Is he the father? DNA paternity testing is the only definitive way to get the answer.
In order to determine paternity, a sample of the alleged father's DNA as well as the child's (the mother's also greatly helps) must be acquired. Since DNA resides deep inside the cells of a person's body, this usually means obtaining whole body cells from which to extract the DNA.
The easiest way to get cells is from the inside of the subject's cheek, but in cases where that issimply not possible (for example, if the alleged father is unwilling to comply with a DNA test), other body specimens may be used.
Everything in your body (just about) is made up of cells, so most pieces of one's body can be used. Most common cell sources used in DNA testing are blood, saliva (don't throw out that plastic cup he drank from!), semen, or even hair—all can be used, with varying degrees of accuracy, to determine paternity.
At the testing lab, the sample taken from the child is compared with the sample taken from theparents. Not the entire molecule is examined—such a test, although it would produce 100% accurate results, would be incredibly complex and incredibly expensive.
DNA testers have narrowed down the "DNA paternity testing zones" to about 15 sections which are compared to each other. This produces results that are only 99.9% accurate.
Legal Paternity Testing
Legal paternity testing refers to a DNA paternity test that can be presented in a court of law—in child support cases, custody cases, inheritance claims, and others, courts now accept DNA paternity testing results as valid evidence of paternity.
Private DNA paternity testing differs from legal paternity testing in that the latter requires witnesses present at the time the samples are taken. Legal DNA paternity testing also has higher standards for accuracy (believe it or not, 99.9% just isn't enough for Uncle Sam). Since the results will be presented in a court of law, government witnesses must be present through all stages of the testing.