Genetic Misconceptions
"Medical Sleuth," about Dr. D. Holmes Morton’s work with genetic diseases among Amish and Mennonite people in Pennsylvania, prompted many readers to ask why such communities do not avail themselves of premarital or prenatal genetic screening to prevent the birth of afflicted children.
Dr. Morton responds:
When the Plain people, which include the Amish and Mennonite, came to Lancaster County from Europe about 300 years ago, they carried a set of recessive gene mutations that are still present in U.S. and European populations. As the two Plain populations expanded from 20 to 50 founding couples to 20,000 to 50,000 people, some recessive mutations became more common, while other genes became less common or even disappeared. The change in gene mutation frequencies within a new, small, rapidly expanding population is called “random genetic drift,” and it—not inbreeding or natural selection—is the major reason for the high incidence of specific recessive disorders in isolated human populations, including the Plain communities. Over the same period, genetic drift has also been at work within the U.S. general population. As a result, cystic fibrosis is a very common recessive disorder in the U.S. Caucasian population, but is absent in the Plain populations of Lancaster County. The carrier rate for this disorder is now 1 in 25 people, 4,000 per 100,000 in the general U.S. population.
Contrary to popular belief, preventing the birth of children with recessive genetic disorders, or allowing affected infants to die from lack of medical care, does not significantly decrease the disease risk in a population over time. That’s because the risk of genetic disease in a population resides in carriers, rather than in a few affected children who have two copies of the abnormal gene.
For the Plain people, prearranged marriages and therapeutic abortions are not acceptable. But they have readily taken advantage of genetic testing and expanded newborn screening for certain inherited disorders, because successful treatment usually depends on early detection and prevention of severe illnesses.
Our clinic seeks to improve methods of diagnosing and treating genetic disorders in the Plain communities. An understanding that many common medical problems arise from genetic causes and are nonetheless treatable may prove to be an important contribution of the Plain people to modern medicine.
D. Holmes Morton, M.D.
Clinic for Special Children
Strasburg, Pennsylvania
Bahamian Blenny
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