UPSC CSE Prelims
Question 45
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
View correct answer and explanation
Correct answer: A
Correct Option: A
Explanation: An AB parent can pass only A or B alleles, while an O parent passes only O; biological offspring can be A or B, not O, under standard ABO inheritance.
Option-wise Analysis:
A: Correct - the O-group boy cannot be the AB × O couple’s biological child, so he must be adopted.
B: Incorrect - A-group children are genetically possible (AO) for these parents.
C: Incorrect - B-group children are genetically possible (BO) for these parents.
D: Incorrect - The genotype constraints determine the answer; the Rh-positive status does not change ABO inheritance.
Review Note: Assumes ordinary ABO inheritance and accurate stated blood typing; exceedingly rare exceptional blood-group phenomena are outside the exam’s scope.