The beacon fish (Hemigrammus ocellifer) exhibits two phenotypes associated with spotting at the base of the caudal fin, with fish either possessing (H. o. ocellifer) or lacking (H. o. falsus) a prominent red spot in this region. Segregation patterns observed from the progenies of 15 different crosses support a hypothesis that caudal spotting in this species is controlled by a single gene with two alleles, for which the caudal spotting allele is completely dominant.
Frankel, Jack, "Caudal Spotting in the Beacon Fish (Hemigrammus ocellifer Characidae)" (2002). Department of Biology Faculty Publications. 133.