Facts to Know

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1. A genetic test examines some aspect of a person's genetic makeup, either directly through gene sequencing or indirectly through the measure of marker chemicals. Such a test usually aims to determine whether a person has, is at above-average risk of having, or is a carrier of a disease-causing genetic mutation.

2. Because the nature of genetic testing is so complex, with implications for both the person being tested and his or her family, genetic counseling is desirable before taking any genetic test and essential if results come back positive or uncertain.

3. Genetic counselors are committed to protecting your privacy. They will not contact other family members without your permission, though they may encourage you to share results that might affect your relatives.

4. A maternal serum screening test indicates whether a fetus is at above-average risk of being born with certain genetic disorders, most notably Down syndrome and open neural tube defects. The test is not diagnostic and a positive result is usually followed up with a diagnostic amniocentesis or chorionic villus sampling test. Out of 1,000 serum screening tests, 25 will suggest increased risk for open neural tube defects, but only one or two of the fetuses will have such a defect. Likewise 70 of 1,000 will test positive for increased risk of Down syndrome, but only one or two will fetuses will actually have the disease

5. Some genetic disorders are recessive and X-linked, which means they are caused by a mutation in a gene that resides on the X chromosome. Females have two X chromosomes, but males have only one. If a mother has a disease-linked recessive gene mutation in one of her X chromosomes, she is a carrier of the disorder but will have no or minimal symptoms herself. If she has a son, he will have a 50 percent risk of inheriting the disorder; a daughter will have a 50 percent chance of being a carrier.

6. In addition to disorders that have surfaced in your family, you may want to consider carrier testing for genetic conditions that occur with greater frequency in your particular ethnic group. For example, Caucasians have a higher risk of cystic fibrosis, while those of African descent are at high risk of carrying a mutation that can cause sickle cell disease. A battery of tests exists for those of Ashkenazi (Eastern European) Jewish descent. Remember that the best time for carrier testing is before a pregnancy.

7. Children should not be screened for carrier status or for diseases that won't trouble them until much later in life because the information is not relevant to their health care. Most geneticists and genetic counselors consider such testing unethical, since children are not in the position to make their own decisions as to whether or not they want the test (known as informed consent).

8. Within a family, two or more incidences of the same type of cancer or related cancers at ages younger than 50 may indicate a hereditary pattern. A genetic counselor can take a closer look at your family history to determine whether an inherited mutation appears to be a possible culprit, and can advise you as to whether testing is available.

9. The best-known cancer predisposition tests look for mutations in the BRCA1 and BRCA2 genes. Women with a BRCA mutation face a lifetime breast cancer risk of up to 85 percent, compared to about 12.5 percent in the general population, and lifetime ovarian cancer risk of up to 44 percent, compared to a population risk of about one percent.

10. If your family has a history of colorectal and related cancers, you may want to consider genetic testing for mutations in two genes strongly associated with hereditary nonpolyposis colon cancer (HNPCC). The syndrome increases lifetime risk of colorectal cancer to 80 percent vs. a two percent population risk, but also boosts risk of endometrial cancer (to 60 percent), ovarian cancer (to 12 percent) and gastric cancer (to 13 percent). Those with HNPCC also face a higher risk of cancers of the kidney and ureter, gallbladder, central nervous system and small bowel.