The results also show how quickly physical adaptations can be reversed when interbreeding occurs between distinct populations.
The genetic deficiencies from each parent's lineage were easily overcome by the strengths of the other.
This means that even though the fish are blind, they basically have functional visual systems that have been deactivated by a few key mutations, said Jeffery, who was unaffiliated with the study.
"Everything must be in place except for the function of these key genes," Jeffery added.
The study also found that cavefish genetics reflect geography.
The farther apart two parents came from, the more likely it was their offspring would be able to see.
This suggests that geographically distant populations are genetically more distant and thus have less overlap in blindness-causing genes.
During the course of their research, the scientists also developed a definitive test for sight in blind cavefish—a boon to future research on the unusual creatures.
Studies from as far back as the 1970s had suggested some hybrid fish could see because their eyes were larger than their parents'—but definitive vision tests have been lacking.
The new test involves immobilizing recently hatched fish and placing them in a cylinder that flashes an alternating pattern of black and white stripes. If the fish can see, they move their eyes according to the color divisions.
"For the first time it really gives us an assay for detecting whether cavefish can see," Jeffrey said.
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