Subject: why does the mutation in Hemoglobin Yakima increase O2 binding affinity?

Hemoglobin Yakima has a mutation where Asp is replaced with His at position 99
so it is just the change in shape that results in the increased affinity? like
how "cooperativity" in normal hemoglobin works? many of the website just say
that it increases the affinity but not why or how this mutation accomplishes 
it.
also are these types of mutations: Hemoglobin Yakima and Hemoglobin Hiroshima
commonly found in the Japanese because of the high altitude areas that they are
living in? so the body has to increase the amount of red blood cells to provide
sufficient O2?
thanks.

Re: why does the mutation in Hemoglobin Yakima increase O2 binding affinity?