Reading paternity test results involves comparing the values at a number of locations along the chromosome, known as loci, tested by the laboratory. Labs provide values for several loci to allow people to see if the subject of a test shares genetic material with another person on all of those loci. Laboratories usually provide information at the top of the paternity test results about whether the two DNA samples were a match, along with offering a detailed breakdown of the testing.
It is important to note that no lab will say with 100% certainty that the provider of one sample was the parent of the the provider of another sample. DNA labs operate on statistical probability; the test will state either that it “includes” or “excludes” the purported father. If the father is included, it means it is strongly statistically probable that he is indeed the child's father. If he is excluded, the test results suggest he cannot be the father.
At each locus, a child will inherit one gene from the mother and one from the father. On paternity test results, the loci tested are named, and people will see a pair of numbers corresponding to each locus for the child and the father, like 1,2 and 4,2. To be a match, one number at each locus must match, showing a genetic relationship between the two samples. In the example above, a 2 is present in both. If some loci match and others do not, the two samples may be related, but they are not parent and child, as children inherit half their DNA from their fathers.
The number of loci tested on a paternity test can vary. The goal with paternity test results is to obtain a statistically relevant sample. A match at a single locus is not as relevant as matches at seven loci, for example. It is possible in rare cases for a child to not match at every locus tested, as sometimes genetic mutations occur or there are problems with the samples that affect the results. DNA labs have very strict procedures for taking and handling samples to avoid issues like contamination.
When reading paternity test results, people can look at the quick answer provided at the top of the test, and they may also choose to examine the results at each locus. Some people find it helpful to highlight or circle the numerical matches to make them easier to spot.