Where's the claim that Moses wrote Genesis come from? That sounds like one of those folklore attributions, like the idea that the gospels were written by the apostles Matthew, Mark, Luke, and John (something there is no basis for and which seems utterly impossible given when they appear to have been written, with John over a century after Christ's birth). Given the multitude of sources, and the fact that the earliest written record of Genesis goes back to 587(ish) BC (at the time the Kingdom of Israel was being overrun by the Babylonians), it seems like at most Moses played his part in the oral history that led up to what ended in the book, rather than that he "wrote" it. One need only look to the first two chapters to see two completely separate creation stories to get past that claim.