I dont think chocolate reapers are unicorns at all. Nor do I think they are random mutations. I am no expert on dna, but it seems to me that if a self pollinating plant has a recessive gene for brown, then brown is going to pop up when pollen from one plant reaches another plant but not when they self pollinate. If that is the what happened, they should be stable from the start because they are not really a hybrid.
Two people with brown eyes can have a blue eyed baby.
Two people with blue eyes can not have a brown eyed baby.
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I had a single chocolate reaper the first year I grew them. If I remember right, they came from Pepper Joe seeds. Saved the seeds and grew again. Chocolate again.   Seems like enough people have run into them that it is likely you got one of the happenstances.Â