1. If the blood groups of mother and father are AB and O, respectively, what are the blood groups possible for their child? (2021)

(A) AB or A

(B) AB

(C) A or B

(D) AB, A, B or O

ABO Blood Group Inheritance Explained: Child Blood Group When Mother is AB and Father is O

Introduction

The ABO blood group system is one of the most important examples of Mendelian inheritance in humans and is frequently asked in competitive examinations. It demonstrates two important genetic principles simultaneously: multiple alleles and codominance. The ABO blood group is determined by a single gene known as the I gene, which exists in three allelic forms: I^A, I^B, and i. While every individual possesses only two alleles, three different alleles are present within the human population.

The alleles I^A and I^B are codominant, meaning both are expressed equally when present together, resulting in blood group AB. In contrast, the allele i is recessive and produces blood group O only when inherited in the homozygous condition (ii). Understanding these inheritance patterns allows geneticists to predict the possible blood groups of offspring using Punnett squares and Mendelian principles.

Correct Answer

Correct Option: (C) A or B

Detailed Explanation

The mother has blood group AB, so her genotype is I^AI^B. The father has blood group O, which means his genotype is ii. During gamete formation, the mother can produce two types of gametes containing either the I^A allele or the I^B allele, whereas the father can produce only one type of gamete carrying the recessive i allele.

When fertilization occurs, the child can inherit either I^Ai or I^Bi. Individuals with genotype I^Ai express blood group A, while individuals with genotype I^Bi express blood group B. Since neither parent contributes both dominant alleles together nor two recessive alleles simultaneously, blood groups AB and O cannot occur in the offspring.

Punnett Square Analysis

The Punnett square clearly shows that only blood groups A and B are possible.

Explanation of Each Option

Option (A): AB or A

This option is incorrect. Although blood group A is possible, blood group AB cannot occur because the father contributes only the recessive i allele and never contributes an I^A or I^B allele.

Option (B): AB

This option is incorrect. Blood group AB requires inheritance of both I^A and I^B alleles. Since the father has genotype ii, this combination is impossible.

Option (C): A or B

This option is correct. The offspring can inherit either I^Ai or I^Bi, producing blood groups A and B, respectively.

Option (D): AB, A, B or O

This option is incorrect. Blood groups AB and O cannot occur because the necessary allele combinations are absent in this parental cross.

Why Option (C) is Correct

The cross between an AB mother (I^AI^B) and an O father (ii) produces only two possible genotypes:

• I^Ai → Blood Group A
• I^Bi → Blood Group B

Therefore, the possible blood groups of the child are only A or B.

Comparison of All Options

ABO Blood Group Alleles and Genotypes

Genetic Principles Demonstrated by the ABO System

Biological Significance

The ABO blood group system has immense clinical importance because it determines compatibility during blood transfusion and organ transplantation. It also plays a significant role in population genetics, forensic science, genetic counseling, and studies of human evolution. Understanding ABO inheritance helps predict offspring blood groups and prevents transfusion reactions that may otherwise be life-threatening.

Final Answer

Mother’s genotype: I^AI^B

Father’s genotype: ii

Possible offspring genotypes:

• I^Ai → Blood Group A
• I^Bi → Blood Group B

Correct Option: (C) A or B