Discussion
In wild-type Arabidopsis thaliana, the four types of floral organs (sepal, petal, stamen, carpel) are arranged in concentric whorls from outside to inside. With reference to the ABC model of floral organ patterning, match the homeotic mutants in Group 1 with their respective arrangements of organs in the four whorls given in Group 2.
| Group 1 | Group 2 |
|---|---|
| (P) A class mutants | (i) sepal, sepal, carpel, carpel |
| (Q) B class mutants | (ii) sepal, petal, petal, sepal |
| (R) C class mutants | (iii) carpel, stamen, stamen, carpel |
| (iv) sepal, sepal, petal, petal |
Answer: B
The correct match is P-iii, Q-i, R-ii.
In the ABC model, A alone gives sepals, A + B petals, B + C stamens and C alone carpels, and A and C repress each other.
- A mutants: C spreads to all whorls, giving carpel, stamen, stamen, carpel.
- B mutants: petals become sepals and stamens become carpels, giving sepal, sepal, carpel, carpel.
- C mutants: A spreads inwards, giving sepal, petal, petal, sepal.
Arrangement (iv), sepal, sepal, petal, petal, does not arise from a single class of mutant, and the other options swap these patterns.
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