Eh. For the Greeks at least the Gods were pretty weird. Athena was born fully armed and armored from Zeus's thigh. Aphrodite was born from sea-foam, I think, which might contextually have been Poseidon's spunk? I wouldn't make assumptions about what people believed about the biology of the gods without looking for ancient primary sources. They were believed to be sustained entirely by nectar and ambrosia, for instance.

Plus I'm not even clear if a concept akin to "genetic disease" existed. I'm pretty sure people were aware that certain traits were heritable, but I don't think there was anything like an awareness of genetics and lots of things were still believed to be the result of supernatural forces.