Why is swapping regulatory regions used to test Pitx1's role in pelvic reduction rather than swapping the coding sequence?

• A. Because Pitx1 coding changes are the main drivers of pelvic variation.
• B. Because regulatory swaps provide a direct test of how expression patterns influence pelvic morphology.
• C. Because swapping regulatory regions is irrelevant to pelvic morphology.
• D. Because pelvic reduction cannot involve any coding changes.

Answer: (C)

The important idea is that where and when a gene is turned on can shape a trait as much as what the gene does. For Pitx1 and pelvic development in sticklebacks, researchers have found that the pelvis shrinks when Pitx1 activity is reduced specifically in the pelvic region during development. By swapping regulatory regions—the DNA elements that control Pitx1’s expression—they directly test whether changes in expression pattern drive the pelvic morphology. If moving a regulatory region alters the pelvis in the expected way, it shows that altered gene expression in that area is responsible. Swapping the coding sequence would change the Pitx1 protein everywhere, across many tissues, which could cause broad, confounding effects and wouldn’t pinpoint the pelvis-specific expression change. So regulatory region swaps are the clearest way to link where Pitx1 is expressed to the pelvictrait outcome.