Q. 65 The basic tenets of the ABC model of Arabidopsis flower development are shown below along with a
diagram.
i. 𝑨 class genes acting alone determine sepal identity
ii. 𝑨 and 𝑩 class genes acting together determine petal identity
iii. 𝑩 and 𝑪 class genes acting together determine stamen identity
iv. 𝑪 class genes acting alone determine carpel identity
v. 𝑨 and 𝑪 class genes mutually inhibit each other
Which of the following organ arrangements is found in an 𝑨 class mutant?
(A) sepal; petal; stamen; carpel
(B) carpel; stamen; stamen; carpel
(C) petal; petal; stamen; carpel
(D) stamen; stamen; stamen; carpe
Introduction
The ABC model of flower development explains how combinations of floral homeotic genes determine organ identity in the four concentric whorls of an Arabidopsis flower. Questions based on mutant phenotypes are extremely common in CSIR-NET, GATE, and competitive life-science exams.
This article explains how an A-class mutant alters floral organ arrangement, using the core principles of the ABC model.
Recap: Basic Tenets of the ABC Model
| Gene Class | Organ Identity |
|---|---|
| A alone | Sepal |
| A + B | Petal |
| B + C | Stamen |
| C alone | Carpel |
Important Rule:
👉 A and C class genes mutually inhibit each other
Normal Floral Organ Arrangement (Wild Type)
| Whorl | Gene Activity | Organ |
|---|---|---|
| 1 | A | Sepal |
| 2 | A + B | Petal |
| 3 | B + C | Stamen |
| 4 | C | Carpel |
What Happens in an A-Class Mutant?
-
A-class gene function is lost
-
C-class genes expand into whorls 1 and 2
-
Mutual inhibition no longer restricts C-gene expression
Floral Organ Identity in an A-Class Mutant
| Whorl | Gene Activity | Resulting Organ |
|---|---|---|
| 1 | C | Carpel |
| 2 | B + C | Stamen |
| 3 | B + C | Stamen |
| 4 | C | Carpel |
Correct Organ Arrangement
Carpel; Stamen; Stamen; Carpel
Correct Answer
✅ Option (B)
Explanation of All Options
Option (A): Sepal; Petal; Stamen; Carpel
❌ Incorrect
-
This is the normal wild-type arrangement
-
Does not represent a mutant phenotype
Option (B): Carpel; Stamen; Stamen; Carpel
✅ Correct
-
Loss of A-class genes
-
Expansion of C-class activity into whorls 1 and 2
-
Matches the predicted ABC model outcome
Option (C): Petal; Petal; Stamen; Carpel
❌ Incorrect
-
Petal identity requires A + B
-
A-class genes are nonfunctional in this mutant
Option (D): Stamen; Stamen; Stamen; Carpel
❌ Incorrect
-
Whorl 1 cannot form stamens because B-class genes are absent
-
Violates basic ABC model logic
Final Takeaway
In an A-class mutant, the floral pattern becomes reproductive-organ dominant, producing:
Carpel → Stamen → Stamen → Carpel
This classic question tests understanding of gene interaction, repression, and floral patterning, making it a high-yield topic for exams.
