Discussion
The following table summarizes the flowering time behavior (days to flower) and the transcript levels in four genotypes of a plant species.
| Genotype | Days to flower | Transcript level of gene A | Transcript level of gene B |
|---|---|---|---|
| Wild type | 30 | Normal | Normal |
| a mutant | 15 | Nil | Increased |
| b mutant | 60 | Normal | Nil |
| ab double mutant | 60 | Nil | Nil |
Which one of the following genetic pathways best explains the observations shown in the table?
Answer: B
The pathway is A gene suppresses B, which promotes flowering transition.
- The b mutant flowers late (60 days against 30), so B promotes flowering.
- The a mutant flowers early (15 days) and has increased B transcript, so A normally represses B; without A, more B speeds flowering.
- The ab double mutant flowers as late as b alone (60 days), so B is epistatic to A: B acts downstream, and the early flowering of a depends entirely on B.
'A activates B' contradicts the rise in B transcript when A is lost. The options in which B regulates A contradict the b mutant's normal A transcript level and the epistasis of b.
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