February 3, 2011
Introduction to Jewish Genetic Diseases: Part 2
In Part 1 on Jewish Genetic Diseases, I described concepts of genetic testing, the Genome Project, preimplantation diagnosis, genetic engineering and gene therapy. The following questions and answers explain in more detail how these programs are implemented.
1. What is genetic testing?
These tests are examinations of an individual’s DNA found in chromosomes which are the chemical alphabets that spell out genes just as letters spell out words. But, what are genes? Genes are the blueprints or instructions used to make the body’s building blocks or cells. Genetic testing enables physicians to check for defects in the DNA that may cause a disease.
2. How do genes cause disease?
Most genes determine critical components that must function correctly in order for your body to be healthy. Genes that spell out a defective component can cause a disease, which can be handed down through generations just like eye color.
3. How are genetic diseases passed from one generation to the other?
Everyone has two copies of each gene. Therefore, when a man and woman have a child, each contributes one copy of their genes to that child. As a general rule, a disease is expressed only if the child has two copies of the defective gene, although there are exceptions. A normal copy of the gene tells the body how to correctly build the product that is controlled by that gene, and this type of gene is called an autosomal recessive gene. Autosomal means that it is not associated with the sex of the offspring, which implies that the risks are equal for boy and girl babies. Recessive means that if one copy of the gene is normal, the damaged gene recedes (does not appear) into the background and cannot cause the disease.
4. What is a carrier?
A carrier is an individual who has one copy of the defective gene and one of the normal gene because, as discussed above, having one normal copy of the gene is enough to prevent the disease. Therefore, the person would not have an autosomal recessive disease. However, if the carrier mates with another carrier who also has one copy of this same defective gene, there is a chance that the baby would have two copies of the defective gene and, therefore, be affected. That is the reason that couples who suspect they could be carriers may decide to undergo genetic testing.
5. What information can genetic testing provide?
Genetic testing can determine whether one or both partners carry a specific autosomal recessive gene defect. In other words, if neither is a carrier or if only one is a carrier, then there is no risk of having a baby with that disease. If, however, both people are carriers, then there is one chance in four (25%) that their baby would be affected by the disease. Therefore, testing before a couple decides to have a baby can determine what chance there is of the offspring having that specific genetic disease. Genetic testing can also be performed during pregnancy using two procedures: amniocentesis (obtaining a specimen of fluid from the vicinity of the fetus) or chorionic villus sampling (obtain actual cells from the fetus).
6. How is testing usually performed?
The test is most often performed on a sample of blood, which is sent to a specialty laboratory for the specific genetic diagnosis.
7. What if you are identified as a carrier?
If you are found to be a carrier, other members of your family could also be carriers, or they may even be at risk of having the disease. Genetic counseling, therefore, is available to determine these risks.
8. Which diseases can be detected?
There are several disorders that have been called “Jewish genetic diseases,” - not because they are specifically Jewish, but because they are much more common in the Jewish community. It is important to note that these diseases are not limited to Jewish individuals, but often occur in people who are or have been in areas from which Jews have emigrated.. Thus, they also occur in the non-Jewish community. Some examples are Gaucher disease, cystic fibrosis, Tay-Sachs disease, Canavan disease, Niemann-Pick disease, and Fanconi anemia. (A more complete list is found at the end of this article.)
9. Are there more genetic diseases that affect Jews?
Over the next several months, we will look at genetic diseases in several subgroups of Jewish people, which include the Samaritans, the Oriental Jews, Yemenite Jews, Karaites, African Jews, Sephardic Jews, and Ashkenazim.
Every resource that I have checked lists some, but not all disorders. I will list the majority of genetic diseases that I am familiar with in the following tables.
B). Additional Genetic Disorders Common Among Ashkenazim
A- beta lipoproteinemia
D). Additional Genetic Disorders Among non-Ashkhenazi Jews including Sephardic and Oriental Jews