Heterozygote advantage.

When a genic disorder is only manifest if there are offending alleles 
at both loci, as in fibrocystic disease, there is a loss 
of two abnormal alleles with each affected birth since the homozygotes 
rarely reproduce. How is this loss compensated? How could some 50,000 
alleles, which can only survive to convey themselves in effective gametes 
if they have a normal allelic partner, become established in the 
British Isles - and how have some half a million or 
more got into the world currency? First, migrations and settlements involve 
the proportion of defective alleles at all loci being conserved, but 
their variety being increased. Secondly, there could be a general tendency 
due to some advantage in health or vitality, which could be 
manifest by disease resistance, cold resistance, and by any number of 
physical or psychological mechanisms. Thirdly, there may be 'compensation' - that 
is the death of an infant may lead to replacement as 
a result of which the replacing sib, if surviving, would have 
a 2/3 chance of being a carrier. Fourthly, the disease may 
be maintained by mutation. Of the four possibilities, three are unrealistic, 
so that substantial selective advantage of heterozygotes over the last few 
hundred years must be postulated. 