Q.39
Which of the following statement(s) is/are CORRECT in the classical ‘ABC model’ for
genetic control of flower development?
(A) ‘Class A’ genes solely determine sepal identity
(B) ‘Class B’ genes solely determine petal identity
(C) ‘Class C’ genes solely determine stamen identity
(D) ‘Class C’ genes solely determine carpel identity
ABC Model Flower Development: Correct Answers Explained
The classical ABC model explains how specific gene classes control floral organ identity in whorls of Arabidopsis flowers. Only option (D) is correct, as Class C genes alone specify carpels, while others require combinations.
Correct Answer
Option (D): ‘Class C’ genes solely determine carpel identity.
In the classical ABC model, floral organs form through combinatorial gene action across four whorls: sepals (whorl 1), petals (whorl 2), stamens (whorl 3), and carpels (whorl 4). Class C genes (e.g., AGAMOUS) act alone in whorl 4 to specify carpels, confirming (D).
Option Analysis
Class A Genes (Option A)
Class A genes (APETALA1, APETALA2) alone specify sepals in whorl 1 but combine with Class B for petals in whorl 2. They do not “solely” determine sepal identity without B-function context, making (A) incorrect.
Class B Genes (Option B)
Class B genes (APETALA3, PISTILLATA) work with Class A for petals (A+B) and with Class C for stamens (B+C). No sole petal determination occurs, so (B) is incorrect.
Class C Genes for Stamens (Option C)
Class C genes combine with Class B for stamen identity in whorl 3 (B+C). They do not act solely for stamens, rendering (C) incorrect.
ABC Model Overview
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Whorl 1: A alone → Sepals
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Whorl 2: A + B → Petals
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Whorl 3: B + C → Stamens
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Whorl 4: C alone → Carpels
This model, derived from Arabidopsis mutants, highlights antagonistic A-C regulation for whorl specificity.
