Section 3: Genetics, Cellular and Molecular Biology
Classical Genetics and Epistasis
Subtopic: Mendelian Genetics & Gene Interactions
Question Statement:
A pedigree of an inheritable disease is shown below.
What type of inheritance does the disease follow?
Options:
(A) Autosomal dominant
(B) X-linked dominant
(C) X-linked recessive
(D) Autosomal recessive
GATE 2018 Biotechnology
Official Answer Key:
C
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2018 Q.48)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (2 Marks)
- **Topic**: Genetics (Pedigree Analysis & Modes of Inheritance)
**2. Detailed Step-by-Step Analysis**:
- Let us analyze the inheritance pattern in the pedigree:
1. **Skipping of generations**: Unaffected parents produce affected offspring (e.g., normal carrier parents produce affected sons), which is characteristic of a **recessive** disorder. This rules out autosomal dominant and X-linked dominant modes.
2. **Sex-bias**: Affected individuals in the pedigree are exclusively **males** ($X^d Y$).
3. **Transmission through carrier females**: An unaffected carrier mother ($X^D X^d$) transmits the mutated allele to 50% of her sons ($X^d Y$), who express the disease because they are hemizygous.
4. An affected father passes his Y chromosome to all sons (hence sons are unaffected if the mother is $X^D X^D$) and passes his mutated X chromosome to all daughters, who become obligate carriers ($X^D X^d$).
- These hallmark features establish the mode of inheritance as **X-linked recessive**.
- **Option-by-Option Evaluation**:
- **Option (A) [INCORRECT]**: Autosomal dominant requires an affected parent in every generation and equal male-female ratio.
- **Option (B) [INCORRECT]**: X-linked dominant would cause all daughters of an affected father to be affected, which is not observed.
- **Option (C) [CORRECT]**: The pedigree displays classic X-linked recessive inheritance with carrier mothers transmitting the trait to affected hemizygous sons.
- **Option (D) [INCORRECT]**: Autosomal recessive typically affects males and females in equal frequencies without sex-linked carrier patterns.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **C**
- **Scoring Criteria**: +2.0 Marks (Negative marking: -0.67 marks).
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