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.