The only thing I can think of is that the temps were a bit too high while he was being incubated. If you notice, the one up for sale on the classifieds also has some extra scutes which also indicates too high of temps while being incubated. I wouldnt say its common or genetic. Just a misshap during incubation. If he's healthy and eating properly I would holed on to him and not cull him. AND I would not sell him either, especially for big money like Turtlesale is doing, its kinda a rip off cus all he is is a regular res just with out yes. He's not a circus freak!
Ricky
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3 Dobermin Pinschers
4 African Grey Parrots
1 Lesser Sulfer-Crested Cockatoo
3 Cockatiels
1 Male Iguana
1 Female RES
1 Juvi YBS
1 Juvi Northern Red-Belly Slider
1 Pleco.
6 Platies