If you are referring to the dangers of inbreeding, that has nothing to do with gene deterioration. It is just that brothers and sisters shouldn't have kids together because it is genetically dangerous, even when both have very healthy genes. It has nothing to do with defects in their genes. It has everything to do with the close relationship, genetically, and the possibility of recessive traits being expressed.
Is it possible that the dangers of close gene association have increased, or possibly weren't present at all originally?
Well, Adam and Eve were genetically identical, except for the boy-girl thing. So their kids would have been genetically identical. Which would mean that all of their kids kids would be genetically identical, being as there was no source for variation. And this would go on, ad infinitum, if it really happened. Not counting real, live, actual mutations. Basically, we should all look alike if we started out with two people only 6,000 years ago. And since the gene pool bottlenecked again, almost as tightly, for the flood, we should all look like Russell Crow. Even our girlfriends.
Of course, other types of genetic change can occur, and since some who study the bible think that the earths population may have been as high as 7 billion when the flood came, there may have actually been opportunities for genetic variation via point mutations, snips, deletions or other ways identified as causing genetic change. But like I said, most of them died, so that is irrelevant. It is the change since the flood that is important. But our population has grown much slower since the flood. The planet didn't hit 2 billion until 1927, so though we humans are a bunch of sinners because of all the sex we have, the pre-flood crowd must have been frickin' minks. Again, they all died, so it doesn't matter, but the idea of 7 billion people in heat is sort of interesting. Especially since a lot of them were 900 years old. Ick.
I'm rambling. If Eden is true, they had to interbreed, because dirt and ribs were apparently in sort supply. That part is no biggie. But since that isn't how things started, we have a huge variation in genetic material in humans because we've had hundreds of thousand of years, and some interbreeding with other closely related species (Neanderthal, for instance), which helped add to the variation.
Genetics are complex. Especially when useless myths like a 6000 year old earth are tossed in to the mix. Which doesn't happen scientifically, but socially, fundamentalists still harm reality by demanding we believe that tripe.