Ashkenazi is the term used to describe Jews who have ancestors from Eastern and Central Europe. Today, there are Ashkenazi Jews all over the world and many are marrying outside of the Jewish community. However, for centuries political and religious factors ensured their genetic isolation from the population at large.

As is the case with other groups who live, or lived, in relative isolation – such as the French Canadians and Icelanders – scientists theorize that most Ashkenazi Jews can trace their ancestry back to a small number of founders.

The founder theory is of particular interest to geneticists because they theorize that even if just a few of these original founders had a mutation, the gene defect would then be passed on to their descendants for generations to come.

This theory is supported by the fact that today people of Ashkenazi Jewish descent have a higher incidence of a number of mutations for specific diseases. Examples of this would be mutations in the genes that increase the risk of developing breast and ovarian cancer as well as mutations that cause Tay-Sachs disease and Gaucher disease.

Who Is Ashkenazi Jewish?
Some Genetic Diseases Are More Common in Ashkenazi Jews
The Founder Effect Theory
The Beneficial Mutation Theory

Who Is Ashkenazi Jewish?

The word Ashkenazi is derived from the Hebrew word for "Germany." Today, the term is used to refer to Jews who have ancestors from Eastern and Central Europe, such as Germany, Poland, Lithuania, Ukraine, and Russia. Today, in the aftermath of World War II and the Holocaust, most of the world's 10 million Ashkenazi Jews live in the U.S., Israel, South America, South Africa, Australia, and New Zealand. About 80 percent of the six million ethnic Jews in the U.S. are of Ashkenazi Jewish descent.

Some Genetic Diseases Are More Common in Ashkenazi Jews

Ashkenazi Jews are at greater risk of developing several genetic diseases that are rarely found in other ethnic populations. For example, Type 1 Gaucher disease – a condition in which the body lacks a gene regulating the breakdown of a particular kind of fat – is one of the more common genetic disorders among Ashkenazi Jews.

An estimated 1 in 10 Ashkenazi Jews is a carrier for Gaucher disease. Carrying one gene mutation for the disease will not cause symptoms. However, if both parents carry the gene, there is a 1 in 4 chance that their children will inherit a Gaucher gene from both of them and develop the disease.

The childhood neurological disorder Tay-Sachs disease is also most commonly found in people of Ashkenazi descent – with an estimated 1 in 31 Ashkenazi Jews carrying a gene for Tay-Sachs. Here again, both unaffected parents must carry a copy of the gene mutation in order to pass the disease on to a child. The child then has a 1 in 4 chance of inheriting two mutated genes and developing the disease.

In addition, recent studies have shown that people of Ashkenazi Jewish descent may be at greater risk for breast and ovarian cancer than the general population. In 1995, scientists from the National Institutes of Health (NIH) discovered that a particular alteration in the breast cancer gene called BRCA1 was present in one percent of the general Jewish population. In comparison, the percentage of people in the general U.S. population that have any mutation in BRCA1 has been estimated to be between 0.1 to 0.6 percent.

A follow-up study in 1996 found two additional mutations (one in the BRCA 1 gene and one in another breast cancer gene called BRCA 2) to be at increased prevalence in the Ashkenazi Jewish population, bringing the overall risk for carrying one of these three mutations to 2.3 percent. More on Breast and Ovarian Cancer in Ashkenazi Jews

The Founder Effect Theory

Stanford University geneticist Neil Risch explains the range of genetic diseases unique to the Ashkenazi Jewish population by theorizing that most of today's Ashkenazi Jews descend from a group of perhaps only a few thousand people – the more privileged Ashkenazi that lived 500 years ago in Eastern Europe.

The poorer Ashkenazi Jews, says Risch, had fewer children that lived to adulthood and, as a result, did not pass on as many genetic conditions. Geneticists refer to this relatively small group of ancestors as founders.

Until recently, both religious and political factors helped to ensure that Ashkenazi Jews married other Ashkenazi Jews. 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 in the population.

The Beneficial Mutation Theory

Other geneticists theorize that diseases in Ashkenazi Jews have arisen because of hidden benefits linked to the inheritance of a single copy of a mutant gene.

For example, people with one copy of the Tay-Sachs gene were discovered to be immune to tuberculosis – a condition common in the crowded ghetto conditions in which Ashkenazi Jews often lived. Geneticists theorize that the people who carried the Tay-Sachs gene may have survived better under crowded, adverse conditions, whereas people without the gene would be more likely to die.

As a result, the Tay-Sachs gene continued to be passed on in greater numbers within the Ashkenazi population, even though it threatened the lifespan of a small number of children who inherited two copies of the gene mutation, resulting in a fatal childhood illness.

Regardless of the origin of the mutations, people of Ashkenazi Jewish descent should be aware that they may be at increased genetic risk for certain conditions. Ashkenazi Jews can also carry gene mutations for conditions that occur in other ethnic populations, but the mutations are specific to the Ashkenazi population.

This knowledge and screening has already decreased the number of children born with the fatal Tay Sachs disease by making carrier testing available to those who want to learn their status. Understanding these risks can lead to more accurate risk assessment, genetic testing, and preventive therapy.