52. Floral organ development is controlled by overlapping expression of ‘A’ class, ‘B’ class and ‘C’ class genes in different whorls. In an Arabidopsis mutant, the flowers
    had sepals, sepals, carpels and carpels in the four whorls. Mutation in which one of the following is the cause for the mutant phenotype?
    (1) ‘A’ class gene alone (2) ‘B’ class gene alone
    (3) ‘A’ and ‘B’ class genes (4) ‘C’ class gene alone

---

Introduction

Floral organ identity in Arabidopsis is specified by the interplay of three classes of homeotic genes: A, B, and C, collectively known as the ABC model. Each gene class is expressed in specific whorls and determines the identity of floral organs. Mutations in these genes lead to characteristic homeotic transformations, providing crucial insight into developmental genetics.

The ABC Model and Gene Class Functions

• A-class genes determine sepal identity in whorl 1 and, with B-class genes, petal identity in whorl 2.

• B-class genes function with A-class genes to specify petals (2nd whorl) and with C-class genes to specify stamens (3rd whorl).

• C-class genes specify stamens in whorl 3 and carpels in whorl 4.

Thus, B-class genes are essential for petals and stamens development.

Effect of B-Class Gene Mutation

• Loss-of-function mutation in B-class genes causes petals to be replaced by sepals, and stamens to be replaced by carpels.

• Sepals and carpels are unaffected, reaffirming that A and C genes control those organs independently.

• Therefore, flowers with whorl composition sepals-sepals-carpels-carpels indicate B-class gene malfunction.

---

Summary Table: Floral Organs and Corresponding Gene Class Control

---

Conclusion

The loss of B-class gene function causes the conversion of petals to sepals and stamens to carpels, resulting in the floral whorl pattern sepals-sepals-carpels-carpels, consistent with option (2).