Hemochromatosis and my weakness

I have a terrible weakness for one kind of evolutionary genetics theories. This is the kind where recessive gene disorders are explained by saying that 1 copy of the gene gives you an advantage over some other disease or condition. For example, 1 copy of the sickle-cell anemia gene is supposed to confer resistance against malaria, etc.

The reason why I call it a weakness is because though these theories are REALLY attractive, evolutionarily-speaking, sometimes they have holes.

So I'm writing my Celtic Societies paper on another one. This one is called hemochromatosis. It's supposedly the most common recessive genetic disorder in Europe and the U.S., although it's most common in people of Northern European descent (aka "Celtic" peoples). 1 copy helps you store more iron; 2 copies and your body builds up so much iron over a lifetime that you get heart and liver failures and maybe even turn a little gray.

It's been said, and I want to say, that the hemochromatosis mutation happened after humans switched to agriculture (about c. 2000 B.C.) because there isn't much iron in grains, so it was evolutionarily useful for people to store more iron.

And yet, geneticists have only traced the hemochromatosis mutation back 60 generations or so, which puts it after 1 A.D. or 2,000 years after that mutation would have been suddenly useful. And if it was so useful, why didn't it explode across all newly agricultural societies?

Sigh - another hole in my favorite brand of theory.