A couple has only A and O blood group children in 3:1 ratio if father's blood group is A, mother's blood group is

Options

A. O
B. A
C. A or O
D. B

Answer

To determine the blood group of the mother based on the information provided, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Blood Groups**:
- Blood groups are determined by the presence of specific antigens. The A blood group has A antigens (genotype can be either I^A I^A or I^A I^O), and the O blood group has no antigens (genotype I^O I^O).
- The A blood group is dominant over the O blood group.

2. **Analyzing the Children's Blood Groups**:
- The couple has children with blood groups A and O in a ratio of 3:1. This means for every 4 children, 3 have blood group A and 1 has blood group O.

3. **Inferring from the Blood Group Ratio**:
- Since O is recessive, for a child to have blood group O, both parents must contribute an O allele (I^O). Therefore, at least one parent must have the O allele.

4. **Father's Blood Group**:
- It is given that the father's blood group is A. This means the father can either be genotype I^A I^A (homozygous A) or I^A I^O (heterozygous A).

5. **Determining the Mother's Blood Group**:
- If the father is I^A I^A, he cannot produce any O blood group children, as he can only pass on I^A alleles. Therefore, the father must be I^A I^O (heterozygous A) to allow for the possibility of having O blood group children.
- For the children to have a 3:1 ratio of A to O, the mother must also contribute an O allele. This means the mother must be either I^A I^O (heterozygous A) or I^O I^O (homozygous O).

6. **Conclusion**:
- Since the children have both A and O blood groups, and the ratio indicates that A is more prevalent, the mother must be A (I^A I^O) to produce the 3:1 ratio of A to O blood groups.
- Therefore, the mother's blood group is A.

### Final Answer:
The mother's blood group is **A** (Option B).