I think Jim has described my experiences very well. I definitely believe that how you react is mostly related to genetics and that living at altitude is the 2nd biggest factor.

A few years ago I went to climb a 14'er with my wife and she got sick and we had to head down at around 12,500 ft. The next weekend I went back to the same peak with her brother and he got sick and had to turn around (I finished as I was tired of coming back and his symptoms were relatively minor). We all lived in the same town but obviously her and her brother had the same genetic background.

I then took my sister who was living in Maine near sea level up a different 14'er and she had no problem. I realize that this is a small test of this theory but it seems to hold true with other experiences I have had with other people.

-Skippy