Inbreeding among Adam's and Noah's Children

Most of what I write below is purely referential postulating. You'll need to find the scriptures I implicitly refer to yourself. I do limit the references to Genesis, in the Bible. Stronger clues can, however, be found in the books of Moses and Abraham in the Pearl of Great Price.

Assuming Adam and Eve were created by God, it's probably safe to assume that He would have created them genetically perfect.

Current understanding of the necessity of genetic diversity does not contemplate genetic perfection. We don't even yet know enough about genetics to be able to define, describe, or theorize genetic perfection.

I believe some geneticists theorize that (because we can't define it) genetic perfection is impossible, but, again, we simply don't know enough to say that.

Incidentally, the rib spoken of in scripture, I consider might be a chromosomal structure. Men are "missing" a "rib" in there. My sometimes interpretation, for your amusement.

Under such conditions of genetic perfection, we should at least consider the possibility that genetic diversity itself developed out of the initial condition of genetic perfection.

Near perfect bodies would mean that it would be possible for them to maintain health to a greater degree for them than for us, including genetic health for enough generations for inbreeding to become a problem.

Early death is not described in the first generations, except in the case of Cain deliberately killing Able. Although the Bible mentions the burden on women due to childbearing, death of the mother in childbirth is also not mentioned. 

Without some attrition due to early death and death in childbirth, the initial population growth is explosive, something on the order of 

p(n+1) = p(n)+((p(n)/2) x 12)

which is a pretty fast growth rate -- a steep exponential curve.

That's assuming ordinary fecundity. Twelve children from one mother is not imposible in our day, and we can consider it as a possible average for the early, near-perfect generations from Adam and Eve. As mentioned, for the first generation, we could even consider Eve capable of triple that number of children. But we don't need to.

The second generation might have been something like just twelve, but the third would have been something like 96 (plus or minus) new individuals. Even if attrition and reduced fecundity began at this point, that's plenty to ensure survival -- if they don't immediately go to war against each other. (That's the reason Cain is promised God's protection, as I understand it, so that what he did doesn't start a war.)

The other problems of sibling marriage -- power issues and such, are fairly clearly described in the Bible's description of the first several generations. (Cain and Able, also, see Lamech, in Cain's line.) 

We also see them recur in the descriptions of the first generations from Noah -- the need for Abraham to leave his father's country, for example.

Noah would be a problem in genetics, unless we assume that, among Adam's descendants, those mentioned as heirs of Adam's instruction in each generation in Genesis deliberately chose a wife for maximum diversity. I have not found clear indication in scripture, but I have found hints, one of which I mention below.

This brings up something else, which I can mention here. We have somewhat of a record of God intervening fairly actively among some of the descendants of both Adam and Noah. Assuming God exists, we do not have any reason to assume He would not intervene as necessary, including the possibility of adjusting the genetic pool by what we would call supernatural means. 

If necessary -- I mention above a way in which in would not have been necessary, but we do not have to discount the possibility.

The beginnings of race really ought to be considered as the result of continued close inbreeding that occurred after the third generation, both from Adam and from Noah. 

Indeed, we might consider the wife that Ham took with him on the ship to have been a deliberate choice to preserve diversity, partially undone in the next generation after Ham, and somewhat carelessly recorded in negative light because of what occurred in the generation after Ham.

Now, even though I can hypothesize the above, I am not going to say that I know that this is the absolute truth, or that I know that other interpretations are wrong, such as that Adam was the first human to be willing to accept God's teachings, and thus the first son of God in that sense. 

I just offer this for those who prefer keep fairly close to the Biblical text, specifically considering Genesis 3: 20, and Genesis 5: 3 and 4.