In a classic stickleback study, how was ploidy variation ruled out as a cause of plate variation?

• A. By sampling only diploid individuals and using genetic markers to confirm ploidy; plate variation correlated with EDA rather than ploidy.
• B. By measuring plate size across populations.
• C. By assuming ploidy is constant in sticklebacks.
• D. By manipulating ploidy experimentally.

Answer: (A)

The key idea is distinguishing variation caused by chromosome number (ploidy) from variation caused by genetic control of plate development. In the classic stickleback work, researchers made sure all fish used were diploid and verified ploidy with genetic markers, so any differences in plate number could not be explained by having extra chromosome sets. They then found that plate variation tracked the EDA genotype—an actual gene effect—rather than ploidy level, showing that plate differences arise from genetic regulation rather than differences in chromosome copies. Simply measuring plate size or assuming ploidy is constant wouldn’t test this distinction, and experimentally changing ploidy isn’t how this study approached the question.