I'm looking at some online articles on Huntington's chorea. In Barranquitas Venezuela almost half of the 10,000 population has it. Most cases apparently trace back 200 years to one woman who had 10 children. The identification of the gene for it found testing the families there. With such a large population of carriers, they are more likely to marry & continue the high concentration. (see Business Week, May 29, 2000 i3683 p4EU2)While predictive testing is possible the Lancet reported in Dec. that only 5-25% (several countries surveyed) of those at risk for the disease decide to receive it. This is mainly because there's no cure, so no point. In the US fear of loss of insurance is also a factor. Prenatal testing is also very low. (The Lancet, Dec 9, 2000 v356 i9246 p1944)