It seems pretty clear that there are (at least) two paths to melanistic morphs.



Model #1 is a traditional homozygous recessive condition, and seemed to be prevalent with early breeder stock. This model applies to what Frank Retes and others observed (as posted on the general Ksnake forum) in the early days of mexicana breeding efforts. Steve Osborne published a note on this in Herp Review (http://www.sierraherps.com/pdf/Osborne1983.pdf) and obtained breeding results that fit a classic Mendelian ratio.



Model #2 follows what John Cherry just described, and is similar to what Brad Alexander, Mike Kochvar, and many others have observed. I have seen Brad’s offspring (the product of pairing a melanistic female with a normal leonis-morph male), all of which are quasi-normal phenotypes—in other words, no melanistics. However, to my eye they are clearly identifiable as the products of at least one melanistic parent. Phenotypically, they have a leonis pattern with a thin veneer of dark gray--like if you stood a few feet away with a can of black spray paint and fired off a brief mist. Other breeders have experienced similar results even when pairing two melanistic parents. Because the non-melanistic offspring were not as clean as other line-bred thayeri, they were more difficult for breeders to sell. I have been told by more than one breeder that this was the reason they no longer work with melanistic thayeri, and likely explains the relative scarcity of this morph for hobbyists. Clearly, this form of melanism does not follow a homozygous recessive pattern.



I am not aware of anyone who currently works with snakes that fit Model #1, and the early breeding stock that carried those genes may be gone from the captive realm.



Finally, it is also conceivable that melanism (in the sense of Model #2) might be the result of multiple genes acting in an additive fashion, such that you get degrees of melanin production.



Certainly a fun project for someone to tackle!



Cheers,



Bob Hansen

www.sierraherps.com

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