^{ A }$).
Case 1: Husband alleles wife alleles
${ I }^{ A }{ I }^{ B }$ X ${ I }^{ A }{ I }^{O}$
Progeny can either be ${ I }^{ A }{ I }^{ A }$, ${ I }^{ O }{ I }^{ A }$, ${ I }^{ A }{ I }^{ B }$, ${ I }^{ O }{ I }^{ B }$
Case 2: Husband alleles wife alleles
${ I }^{ A }{ I }^{ B }$ X ${ I }^{ A }{ I }^{ A }$
Progeny can either be ${ I }^{ A }{ I }^{ A }$, ${ I }^{ A }{ I }^{ B }$

The husband has both A and B alleles. If the progeny has to inherit the O group, then the child should receive the ${ I }^{ O }$ allele from both the parents. So, according to the law of dominance, the father should have at least one ${ I }^{ O }$ allele to inherit the O group by the progeny.
So, the correct answer is ‘O’.

Note: Apart from ABO blood group system, the Rh (rhesus) typing determines the positive or negative blood group. The presence of Rh antigen denotes that the blood group is positive (Rh-positive) and the absence of Rh factor denotes that the blood group is negative (Rh-negative).