In Guns, Germs, and Steel, Diamond argues that geography is the major cause of cultural diversity. Does not genetics play a major role...

I think, first of all, that you are starting this off with
a faulty assumption.  In your explanation, you tell us that people migrated to Europe
where

...
centuries of learning to adapt and survive in a harsh environment brought about the now
well established practices of
agriculture.

This is simply
not true.  Agriculture, of course, did not begin in Europe.  Instead, it began
independently in a number of places, none of which was in Europe.  The first of these
appear to have been the Fertile Crescent and China.  It was only later that agriculture
diffused to Europe.  Therefore, you have a mistake in the premise of your
question.

Second, the time between the discovery of
agriculture and the present is in no way sufficient to allow for serious genetic change
in populations, even if those populations had been kept separate from one another. 
Agriculture has only existed for 10,000 years, which is very little time in terms of
evolution and genetic change.  Furthermore, for much of that time, various populations
have mixed and people in different parts of the world are not terribly genetically
distinctive.

So I have a hard time seeing how genetics
could play a part in the equation you posit here.  Instead, it seems much more likely
that geography had a great deal to do with the ways in which people had to/chose to
live.