Paternity Index Explained

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)^[1] 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.^[2]

PI calculations utilize allele frequencies generated from established population databases^[3] 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.

Calculations

The PI is a likelihood ratio^[4] 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.

There are 14 possible trio paternity combinations and 5 possible duo paternity combinations.^[5]

See also

• Combined DNA Index System
• International Society for Forensic Genetics

Further reading

• 8652419 . 1996 . Budowle . B . Monson . KL . Chakraborty . R . Estimating minimum allele frequencies for DNA profile frequency estimates for PCR-based loci . 108 . 4 . 173–6 . International Journal of Legal Medicine . 10.1007/BF01369786. 11097755 .
• 11373002 . 2001 . Budowle . B . Sprecher . CJ . Concordance study on population database samples using the PowerPlex 16 kit and AmpFlSTR Profiler Plus kit and AmpFlSTR COfiler kit . 46 . 3 . 637–41 . Journal of Forensic Sciences. 10.1520/JFS15016J .
• Bruce . Budowle . Patrick J. . Collins . Pero . Dimsoski . Constance K. . Ganong . Lori K. . Hennessy . Craig S. . Leibelt . Sulekha . Rao-Coticone . Farideh . Shadravan . Dennis J. . Reeder . Population Data on the STR Loci D2S1338 and D19S433 . Forensic Science Communications . 2001 . 3 . 3 .
• 11373021 . 2001 . Levadokou . EN . Freeman . DA . Budzynski . MJ . Early . BE . McElfresh . KC . Schumm . JW . Amin . AS . Kim . YK . Sprecher . CJ . Krenke . B. E. . Silva . D. A. . McIntosh . T. M. . Grubb . J. C. . Johnston . L. J. . Sailus . J. S. . Ban . J. D. . Crouse . C. A. . Nelson . M. S. . Allele frequencies for fourteen STR loci of the PowerPlex 1.1 and 2.1 multiplex systems and Penta D locus in Caucasians, African-Americans, Hispanics, and other populations of the United States of America and Brazil . 46 . 3 . 736–61 . Journal of Forensic Sciences. 8 .
• 16410169 . 2006 . Butler . JM . Decker . AE . Vallone . PM . Kline . MC . Allele frequencies for 27 Y-STR loci with U.S. Caucasian, African American, and Hispanic samples . 156 . 2–3 . 250–60 . 10.1016/j.forsciint.2005.02.011 . Forensic Science International.
• 12877323 . 2003 . Butler . JM . Schoske . R . Vallone . PM . Redman . JW . Kline . MC . Allele frequencies for 15 autosomal STR loci on U.S. Caucasian, African American, and Hispanic populations . 48 . 4 . 908–11 . Journal of Forensic Sciences. 10.1520/JFS2003045 .
• 15749374 . 2005 . Vallone . PM . Decker . AE . Butler . JM . Allele frequencies for 70 autosomal SNP loci with U.S. Caucasian, African-American, and Hispanic samples . 149 . 2–3 . 279–86 . 10.1016/j.forsciint.2004.07.014 . Forensic Science International.
• Brenner CH (1999) Kinship Analysis by DNA When There Are Many Possibilities, Progress in Forensic Genetics 8, Eds G Sensabaugh et al.
• 19697775 . 2009 . Que . TZ . Yan . PH . Lin . Y . Liu . Y . Li . L . The evaluation of Identifiler system in paternity testing . 25 . 3 . 184–6 . Fa Yi Xue Za Zhi.
• 10356771 . 1999 . Thomson . JA . Pilotti . V . Stevens . P . Ayres . KL . Debenham . PG . Validation of short tandem repeat analysis for the investigation of cases of disputed paternity . 100 . 1–2 . 1–16 . Forensic Science International . 10.1016/S0379-0738(98)00199-6.

External links

• aabb.org
• International Society of Forensic Genetics
• Howstuffworks.com - How DNA Testing Works
• STRBase
• US Department of State
• University of North Texas Health Sciences Center Graduate School of Biomedical Sciences Forensic and Investigative Genetics

Notes and References

1. Book: Guidance for Standards for Relationship Testing Laboratories . June 2009 . 9th . 978-1-56395-293-7.
2. Web site: DNA Relationship Testing Procedures . Bureau of Consular Affairs.
3. 11372982 . 2001 . Budowle . B . Shea . B . Niezgoda . S . Chakraborty . R . CODIS STR loci data from 41 sample populations . 46 . 3 . 453–89 . Journal of Forensic Sciences. 10.1520/JFS14996J .
4. 3766545 . 1986 . Baur . MP . Elston . RC . Gürtler . H . Henningsen . K . Hummel . K . Matsumoto . H . Mayr . W . Moris . JW . Niejenhuis . L . Polesky . H . No fallacies in the formulation of the paternity index . 39 . 4 . 528–36 . 1683973 . American Journal of Human Genetics. 8 .
5. Web site: Paternity index formulas.