Section 3: Genetics, Cellular and Molecular Biology
Classical Genetics and Epistasis
Subtopic: Mendelian Genetics & Gene Interactions
Question Statement:
Which one of the following is coded by the ABO blood group locus in the human genome?
Options:
(A) Acyl transferase
(B) Galactosyltransferase
(C) Transposase
(D) B-Galactosidase
9 ---
GATE 2019
Biotechnology
Official Answer Key:
B
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2019 Q.40)**
**1. Governing Concept**:
- **Topic**: Genetics & ABO Blood Group Biochemistry
- The human *ABO* gene locus on chromosome 9 codes for a **glycosyltransferase** enzyme:
- The precursor substrate is the **H antigen** (fucose attached to galactose by $\alpha(1,2)$-fucosyltransferase).
- The $I^A$ allele encodes **$\alpha(1,3)-N$-acetylgalactosaminyltransferase**, which adds $N$-acetylgalactosamine (GalNAc) to the H antigen, creating the **A antigen**.
- The $I^B$ allele encodes **$\alpha(1,3)$-galactosyltransferase**, which adds D-galactose to the H antigen, creating the **B antigen**.
- The $i$ ($O$) allele has a single-base deletion resulting in a frameshift and inactive protein, leaving the unmodified H antigen.
- Hence, the ABO locus encodes **glycosyltransferases**.
**2. Option Analysis**:
- **Option (A) is INCORRECT**: Fucosyltransferase is encoded by the separate *H* (FUT1) and *Se* (FUT2) gene loci, not ABO.
- **Option (B) [Glycosyltransferase] is CORRECT**: The biochemical class of enzymes encoded by the functional ABO alleles.
- **Option (C) is INCORRECT**: Protease is not involved in ABO antigen determination.
- **Option (D) is INCORRECT**: Phosphatase plays no role.
**3. Final Answer**:
- **Correct Choice**: **B**
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