What does the percentage for probability of paternity mean?

In paternity testing, Paternity Index (PI) is a calculated value generated for a single genetic marker or locus (chromosomal location or site of DNA sequence of interest) and is associated with the statistical strength or weight of that locus in favor of or against parentage given the phenotypes of the tested participants and the inheritance scenario. Phenotype typically refers to physical characteristics such as body plan, color, behavior, etc. in organisms. However, the term used in the area of DNA paternity testing refers to what is observed directly in the laboratory. Laboratories involved in parentage testing and other fields of human identity employ genetic testing panels that contain a battery of loci (plural for locus) each of which is selected due to extensive allelic variations within and between populations. These genetic variations are not assumed to bestow physical and/or behavioral attributes to the person carrying the allelic arrangement(s) and therefore are not subject to selective pressure and follow Hardy Weinberg inheritance patterns.

The product of the individual PIs is the CPI (Combined Paternity Index) which is ultimately used to calculate the Probability of Paternity seen on paternity test reports. Minimum Probability of Paternity value requirements for state cases differ between states but the AABB requires in their Standards for Relationship Testing Laboratories (currently in the 9th edition) a minimum of 99.0% be reported where the tested man is ‘not excluded’ as the biological father of the child in question. U.S. Department of State requires a minimum Probability of Paternity of 99.5% for all immigration cases.

PI calculations utilize allele frequencies generated from established population databases most commonly using Short Tandem Repeats.

Because allele frequencies can be either generated in-house or published, PI’s can differ between companies. This is an understood phenomenon and justifiable amongst members of the testing community.[citation needed]

Calculations[edit]

The PI is a likelihood ratio that is generated by comparing two probabilities where PI = X / Y:

1. Numerator (“X”) – The probability that we observe the phenotypes of the tested participants in the inheritance scenario given that the tested man is the true biological father. More simply, the probability that some event will occur given a certain set of circumstances or conditions. This calculation assumes that the individuals tested are a “true trio/duo” (which is explained two paragraphs down) or in other words, the parent(s) tested are the true biological parents.
2. Denominator (“Y”) – The probability that we observe the phenotypes of the tested participants in the inheritance scenario given that a random man is the true biological father. More simply, the probability that some event will occur given a different set of circumstances or conditions. This calculation assumes that the individuals tested are a “false trio/duo” or in other words, the parent(s) tested are not the true biological parents.

In general, X / Y can be translated as: It is X / Y times more likely to see the observed phenotypes if the tested man is the true biological father than if an untested, unrelated randomly selected man from the same racial population was the true biological father.

When you perform a DNA paternity test the result will be conclusive in most cases with the result either confirming that the father is ‘not excluded’ from being the biological father or alternatively it will confirm that he is ‘excluded’ with 100% certainty. All our DNA tests are performed by analyzing 21 genetic loci. These loci are analysed individually and the resulting index of relationship used to obtain a paternity index. Once all the indices are available they are analysed together to determine the Combined Paternity Index (CPI).

What is the combined paternity index?

The Combined Paternity Index (CPI) is calculated from the Paternity Indices generated from each individual locus and gives an indication of the likelihood of Paternity between the alleged father and child as compared to analysis with the general population. In our tests we generate 20 different paternity indexes; one index is calculated for every one of the 20 genetic loci we analyse. We use these indexes to:

• calculate the combined paternity index
• calculate the probability of paternity

The 21st genetic locus is the amelogenin sex gene and we do not generate a paternity index for this genetic marker. This is because we consider testing this a locus a simple measure to make sure that no swabs have been misplaced. Sometimes clients absentmindedly swtich swabs; they might place the mother’s DNA samples in the blue envelope we allocate for the samples of the alleged father. Once in the laboratory, we wil immediately know whether the swabs contain male DNA or female DNA and hence, whether there has been any mix up.

The Probability of Paternity or POP

The probability of paternity is simple a percentage. It is the most straightforward part of your paternity test and the quickest way of knowing whether the tested man is the biological father of the child. The probability of paternity can express the following:

A 99.99% would signify that the alleged father is the biological of the child.

A 0% probability of paternity would signify that the alleged father is not the biological father of the child.

Results

In the sample result templates we have provided, you can see that the DNA test report will show on the left hand column each of the 21 loci analsyed. You can also see the paternity indexes, the combined paternity index and the probability of paternity.

The first thing you will notice when you look at the table is the 20 genetic markers tested on the far left. You will also notice that in most cases, for each person tested, every genetic marker will be represented by 2 numbers. This is because of the genetic markers is inherited from the father and the other from the mother. If you only notice one number rather than 2, this means that the person has inherited the same genetic marker from both parents.

The biological father will need matching numbers for every one of the 20 genetic markers tested if he is to be included as the biological father. If you see two or more genetic markers that do not match, it means that the tested man is not the biological of the child. There are rare exceptions: a single mismatch can occur but this may not necessarily mean that the tested father is not the biological father. We will need to include the mother’s sample in such cases or carry out some further tests to extract more genetic markers. This will help us provide an accurate result. We do not issue paternity inclusions with results lower than 99.9%.

If you have any queries about how to interpret your DNA paternity test result kindly Contact us directly.

What is the meaning of probability of paternity?

What is the percentage of DNA paternity?

What probability of paternity (%) threshold would be used for conclusive identification?

How do you calculate probability of paternity?