49. Three classed of genes A, B and C regulates the development of flower in Arabidopsis. If a loss-of-function mutation occurs in the B-type genes, what will
    be the composition of the flower whorls?
    (1) sepals-petals-stamens-carpels
    (2) sepals-sepals-stamens-carpels
    (3) sepals-sepals-carpels-carpels
    (4) petals-petals-stamens-stamens

The correct answer is:

(3) sepals-sepals-carpels-carpels

Explanation:

According to the ABC model of flower development:

• Class A genes alone specify sepals in the first whorl.

• Class A and B genes together specify petals in the second whorl.

• Class B and C genes together specify stamens in the third whorl.

• Class C genes alone specify carpels in the fourth whorl.

If there is a loss-of-function mutation in the B-type genes (which contribute to petals and stamens):

• The second whorl develops as sepals instead of petals (because only class A activity remains).

• The third whorl develops as carpels instead of stamens (because only class C is active).

• The first and fourth whorls remain unchanged (sepals and carpels, respectively).

Thus, the flower will have:

• 1st whorl: sepals

• 2nd whorl: sepals (instead of petals)

• 3rd whorl: carpels (instead of stamens)

• 4th whorl: carpels

This corresponds to option (3).

Introduction

The ABC model of flower development explains how three classes of genes (A, B, and C) specify the identity and positioning of floral organs in angiosperms. Mutations in these genes cause homeotic transformations where one organ type is replaced by another. Particularly, B-type gene mutations alter petals and stamens, providing insight into fundamental floral organ patterning.

The ABC Model and Floral Organ Specification

• Class A genes drive sepal development (1st whorl).

• Class A + B genes specify petals (2nd whorl).

• Class B + C genes specify stamens (3rd whorl).

• Class C genes specify carpels (4th whorl).

Loss of function in B-type genes leads to inability to specify petals and stamens, which are replaced by the organs determined by the remaining active gene classes.

Effect of B-Type Gene Mutation

• The second whorl forms sepals instead of petals because only A function remains.

• The third whorl forms carpels instead of stamens because only C function is active.

• The first (sepals) and fourth (carpels) whorls remain unaffected.

This creates a flower with floral whorl composition: sepals–sepals–carpels–carpels.

Summary Table: Floral Whorl Organ Identity in B-Type Mutants

Conclusion

Loss-of-function mutation in B-type genes causes petals to be replaced by sepals and stamens to be replaced by carpels while sepals and carpels themselves remain unchanged, consistent with the ABC model and option (3).