Nice find Buckgator! Okay so it is polygenetic and both the top and lower jaw each receive their own set of genes that determines the jaw length independently of each other. Now my female is from Expresso who is undershot and has thrown some undershot dogs so does that mean she is throwing a long lower jaw or a short upper jaw I wonder? After reading the article I understand it isn't just a single recessive gene that causes underbite but actually several genes but is all the information for say one given jaw (top or bottom) coming from one parent or are the genes that are received for lets say the lower lower jaw a mixture of genes coming from both parents? Lastly, it still seems like the gene combination for an undershot dog is recessive......I think lol. It seems when an undershot gyp is bred to a dog with a good scissor bite most of the litter comes out with decent bites, not undershot jaws. I suppose that is maybe a reflection of each dogs ancestry though and not just the individual.