Technical Information

We have 46 chromosomes inside every cell in our body. Twenty-three of these chromosomes came from our biological mother and 23 chromosomes came from our biological father. One pair of these chromosomes contains DNA that determines the sex of the child. In a male child, this sex chromosome is XY; and in the female child, this sex chromosome is XX. DNA parentage testing examines the genetic material found inside the nucleus, and includes the examination of a portion of the sex chromosome that determines the sex of the child.

DNA is also found in the mitochondria contained in every cell cytoplasm, outside the nucleus. Mitochondrial DNA passes without change (except for rare mutations) from mother to all children, sons and daughters, but only daughters pass it to the next generation. Therefore, mitochondrial DNA testing is used to determine whether two or more people are related through their maternal lineage.

The genetic sites that are examined in DNA parentage testing are called Short Tandem Repeats (STRs). These STRs are short fragments of DNA that are repeated several times at a certain location on the DNA. Different individuals can have a different number of repeats at the same location on the DNA, . Thus, these different number of repeats represent different genetic variants, or alleles.

All children have a biological mother and a biological father. For each genetic site in the child, there is one genetic variant (allele) that came from the father and one genetic variant (allele) that came from the mother. DNA Parentage Test determines and examines the genetic variants [i.e. allele sizes] found in the mother, the child and the alleged father.

The Figure #1 below shows the actual DNA results data from an STR testing of the FGA genetic site identified in specimens analyzed from the mother, her child and from an alleged father in DNA Paternity Test. Each tested individual (i.e, the mother, the child, and the alleged father) have two peaks at the FGA genetic site. The size of each peak represents the number of times that the STR is repeated in this person. In the example depicted below, the mother and the child have the STR allele that is repeated 21 times. This means that the child received the STR that is repeated 21 times from the mother. The picture below also shows that the child also has the STR that is repeated 19 times. The child's STR that is repeated 19 times has to come from the child's biological father (Note that the mother does not have the STR that is repeated 19 times; and therefore, the mother could not give this STR to the child). The picture below shows that the tested alleged father does have the STR that is repeated 19 times, and he shares this STR with the child. This means that the tested man could be the child's biological father. However, there are other men in the population who also have the STR that is repeated 19 times at the FGA genetic site. If this identified STR is rarely found in other men in the population, the likelihood that the tested alleged father is this child's biological father is high. If this identified STR is commonly found within men in the population, the likelihood that the tested alleged father is this child's biological father is low since many of these men match this child. Using gene frequencies obtained from race specific population databases, the laboratory calculates the Paternity Index (PI) number for each STR system. This PI number represents the likelihood, or odds, that the tested man passed the matching allele to the child, in comparison to an untested, unrelated man of the same race. The value of PI reflects the frequency of the matching allele in the population specific to the tested alleged parent's race. Typically, numerous genetic sites are examined during DNA paternity testing in the manner described here. If, at the end of the test, the laboratory showed only matches between the child and the tested alleged father, the laboratory calculated the Combined Paternity Index that represents the likelihood that the tested alleged father passed all of the tested matching alleles to the child, in comparison to an untested, unrelated person of the same race. Additionally, the laboratory calculates the probability of paternity, expressed as a percentage (%), and it represents the probability that the tested alleged father is the biological father of the child. (See a  Sample DNA Paternity Test report, Paternity Inclusion).

Figure #1

The Figure #2 below shows the actual DNA results data from an STR testing of the FGA genetic site identified in specimens analyzed from the mother, her child and from an alleged father in DNA Paternity Test. However, in the case, the child's STR that is repeated 23 times came from the child's biological father (Note that the mother does not have the STR that is repeated 23 times; and therefore, the mother could not give this STR to the child). The picture below shows that the tested alleged father does NOT have the STR that is repeated 23 times. This alleged father has allele sizes that are different from those found in the child. Typically, numerous genetic sites are examined during DNA paternity testing in the manner described here. If, at the end of the test, the laboratory shows that the tested alleged father does not match the child on 3 or more genetic sites, the alleged father is NOT the child's biological father. (See a  Sample DNA Paternity Test report, Paternity Exclusion).

Figure #2

In general, the more genetic sites are tested by the laboratory, the more rare is the total genetic profile, and the fewer men will have the same exact DNA profile. Thus, the accuracy of the DNA test results largely depends on the extent of the DNA testing process. With sufficient testing, DNA technology provides an extremely powerful method of discriminating between fathers and non-fathers.