Question

A man with A blood group marries a woman with B blood group and both show the presence of homozygous alleles. The possible genotypes of the offsprings would be
I. Type A
II. Type B
III.Type O
IV. Type AB
(a)II only
(b)I and II
(c)I, II, and IV
(d)I, II, III, and IV
(e)IV only

Answer 1

Hint: Human blood group system has two antigens A and B. Their production is governed by the gene I which exists in three allelic forms, namely ${ I }^{ A }$, ${ I }^{ B }$, and ${ I }^{ O }$. ${ I }^{ A }$ and ${ I }^{ B }$ are codominant to each other, but both are dominant over ${ I }^{ O }$. When a person with Blood group A has homozygous alleles i.e. $\left( { I }^{ A }{ I }^{ A } \right) $ and a person with blood group B has homozygous alleles $\left( { I }^{ B }{ I }^{ B } \right) $.

Complete answer:
When ${ I }^{ A }$ is present in homozygous $\left( { I }^{ A }{ I }^{ A } \right) $ or heterozygous $\left( { I }^{ A }{ I }^{ O } \right) $, antigen A would be produced and the blood group would be A. Similarly when ${ I }^{ B }$ is present in homozygous or heterozygous condition antigen B would be produced and the blood group will be B. When ${ I }^{ O }$ is present in the homozygous condition, no antigen would be produced and the blood group will be O. Here the person with Blood group A has homozygous alleles i.e. $\left( { I }^{ A }{ I }^{ A } \right) $ and the person with blood group B have homozygous alleles $\left( { I }^{ B }{ I }^{ B } \right) $. So, the resulting offsprings will have blood group AB.
P1-A group $\left( { I }^{ A }{ I }^{ A } \right) $ $\hspace{5cm}$ B group$\left( { I }^{ B }{ I }^{ B } \right) $
  G- ${ I }^{ A }$ $\hspace{8cm}$ ${ I }^{ B }$

Additional Information: -Human ABO blood group system provides an excellent example of multiple allelism.
-A blood group contains antigen A and antibody b. B blood group has antigen B and 'a' antibody.
-AB group has antigens A and B, but no antibodies. O blood group has antibodies a and b, but no antigens.
So, the correct answer is ‘IV only’

Note: In 1900, the Austrian physician Karl Landsteiner discovered two kinds of antigens and their antibodies. The antigens were termed A and B, and the antibodies a and b. These antibodies are not acquired but are constitutive or natural, present in blood as natural constituents.
-Antigens occur in the plasma membrane of RBCs, and antibodies in blood plasma. He could also realize that a particular type of blood may contain either one or both or none of these antigens and antibodies.