there are not many known genetic issues.
and you can find most of the known issues here:
http://www.owalreptiles.com/issues.php
apart from that, deformities and defects often occur when something goes wrong during incubation, like a temperature spike, when some equipment failure causes the eggs to get too hot or too cold.
like with all living things, sometimes a defect or issue just randomly occurs for no known reason, but thats quite rare.
just avoid the combos that are marked as "lethal" or "severe wobble"; and when you know what you want to pair with what or have a specific question just ask here in the forum, people will point it out if they see a problem. and then try to get the incubation done as perfectly as possible, the right temperature and humidity are really important for the eggs.
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