That is essentially what we have, but at a species level rather than within a species. Of the two species of the genus Osteoglossum one ends up with DE 99% of the time, the other doesn't get DE 99.9% of the time. Environmental factors may make DE worse or minimize it, but comparing the frequency of DE in silvers to blacks to me it seems certain that silvers have some sort of genetic predisposition towards getting DE that blacks don't.
