What are two examples of major-effect loci mentioned for stickleback divergence?

• A. MC1R and EDNRB
• B. EDA and Pitx1_Pel
• C. PAX6 and HOXA
• D. GHR and IGF1

Answer: (B)

In sticklebacks, big evolutionary shifts between marine and freshwater forms are often driven by a small number of loci with large effects. Two classic examples are EDA, which controls the amount of protective armor plates, and Pitx1_Pel, a regulatory variant associated with pelvic reduction. Freshwater populations frequently carry the EDA variant that reduces plating, and the Pitx1_Pel changes reduce pelvic structures, both providing advantages in freshwater environments. These two loci are repeatedly found across independent stickleback lineages, illustrating how a couple of major-effect genes can drive repeated divergence. Other gene pairs listed tend to be linked to different traits (like pigmentation or growth) and are not the well-established major-effect loci tied to armor and pelvis in sticklebacks.