Discussion
Which one of the following class(es) of genes, when deleted, would result in sepal formation in all the floral whorls of Arabidopsis, as per the rules of the typical ABC model of floral organ patterning?
Answer: C
Deleting class B and class C genes gives sepals in every whorl.
In the ABC model, A alone specifies sepals (whorl 1), A with B petals (whorl 2), B with C stamens (whorl 3) and C alone carpels (whorl 4). A and C repress each other, so when one is lost the other spreads into all four whorls.
Without B and C, A is expressed throughout and is the only function left, so every whorl forms sepals.
Losing A alone lets C spread: carpel, stamen, stamen, carpel. Losing A and B leaves C alone everywhere: carpels throughout. Losing B alone gives sepal, sepal, carpel, carpel.
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