Several other diseases, like Tay-Sachs and cystic fibrosis, are also more common among Ashkenazis. The higher incidence is thought to be due what is called the founder effect. During that time the population contracted well below two million and was broken into small communities with intermarriage. The mutations that happened to be common among the founders of the community are still common and occur at higher frequencies than do mutations in larger populations among whom genes are more widely mixed. Other present-day populations with strong founder effects include the Finns, who seem to have descended from a group of merely 2,000 people, Icelanders and Pacific Islanders.
Larger populations tend to have a greater number of founder mutations but none very common.
Other groups may have a larger number of founders and a greater range of mutations, so that in the aggregate they carry just the same burden of disease. Nonetheless, two clinical geneticists who co-signed Vogelstein's report expressed concern at how an expanded testing program might be perceived both outside and within the Ashkenazi community. As more founder-effect genetic diseases are discovered, he said, some patients fear that they will be coerced into taking genetic tests before marriage. Others have concealed their diseases rather than face the pressure to be tested, he said. Both he and Ostrer said that only people with a family history of colon cancer should be offered the new test until the risks associated with the disease have been better measured.