Not sure on pleco genetics but with all breeding there is "3 legs" as I refer to it.
1 is mom ,2 is dad and the third leg is one of mom or dads parents.Some fish also don't breed true and crossing color variations can yield random results.I had a pair of "Red finned " angels that shot every color under the sun except albino and black..For real...
I kept a pair of platinums from them !