Most would be fairly obvious, deformed body parts or something like eyes or jaws missing. I have also heard about snakes that show symptoms like train wreak spiders, but are not spiders. Any of these could be contributed to non-genetic factors also tho, so don't let one negative animal or even one clutch make you panic. Incubation and environmental factor can also cause it. However, if you produce a few negative animals in the same line of animals, you might want to try some outcrossing and hope to remove the negative genes.