20. 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 of Flower Development Explained | Genetic Control of Floral Organ Identity
Introduction
The formation of flowers is one of the most remarkable developmental processes in flowering plants. Every flower develops four concentric whorls consisting of sepals, petals, stamens, and carpels. Although these floral organs appear structurally different, they originate from similar groups of undifferentiated cells present in the floral meristem. Their identity is determined by a specific combination of regulatory genes that control organ differentiation during development.
The classical ABC model of flower development, proposed from genetic studies in Arabidopsis thaliana and Antirrhinum majus, explains how combinations of three classes of homeotic genes specify floral organ identity. These genes encode transcription factors, most of which belong to the MADS-box gene family. According to the model, no single gene class determines every floral organ independently. Instead, floral organs are specified through combinations of gene activities.
Correct Answer
Correct Answers: (A) and (D)
Detailed Explanation
The classical ABC model states that the identity of each floral organ depends on the combination of active gene classes rather than on the action of a single gene class in most cases.
In the first floral whorl, only Class A genes are active, and this activity alone specifies the formation of sepals. Therefore, Class A genes solely determine sepal identity.
In the second whorl, Class A and Class B genes work together to specify petals. Since petals require the combined action of both A and B genes, Class B genes alone cannot determine petal identity.
In the third whorl, Class B and Class C genes together specify stamens. Thus, Class C genes alone are insufficient for stamen identity.
In the fourth whorl, only Class C genes remain active, and this activity alone specifies the formation of carpels. Therefore, Class C genes solely determine carpel identity.
Consequently, only statements (A) and (D) are correct.
Explanation of Each Option
Option (A): ‘Class A’ Genes Solely Determine Sepal Identity
This statement is correct. Sepals develop in the outermost floral whorl where only Class A genes are expressed. No participation of Class B or Class C genes is required.
Option (B): ‘Class B’ Genes Solely Determine Petal Identity
This statement is incorrect. Petals are produced only when Class A and Class B genes function together. Class B genes alone cannot specify petals.
Option (C): ‘Class C’ Genes Solely Determine Stamen Identity
This statement is incorrect. Stamens develop through the combined action of Class B and Class C genes. Class C genes alone are insufficient.
Option (D): ‘Class C’ Genes Solely Determine Carpel Identity
This statement is correct. Carpels develop in the innermost floral whorl where only Class C genes are active.
Why Options (A) and (D) are Correct
The ABC model predicts that sepals require only Class A gene activity, whereas carpels require only Class C gene activity. In contrast, petals and stamens always require the combined action of two different gene classes.
Why the Other Options are Incorrect
Why Option (B) is Incorrect
Petal identity depends on the combined expression of Class A + Class B genes, not Class B genes alone.
Why Option (C) is Incorrect
Stamen identity depends on the combined expression of Class B + Class C genes, not Class C genes alone.
Comparison of All Options
| Option | Statement | Correct or Incorrect |
|---|---|---|
| A | Class A genes solely determine sepal identity | Correct |
| B | Class B genes solely determine petal identity | Incorrect |
| C | Class C genes solely determine stamen identity | Incorrect |
| D | Class C genes solely determine carpel identity | Correct |
The Classical ABC Model
| Floral Whorl | Gene Combination | Floral Organ |
|---|---|---|
| Whorl 1 | Class A | Sepal |
| Whorl 2 | Class A + Class B | Petal |
| Whorl 3 | Class B + Class C | Stamen |
| Whorl 4 | Class C | Carpel |
Major Floral Homeotic Genes
| Gene Class | Representative Genes | Main Function |
|---|---|---|
| Class A | APETALA1 (AP1), APETALA2 (AP2) | Sepal formation and petal specification |
| Class B | APETALA3 (AP3), PISTILLATA (PI) | Petal and stamen formation |
| Class C | AGAMOUS (AG) | Stamen and carpel formation |
Gene Activity in Different Floral Organs
| Organ | Required Gene Activity |
|---|---|
| Sepal | Class A only |
| Petal | Class A + Class B |
| Stamen | Class B + Class C |
| Carpel | Class C only |
ABC Gene Loss Mutants
| Gene Mutated | Major Effect |
|---|---|
| Class A | Sepals become carpels; petals become stamens |
| Class B | Petals become sepals; stamens become carpels |
| Class C | Stamens become petals; carpels become sepals |
Biological Significance
The ABC model provides a molecular explanation for how genetically identical floral meristem cells differentiate into four distinct floral organs. It demonstrates that organ identity depends on combinations of regulatory genes rather than individual genes acting independently. This discovery transformed developmental biology and has become the foundation for understanding floral evolution, plant breeding, and genetic engineering of ornamental and crop plants.
Final Answer
Correct Answers: (A) and (D)
According to the classical ABC model of flower development, Class A genes alone specify sepal identity, while Class C genes alone specify carpel identity. Petals require the combined activity of Class A + Class B genes, and stamens require the combined activity of Class B + Class C genes.
