The result of the DNA testing process is a tracing that has a spike for every DNA fragment that is detected. Each one of these fragments is measured very accurately and they can be compared across lanes since all fragments are measured using the same in-lane standards. Each fragment represents a gene.

DNA Paternity testing is based on this very simple principle: For any genetic system, an individual must obtain one gene from his mother and one gene from his father.

To use this principle, we first compare the mother with the child and determine which genes in the child came from the mother. The child’s biological father must have contributed the remaining genes. If the alleged father is missing two or more, he cannot be the father of the child. If he matches them all he cannot be excluded.

For each STR system, a paternity index is calculated. The paternity index is a statistical parameter called a likelihood ratio. It compares the likelihood that the alleged father could have produced a child with this genotype by mating with the mother with the likelihood that another (randomly chosen) man could have done so. To understand how the paternity index is produced, let’s use some real data (shown below).

The figure shows the results produced when each lane is scanned for the STR D22S683 on chromosome 22. Each peak represents an allele of this STR system. Note that each individual possesses two alleles. The mother has an 18 repeat allele and a 20 repeat allele. The child has a 19.2 repeat allele (19 full GATA repeats plus two extra base pairs) and a 20 repeat allele. The alleged father has a 14 repeat allele and a 19.2 repeat allele. We first compare the mother to the child and determine that she must have contributed the 20 repeat allele to the child. Thus the child’s other allele, the 19.2 repeat allele, must have been received from his biological father. Note that the alleged father also possesses this 19.2 repeat allele, so we cannot exclude him as the biological father of the child. We then proceed to calculate a Paternity Index (PI) for this STR system. The alleged father’s likelihood of contributing a 19.2 repeat allele is simply one-half. He has two alleles and there is an equal chance of him passing on a 19.2 or a 14 repeat allele. A randomly chosen untested man of the father’s race would have a likelihood of passing on the 19.2 repeat allele that is equal to the frequency of that allele in individuals of the alleged father’s race. Let’s say that the alleged father is Caucasian and the frequency of that allele in Caucasians is 0.1. The PI would then be calculated by dividing 0.5 by 0.1. Thus the PI is 5.0.

These same calculations are performed for each STR system and if the alleged father is not excluded, a Combined Paternity Index (CPI) is calculated by multiplying all the individual PI’s since the individual genes at all loci are inherited independently of one another. The table below lists mean and median CPI’s as well as minimum and maximum CPI’s for the major racial groups tested at Identigene.

IDENTIPLEX PATERNITY INDEX (PI) DATA BY RACE


The PI is determined strictly from the genetic evidence. Genetic evidence is combined with social evidence (the prior probability of paternity) using Bayes’ Theorem of Conditional Probability to calculate the Probability of Paternity. Paternity laboratories typically remain neutral in the assessment of the prior probability by using the value 0.5. By using this value, we assume the alleged father and a randomly chosen untested man have an equal likelihood of fathering the child before any testing is carried out. Using this value, we simply divide the paternity index by one plus the paternity index to obtain the Probability of Paternity. For example, if the Combined Paternity index were 999, the Probability of Paternity would be 0.999 or 99.9%.

Sample Exclusionary Report (79.5K)
Sample Inclusionary Report (81.2K)

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