DNA (Deoxyribonucleic acid) is the genetic material in the cells of your body. Every nucleated cell has 46 chromosomes, except the sperm cells from the man and the egg of the woman which only contain 23 chromosomes each. At the moment of conception, there are 46 chromosomes necessary to create a person. Thus, a person receives half of his/her genetic material or DNA from the biological mother, and the other half from the biological father. By comparing the DNA profiles of the mother, father, and child, one can determine the genetic relatedness of a group of individuals.

In a parentage test, predetermined regions of DNA located on specific chromosomes (genetic loci) are analyzed. Particular regions are chosen because variability of DNA sequence occurs at each of them.

At each chromosomal location, individuals will typically have two differently sized DNA markers, one coming from their mother and the other from their father. The DNA profiles of the mother and child are examined first. Typically for a test including a mother, the comparison will demonstrate a match between the mother and child for one of the markers at all genetic loci. Knowing which markers come from the mother automatically indicate which markers the child must have received from the biological father. These markers are referred to as the "obligate paternal alleles" because the true biological father is obligated to have that same of DNA markers. The alleged father’s DNA is compared to the obligate paternal alleles of the child.

A man who is the biological father of a child will have a DNA marker that correctly matches the obligate paternal allele at every chromosomal location that is examined. If the alleged father has a marker that correctly matches the obligate paternal allele at every genetic location tested, the individual is included as the potential father.

If the alleged father is not the biological father of the child, his DNA profile typically will not include the obligate paternal alleles of the child at three or more of the genetic loci analyzed. If an alleged father does not match the obligate paternal alleles of the child at typically three or more genetic loci, that individual is excluded as being the biological father of the child. When the profile does not match at one or two genetic loci, further testing is normally required to determine if the mismatches are genetic mutations. Most laboratories require 3 exclusions to eliminate the alleged father from being the biological father.

The frequency at which the child’s obligate paternal allele occurs in the general population is determined for each genetic locus. These values are then used to calculate statistics that present the likelihood of the alleged father being the biological father of the child, and the relative power of the test.

If the alleged father is not excluded as being the biological father, a number is reported which tells how many times more likely he is to being the biological father than an unrelated random male of similar ethnic background. This number is referred to as the Paternity Index (PI) and is a calculation based on the frequency of the collection of genetic markers shared by the alleged father and child.

Normally the burden of proof is on the woman to prove that the man she asserts to be the father is the biological father of her child. In order to shift the burden of proof from the mother to the alleged father courts typically require:

1. The alleged father cannot be excluded by the paternity test.
2. The test performed must be capable of producing a Probability of Paternity of 99% or greater.