Which trait shows the strongest repeatable genetic basis across freshwater sticklebacks?

• A. Dorsal fin length controlled by BMP signaling.
• B. Gill raker length controlled by Hedgehog pathway.
• C. Body size controlled by growth hormone receptor.
• D. Lateral plate number controlled primarily by EDA; pelvic spine variation controlled by Pitx1 pelvic enhancer.

Answer: (D)

The key idea is that some traits show parallel, repeatable genetic changes across independent populations because a few large-effect genes drive the difference. In freshwater sticklebacks, the number of lateral armor plates is predominantly controlled by the EDA gene. Across many separate freshwater populations, reduced plate number has evolved repeatedly, and this trait changes largely due to regulatory changes at EDA. That makes EDA a single, powerful target that yields consistent phenotypic shifts in many lineages, so the genetic basis is highly repeatable.

Pelvic spine variation, while also tied to a major locus (the Pitx1 pelvic enhancer), is less uniformly observed across freshwater populations, and its presence depends more on genetic background and context. The other traits involve different pathways and modest effects that don’t show the same universal repeatability. So the lateral plate number governed by EDA stands out as the most consistently repeatable genetic change in freshwater sticklebacks.