Chicago Center for Jewish Genetics Disorders - About Jewish Genetic Disorders

Carrier Screening

What is it: Carrier screening is performed on individuals who might carry one copy of a disease-causing recessive gene that could be inherited by their children. If one or both of the parents is a carrier, the risk of passing the disease to a child can be predicted. Carrier screening is intended for healthy people who have no symptoms of disease, but who are known to be at high risk because of family history or ethnicity.

When is it done: Carrier screening is ideally done before a pregnancy occurs. It is offered to individuals who have family members with a genetic condition, family members of an identified carrier, and individuals in ethnic or racial groups known to have a higher carrier rate for a particular condition.

How is it done: Screening usually involves obtaining a blood sample for analysis. The serum, or liquid part of the blood, and the cells in the blood contain a large number of enzymes and other proteins that can be analyzed to see if they have been altered by genetic mutation. Alternatively, DNA, which contains all the genes, can be isolated from the cells in the blood and analyzed to directly determine if there is a mutation in a specific gene.

Benefits of testing: This type of screening allows for the identification of carriers for a genetic disorder who, while they themselves are unaffected, may pass along the disorder to their children. Once it has been established that both members of the a couple are carriers for the same disorder, several reproductive options are available including pregnancy with and without testing, sperm or egg donor, adoption, or not having children. See Prenatal Diagnosis for more in-depth discussion of some of these options.

Other things to consider:

For many disorders, enzyme analysis may be possible to determine carrier status. However, the measured enzyme activity levels may fall in an "indeterminate range" or between the normal (non-carrier) range and the carrier range, and it would thus be unclear if an individual is a carrier or not for that disorder. When molecular genetic testing is being done on a disorder in which numerous mutations have been found to be disease-causing, testing may first need to be performed on an affected family member in order to determine the mutation(s) present in the family. A disease for which this is true is phenylketonuria. Also, for certain disorders such as Tay-Sachs disease, carrier status may more accurately be determined by enzyme analysis but there might also be potential problems with false negative or false positive results. In such situations, both DNA mutational analysis for the more common mutations and enzyme assays both may be done to more accurately determine carrier status.
Certain ethnic groups are at higher risk for particular autosomal recessive disorders. In these situations, carrier screening may be offered to individuals who do not have a family history. It is important to note that these disorders still occur in the general population, but much less frequently. Some examples of these disorders are:
- 1 in 10 to 1 in 14 African-Americans in the United States are carriers for sickle-cell disease; 1 in 600 are affected.
- 1 in 30 to 1 in 40 individuals of Mediterranean ancestry, including Sephardic Jews, are carriers for beta-thalassemia; 1 in 3,600-6,000 are affected.
- 1 in 25 Southeast Asians and Chinese are carriers for alpha-thalassemia; 1 in 2,500 are affected
Carrier screening can improve risk assessment for members of racial and ethnic groups more likely to be carriers for certain genetic conditions. In contrast, carrier screening may be less accurate in populations in which the disorder is NOT as common. An example of this would be for DNA carrier testing for cystic fibrosis. Because cystic fibrosis occurs most commonly in individuals of Northern European ancestry, the most common mode of testing yields a carrier detection rate of 90% in Caucasians and 97% in Ashkenazi Jews. However, if a Hispanic individual has a family history of cystic fibrosis, this same test procedure has a detection rate of 57%. This means that 43% of all Hispanic individuals who are carriers for cystic fibrosis would not be detected by this test. It may be possible to modify testing in such situations to increase the detection rate.
Carrier screening is available for each of the nine disorders found in higher frequency in the Ashkenazi Jewish population. However, due to the difference in carrier frequency, detection rate, and disease severity among these disorders, Ashkenazi Jews are not automatically screened for all of these disorders. Cost also factors into this decision for a number of individuals and couples whose health insurance may not cover any or all of the screening. The American College of Obstetrics and Gynecology (ACOG) recommends that Ashkenazi Jews be offered carrier screening for four of the disorders: Tay-Sachs, Canavan disease, cystic fibrosis and familial dysautonomia. These four disorders have a similar carrier frequency and have a high detection rate in this population. ACOG states that:
1. Taking a family history will help to identify individuals of Ashkenazi Jewish ancestry
2. Carrier screening is ideally offered when planning a family or in early pregnancy.
3. Individuals may inquire about carrier screening for other disorders not included in the ACOG recommendations
4. If only one member of the couple is Ashkenazi Jewish, that member should be screened first and those results will determine if the partner is screened
5. Those with a family history of an Ashkenazi Jewish genetic disorder should be offered screening for that disorder
6. If both partners are determined to be carriers of the same disorder, they should be offered genetic counseling with the option of prenatal diagnosis
7. Identifying carriers has implications for family members.

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