Q.No. 30 W, X and Y are the intermediates in a biochemical pathway as shown below:
S → W → X → Y → Z

Mutants auxotrophic for Z are found in four different complementation groups, namely Z1, Z2, Z3 and Z4.
The growth of these mutants on media supplemented with W, X, Y or Z is shown below (Yes: growth observed; No: growth not observed):

Mutants	W	X	Y	Z
Z1	No	No	Yes	Yes
Z2	No	Yes	Yes	Yes
Z3	No	No	No	Yes
Z4	Yes	Yes	Yes	Yes

What is the order of the four complementation groups in terms of the step they block?

• (A) S → Z1 → W → Z2 → Z3 → Y → Z4 → Z
• (B) S → Z4 → W → Z2 → Z1 → X → Z3 → Y → Z
• (C) S → Z3 → Z1 → Z2 → X → Z4 → Y → Z
• (D) S → Z4 → W → Z1 → X → Z2 → Y → Z3 → Z
  

  ✅ Correct Answer: Option (A)

  Order of mutants (site of block):
  S → Z1 → W → Z2 → X → Z3 → Y → Z4 → Z

  ---

  🔍 Step-by-Step Reasoning

  📌 Pathway given

  S → W → X → Y → Z


  W, X, Y = intermediates.
  Mutants need Z for survival (auxotrophs).

  📌 Reading supplementation pattern

  Mutants	+W	+X	+Y	+Z
  Z1	No	No	Yes	Yes
  Z2	No	Yes	Yes	Yes
  Z3	No	No	No	Yes
  Z4	Yes	Yes	Yes	Yes

  Rule to infer position:
  A mutant grows only if provided its downstream product.

  ---

  🧬 Mutant Z4

  Growth with W, X, Y & Z

  👉 Means giving W rescues it
  ➡️ Block must be before W
  So Z4 = defect in S → W step

  ---

  🧬 Mutant Z3

  Growth only with Z

  👉 Even Y cannot rescue
  ➡️ Block must be right before Z
  So Z3 = defect in Y → Z

  ---

  🧬 Mutant Z2

  Growth with X, Y, Z, not with W

  👉 Giving X works → W→X is intact
  ❌ Block must be W → X

  So Z2 = defect in W → X

  ---

  🧬 Mutant Z1

  Growth with Y & Z, not W or X

  👉 Giving Y works
  ❌ Block must be X → Y

  So Z1 = defect in X → Y

  ---

  🧩 Combine positions

  S —Z4→ W —Z2→ X —Z1→ Y —Z3→ Z


  ---

  🟢 Correct Option

  ✔ (A) Z1 Z2 Z3 Z4 in correct order

  ---

  ❌ Why the other options are wrong

  ❌ Option B: Z4 → Z2 → Z1 → Z3

  Positions mismatched. Z3 must be last; Z4 must be first.

  ❌ Option C: Z3 → Z1 → Z2 → Z4

  Claims Z3 is earliest block—but Z3 needs Z (total last block).

  ❌ Option D: Z4 → W → X → Y → Z3

  Puts Z3 wrong & splits others incorrectly.

  Only (A) respects metabolic logic & data.

  ---

  ---

  📘 Introduction

  In metabolic genetics, pathway block mapping is a powerful method to place mutants along a biosynthetic route. This IIT JAM and GATE-style question uses complementation groups Z1–Z4 and their growth response to supplemented intermediates W, X, Y and Z to identify where each mutant blocks synthesis along the pathway S → W → X → Y → Z.

  ---

  🧪 Mutant Supplementation Strategy

  Auxotrophic mutants require the end product (Z) to survive. If intermediate nutrients rescue growth, the mutant’s block lies before that supplement in the pathway.

  ---

  📊 Data Interpretation

  • Z4 grows with W → block before W

  • Z2 grows with X → block before X

  • Z1 grows with Y → block before Y

  • Z3 grows only with Z → block just before Z

  Ordering these steps maps the pathway disruptions directly.

  ---

  🏁 Final Ordered Pathway

  S — Z4 → W — Z2 → X — Z1 → Y — Z3 → Z


  Thus, the correct order of complementation groups from first to last block is:

  ⭐ Z4 → Z2 → Z1 → Z3

  Which matches Option (A).