You seem to be thinking in current terms as genetic defects are rife today and inbreeding leads to deleterious genes. The gene pool started out perfect with the addition of the death factor after Adam sinned. The faults have increased slowly over time but the curve seems to be somewhat of an exponential one. People in general are becoming very polluted with genetic faults. At this rate we may not have much time save The LORD’s eminent return. Another good reason to say “Come LORD Jesus”.
If your model made any sense, then there would be a gradual progression towards increased genetic abnormalities, to such an extent that from the gene pool of Adam and Eve, to the current generation, barely anyone born today could have escaped drastic genetic defects. This, however, isn’t the case, thus implying that no “perfect” genetic source originally existed to be corrupted later on, least of all, out of a gene-pool of two members, propagating their generations, to the multi-billions, by incest.