A standard paternity test involves the participation of the child, alleged father, and the mother (called a trio). To test paternity, the laboratory examines 16 specific areas of each individual's DNA sample. These areas are called loci (singular=locus). Since every individual has 2 copies of each chromosome, they will have two readings for each locus tested. Once testing is completed, DNA loci are compared. Readings for each tested individual will have two numbers. For each locus, one of the child's numbers must match one of the mother's numbers for that locus, and the other number, the biological father's. To illustrate:
If the Mother=(A,B) and the Father=(C,D),
then the Child can be (A,C); (A,D); (B,C) or (B,D).
A paternity index is determined for each match. The paternity index is a calculation of how frequently that match occurs in a specific population; it is the likelihood that the tested man is the biological father of the tested child based on that loci. But one locus is not enough to conclusively determine paternity. That is why we conduct testing on 16 different loci.
Each locus has its own paternity index. If all the loci match, the paternity indices for each are combined, and a combined probability of paternity is calculated.
Our laboratory routinely generates probabilities of 99.999% and higher, meaning the tested man is 99.999% more likely than any other man in his race population to be the biological father of the tested child.
If the tested man does not match the tested child on a locus, the paternity index for that locus is 0. If there are 3 or less non-matches, the samples are examined further to obtain conclusive results (additional testing of up to 25 total markers). If there are 4 or more non-matches, it is concluded that the tested man cannot be the biological father of the tested child. The probability of paternity is 0%.
