PlanetCatfish.com

The long fin mutation in the common bushy nose ancistrus pleco is an autosomal dominant mutation.
This means that if a pleco carries the long fin mutation (heterozygous for long fin), then it will have long fins.
Conversely, if the pleco only has short fins, then there is absolutely no way it is carrying the long fin mutation.

I am curious if any pleco breeders out there have ever come across a long fin pair that produces 100% long fin offspring.
I am getting a sense that long fin is a homozygous lethal mutation, and that every long fin we see is heterozygous for long fin.
I asked around several pleco breeders on aquabid and nobody has come across a pair that produces 100% long fin fry.
I suppose it's possible that long fin has variable expressivity or incomplete penetrance, which might explain why
a long fin pair cannot produce 100% long fin offspring. However this is less likely because a pair of short fin plecos
(born from long fin parents) cannot produce any long fin offspring.

I think there is something to your hypothesis. My experience with breeding LF Albino BN produced similar results.