UPSC Prelims 2011 · Question 76 of 100

UPSC Prelims 2011 question on ABO Blood Group Inheritance

A married couple adopted a male child. A few years later, twin boys were born to them. The blood group of the couple is AB positive and O negative. The blood group of the three sons is A positive, B positive, and O positive. The blood group of the adopted son is

  1. O positive
  2. A positive
  3. B positive
  4. Cannot be determined on the basis of the given data
Show answer

Answer: A. O positive

Verdict

Correct Answer: O positive

Analysis

This question is based on ABO blood group inheritance. A couple with AB and O blood groups can have only A or B blood group children, not O or AB.

1. A positive — Biologically possible

The AB parent has genotype IAIB.

The O parent has genotype ii.

A child can receive IA from the AB parent and i from the O parent, producing blood group A.

2. B positive — Biologically possible

A child can also receive IB from the AB parent and i from the O parent.

This produces blood group B, so B positive is also biologically possible.

3. O positive — Biologically impossible

For a child to have blood group O, the genotype must be ii.

But the AB parent cannot contribute i; they can contribute only IA or IB.

Therefore, an O positive child cannot be the biological child of an AB and O couple.

Extra UPSC info

* Blood group AB has genotype IAIB.

* Blood group O has genotype ii.

* Cross between AB and O gives only A or B blood groups.

* Rh factor is inherited separately from ABO blood group.

* Since positive Rh is possible in the children, the issue here is ABO group, not Rh factor.

* Blood group inheritance questions are solved by checking which allele each parent can pass.

How to crack it

The adopted son is O positive because an AB and O couple cannot biologically produce a child with O blood group.

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