What can your spit tell you about your DNA?

­Genetic tests analyze DNA present in blood and other tissue to find genetic disorders -- diseases linked to specific gene variations or mutations. About 900 such tests exist, ranging from more invasive procedures that require a trip to the hospital to the new generation of at-home tests that demand nothing more than spitting into a sterile, mini-sized spittoon. Here's a quick overview.

Prenatal testing may involve sampling and testing the DNA of a fetus. One common test under this umbrella is amniocentesis, which requires a physician to insert a needle into the water-filled sac surrounding the fetus to withdraw a small amount of fluid. In a lab, workers culture fetal cells from the amniotic fluid to obtain a sufficient quantity of DNA. Then they analyze the DNA for chromosome abnormalities that can lead to diseases or conditions such as Down s­yndrome and spina bifida.

Another approach to genetic testing is gene sequencing, which identifies all of the building blocks, or nucleotides, of a specific gene. Once a person's gene has been sequenced, doctors can compare the gene against all known variations to see if it is normal or defective. For example, inherited alterations in the genes called BRCA1 and BRCA2 (short for "breast cancer 1" and "breast cancer 2") are associated with many cases of breast cancer.

Next up is single nucleotide polymorphism (SNP) testing. Nucleotides have three parts: a phosphate group, joined to a pentose sugar, bonded to a nitrogenous base. You can probably recite the various bases along with us -- adenine, guanine, cytosine, thymine and uracil (in RNA). Together, these nucleotides can combine in nearly infinite ways to account for all of the variation we see within and between species. Interestingly, the sequence of nucleotides in any two people is more than 99 percent identical [source: 23andMe]. Only a few nucleotides separate you from a complete stranger. These variations are called single nucleotide polymorphisms, or SNPs (pronounced "snips").

To run a SNP test, scientists embed a subject's DNA into a small silicon chip containing reference DNA from both healthy individuals and individuals with certain diseases. By analyzing how the SNPs from the subject's DNA match up with SNPs from the reference DNA, the scientists can determine if the subject might be predisposed to certain diseases or disorders.

­SNP testing is the technique used by almost all at-home genetic testing companies. It doesn't, however, provide absolute, undisputed results.

­Unlik­e at-home pregnancy testing, which delivers results to the user as she watches, at-home genetic testing isn't so simple or home-based. You do get to provide the sample at home, but everything else requires the help of off-site trained scientists. You can't simply spit in a cup, dip in a wand and read the results. This is how it works:

­All of the service providers offer security measures to protect your data and allow you to choose how much data is made available to you.

­­­So what, exactly, do a few milliliters of spit tell you? The most important thing you'll learn is what kind of genetic markers you carry.

A genetic marker is any alteration in your DNA that may indicate an increased risk of developing a specific disea­se or disorder. Because SNPs are, by their very definition, variations in DNA, they can be used as flags or markers for nearby DNA that affects your health.

Different se­rvice providers identify different genetic markers. For example, 23andMe identifies your risk for developing lupus, while deCODEme and Navigenics don't. These latter companies, however, test for markers of Alzheimer's disease, which isn't included in 23andMe's repertoire. The table below compares the service providers across 10 other important diseases and disorders [source: Genetics and Public Policy Center].

Understanding your susceptibility to certain diseases is a powerful tool. Say, for instance, your at-home genetic test reveals you're predisposed to develop osteoporosis. You can use this information to take a more proactive role in your own health care. You might decide to take supplements to ensure you're getting enough calcium and vitamin D. You might also engage in regular weight-bearing exercise and opt to have a bone density test to determine your risk for future fracture. These kinds of decisions are part of a growing trend that finds more patients -- well educated and computer-literate -- assuming control of their own health care.

­Better health is not the only thing you can get out of your spit. You can also trace your ancestral roots. This is possible because closely related individuals have more similarities in their DNA. By comparing your genetic information to that of people from around the world, you can fill out a comprehensive family tree, tracing your lineage through either your mother or your father.

­Some people char­acterize at-home genetic testing as a revolution in health care. Others are more cautious. A few are deeply troubled. Much of the concern centers on the uncertainty of results that come back from the testing companies.

Having a genetic marker for a disease is not a guarantee that a person will actually get that disease. Few conditions are purely genetic. Most have both inherited and environmental causes. Many cancers, for example, require a genetic predisposition, as well as an environmental trigger to set the disease in motion. As a result, consumers who make decisions based solely on the results of a genetic test may receive unnecessary treatment or make unnecessary lifestyle changes.

A few states require consumers to obtain authorization from a health care provider before they undergo genetic testing. Some states prohibit at-home testing altogether. Currently, 25 states and the District of Columbia permit at-home genetic testing without restriction [source: Genetics and Public Policy Center]. The U.S. Food and Drug Administration doesn't regulate direct-to-consumer tests, although the labs themselves must comply with the Clinical Laboratory Improvement Amendments (CLIA), which ensure quality laboratory testing. CLIA doesn't, however, require genetic testing labs to enroll in programs designed to assess their analytic validity -- their ability to deliver correct answers when performing tests.

Finally, there's the issue of genetic discrimination. Many people worry that their genetic information will interfere with their ability to get insurance coverage. In May 2008, the Bush administration took one major step to prevent this by passing the Genetic Information Nondiscrimination Act, or GINA. GINA protects Americans by making it illegal to use genetic information to influence health insurance and employment decisions. It does not extend to genetic information-based discrimination in life or long-term care insurance.

So before you spit into a cup and send it off for testing, you had better make sure you're fully prepared for all consequences, good and bad. Perhaps it's best to remember an old proverb that warns, "Don't spit into the well -- you might drink from it later."