56. In Agamous mutant (flower within flower phenotype) which of the following statements is valid?
(1) Class A genes are expressed in the first two whorls, Class B genes are expressed in the second and third whorls and Class C genes are expressed in the third and fourth whorls.
(2) Class A genes are not expressed. Class B and C genes are expressed in all the whorls.
(3) Class A genes are not expressed. Class B genes are expressed in the second and the third

Class A genes are expressed in the first two whorls, Class B genes are expressed in the second and third whorls and Class C genes are expressed in the third and fourth whorls.

Introduction

The Arabidopsis AGAMOUS (AG) mutant demonstrates a classic homeotic phenotype called “flower within flower,” caused by defects in floral organ identity genes. The ABC model defines spatial expression of Class A, B, and C genes within floral whorls directing organ differentiation. Understanding gene expression patterns clarifies the morphological changes seen in AG mutants.

Expression Patterns in Wild-Type and ag Mutants

• Class A genes (e.g., APETALA1/AP2) are expressed in the first two whorls, specifying sepals and petals.

• Class B genes (e.g., APETALA3, PISTILLATA) express in the second and third whorls, defining petals and stamens.

• Class C gene (AGAMOUS) expresses in the third and fourth whorls, specifying stamens and carpels and repressing A class gene expression in inner whorls.​

In agamous mutants:

• Class A gene expression expands into inner whorls normally occupied by Class C activity.

• Loss of AG activity leads to transformation of stamens to petals and carpels to additional flowers.

• Class A, B, and C genes retain their characteristic spatial expression domains although the repression by class C is lost resulting in phenotypic changes.​

Flower Within Flower Phenotype Explained

• The mutation causes the floral meristem to remain indeterminate, continuing to produce additional floral organs in place of carpels.​

• The first two whorls keep A gene expression; B genes remain in whorls 2 and 3; C gene expression is lost or reduced causing homeotic transformations.​

Summary Table: Gene Expression Domains

Conclusion

The gene expression and mutant phenotype data confirm that in the AGAMOUS mutant, Class A genes are expressed in the first two whorls, Class B genes in the second and third whorls, and Class C genes in the third and fourth whorls. This pattern corresponds to option