# DBT JRF 2020 — Question 107

Exam: DBT JRF 2020 · Section: B · Topic: Plant Biology · Difficulty: Hard

## Question

Which of the following floral whorls are absent in agamous (ag) mutant of Arabidopsis?

## Options

- **A.** sepals and carpels
- **B.** stamens and carpels
- **C.** petals and stamens
- **D.** sepals and petals

## Answer

Option B

## Explanation

ABC (Homeotic gene) model of flower organ identity: Three classes of genes (A, B, C) acting in overlapping domains determine floral organ identity. A alone → sepals; A+B → petals; B+C → stamens; C alone → carpels. A and C are mutually exclusive (A represses C and vice versa). AGAMOUS (AG) provides C function in Arabidopsis (inner two whorls: stamens and carpels). In agamous (ag) loss-of-function mutants: C activity is lost; the mutual repression fails and A expands into all whorls. Without C: whorl 3 (normally B+C = stamens) becomes B+A = petals; whorl 4 (normally C = carpels) becomes A = sepals. Result: ag flowers are sepal-petal-petal-sepal... inflorescence (indeterminate, repeating). Stamens and carpels are ABSENT from ag mutants.

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