[This story was reviewed by About.com's Medical Review Board on March 21, 2013]
It is estimated that everybody carries 6-8 disease-producing genes. If both the mother and father carry the same disease-producing gene, their child may be affected by an autosomal recessive genetic disorders. In autosomal dominant disorders, one gene from one parent is enough to make the disease manifest. Many racial and ethnic groups, especially those that encourage marrying within the group, have genetic disorders which occur more frequently in the group.
Jewish Genetic Disorders
Jewish Genetic Disorders are a group of conditions which are unusually common among Ashkenazi Jews -- Jews who have ancestors from Eastern and Central Europe. These same diseases can affect Sephardi Jews and non-Jews, but they afflict Ashkenazi Jews more often - as much as 20 to 100 times more frequently.
Most Common Jewish Genetic Disorders
- Mendelian Disorders are the direct result of mutated genes
- Bloom Syndrome
- Canavan Disease
- Factor XI Deficiency
- Familial Dysautonomia (Riley-Day syndrome)
- Fanconi Anemia
- Gaucher Disease
- Mucolipidosis IV
- Niemann-Pick Disease
- Non-Classical Adrenal Hyperplasia
- Nonsyndromic Hearing Loss
- Tay-Sachs Disease
- Torsion Dystonia
- Breast Cancer (BRCA1 and BRCA2)
- Crohn's Disease
- Familial Colon Cancer
Certain disorders tend to be more common among Ashkenazi Jews due to the "founder effect" and "genetic drift." Today's Ashkenazi Jews descended from a small group of founders. And for centuries, for political and religious reasons, Ashkenazi Jews were genetically isolated from the population at large.
The founder effect occurs when populations are started from a small number of individuals of an original population. Geneticists refer to this relatively small group of ancestors as founders. It is believed that most of todays Ashkenazi Jews descended from a group of perhaps only a few thousand privileged Ashkenazi Jews that lived 500 years ago in Eastern Europe. Today millions of people may be able to trace their ancestry directly to these founders. Thus, even if just a few founders had a mutation, the gene defect would become amplified over time. The Founder Effect of Jewish Genetic Disorders refers to the chance presence of certain genes among the founders of today's Ashkenazi Jewish population.
Genetic drift refers to a mechanism of evolution in which the prevalence of a particular gene (within a population) is increased or decreased not through natural selection, but by mere random chance. If natural selection were the only active mechanism of evolution, presumably only good genes would persist. But in a circumscribed population like the Ashkenazi Jews, the random action of genetic inheritance has a somewhat higher probability (than in a much larger population) of allowing certain mutations that do not confer any evolutionary advantage (like these diseases) to become more prevalent. Genetic drift is a general theory that explains why at least some bad genes have persisted.