Q.78 A woman with blood group A gave birth to a baby with blood group AB. The blood group of the father
would be
(A) only AB
(B) only B
(C) either AB or B
(D) blood group O

A woman with blood group A can give birth to a baby with
blood group AB only if the father contributes the
B allele (I^B).
Therefore, the correct answer is:
(C) Either AB or B, as both paternal blood groups allow this inheritance outcome.

ABO Blood Group Basics

The ABO blood group system is controlled by three alleles:

• I^A – produces A antigen
• I^B – produces B antigen
• i – recessive allele producing no antigen

Alleles I^A and I^B are
codominant, while i is recessive.

Blood group A corresponds to genotypes
I^AI^A or
I^Ai.
Blood group AB corresponds to genotype
I^AI^B.

Inheritance Logic Explained

A mother with blood group A can pass either:

• I^A allele, or
• i allele (if heterozygous)

To produce an AB child (I^AI^B):

• The mother must contribute I^A
• The father must contribute I^B

This immediately rules out fathers with blood groups
A or O, as they do not carry the
I^B allele.

Correct Answer: (C) Either AB or B

Case 1: Father with Blood Group AB (I^AI^B)

The father has a 50% chance of passing the
I^B allele.
If the mother contributes I^A,
the child will be AB.

Case 2: Father with Blood Group B (I^BI^B or I^Bi)

The father will always pass I^B.
If the mother passes I^A,
an AB child is produced.

Example Punnett square (Mother I^Ai × Father I^Bi):

• 25% A
• 25% B
• 25% AB
• 25% O

Thus, AB offspring is clearly possible.

Why Not Other Options?

• (A) Only AB: Incorrect. This excludes fathers with blood group B,
  who can also contribute the I^B allele.
• (B) Only B: Incorrect. This excludes AB fathers,
  who are equally capable of producing an AB child.
• (D) Blood group O: Incorrect. Blood group O (ii) fathers
  pass only the i allele, so AB offspring is impossible.

Option-wise Summary Table