Pelvic reduction in freshwater sticklebacks is associated with what genetic change in the pelvis?

• A. Pitx1 activation in the pelvis
• B. Pitx1 inactivation in the pelvis
• C. Pitx2 inactivation in the pelvis
• D. Pitx1 duplication

Answer: (B)

Changes in where a gene is active during development can drive big changes in anatomy. For the pelvis in freshwater sticklebacks, Pitx1 is a key driver of pelvic structure formation. In these fish, the pelvis is reduced because Pitx1 is not turned on in the developing pelvic region. This isn’t about changing the Pitx1 protein itself, but about losing its expression specifically in the pelvis due to regulatory changes (such as deletion of a pelvic-specific enhancer). With Pitx1 inactive in that area, the pelvis doesn’t form as it does in other populations, leading to reduced pelvic structures. If Pitx1 were activated in the pelvis, or if another gene like Pitx2 were involved or Pitx1 were simply duplicated, you wouldn’t see this same pelvic reduction pattern. So the correct interpretation is that pelvic reduction is associated with inactivation of Pitx1 expression in the pelvis due to regulatory changes.