Question

As per the Mendelian inheritance pattern for the ABO blood group, which of these is incorrect with respect to the blood group of the child?

• A. Mother and Father both have "O" blood group and Child has "A" blood group
• B. Mother, Father and Child all have "A" blood group
• C. Mother has "A" blood group, Father has "B" blood group and Child has "AB" blood group
• D. Mother and child has "O" blood group and Father has "B" blood group

Hint:

The golden rule for ABO inheritance: \textbf{Two Type O parents can ONLY have Type O children.} This is because they both have the genotype \(ii\) and can only pass on the recessive \(i\) allele.

Answer 1

The Correct Option is A

The ABO blood groups are determined by three alleles: \(I^A\), \(I^B\), and \(i\). A child inherits one allele from each parent.
- Genotype for Type O is \(ii\). - Genotype for Type A is \(I^A I^A\) or \(I^A i\). - Genotype for Type B is \(I^B I^B\) or \(I^B i\). - Genotype for Type AB is \(I^A I^B\).
Step 1: Evaluate each option.
(1) Mother (O) has genotype \(ii\). Father (O) has genotype \(ii\). They can only pass on the \(i\) allele to their child. Therefore, their child MUST have genotype \(ii\) and be blood type O. A child with blood group "A" is impossible. This statement is incorrect.
(2) Mother, Father, and Child all have "A". This is possible. For example, if both parents have genotype \(I^A i\), they can have a child with genotype \(I^A I^A\) or \(I^A i\) (Type A).
(3) Mother (A) and Father (B). This is possible. If Mother is \(I^A i\) and Father is \(I^B i\), they can have a child with genotype \(I^A I^B\) (Type AB).
(4) Mother (O) and Child (O), Father (B). This is possible. Mother (\(ii\)) gives an \(i\) allele. For the child to be O (\(ii\)), they must also get an \(i\) allele from the father. This is possible if the father has genotype \(I^B i\).
Conclusion: The scenario in option (1) violates the laws of Mendelian inheritance.