Heterozygote disadvantage in cystic fibrosis [letter].

I have found that cystic fibrosis (C.F.), which is very common in 
South West Africa, occurs almost exclusively in white Afrikaners, probably as 
a result of inbreeding, the drift from the Northern Cape Colony, 
and religious and geographical separation. However, I have not found C.F. 
in the descendants of the Angola Boers, a small proportion (6 
- 7%) of the White population who belong to a small 
branch of the Dutch Reformed Church. The absence of C.F. may 
stem from their limited numbers (7000) but I suggest that it 
may have at least partially a genetic basis. Homozygotes with C.F. 
are subject to salt-losing crises which can be fatal in hot 
weather and heterozygotes often have an increased concentration of sodium and 
chlorine ions in their sweat. The influence of environmental temperature and 
drought may explain why C.F. occurs among Blacks in temperate zones 
of the United States but not in Africa, and is less 
frequent in the hot southern regions of Europe than in the 
colder, more northern areas. Negative selection of this type may have 
operated only in hot, arid environments and the possibility that other 
environments offered advantages to C.F. heterozygotes cannot be excluded. 