Answer:
A polygenic trait.
Explanation:
A polygenic trait is where multiple different genes control a single trait. (Example: Eye color)
https://humanbodyanatomy.co/parts-of-the-body-anatomy/parts-of-the-body-anatomy-anterior-and-posterior-parts-of-the-body-anatomy-human-anatomy-chart/
this link has a picture with the posterior region.
it might cancel out large quantities of salt that are not needed
