Paternity Index
Encyclopedia
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 a 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.
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.
Paternity testing
Parental testing is the use of genetic fingerprinting to determine whether two individuals have a biological parent-child relationship. A paternity test establishes genetic proof as to whether a man is the biological father of an individual, and a maternity test establishes whether a woman is the...
,Paternity Index (PI) is a calculated value generated for a single genetic marker or locus
Locus (genetics)
In the fields of genetics and genetic computation, a locus is the specific location of a gene or DNA sequence on a chromosome. A variant of the DNA sequence at a given locus is called an allele. The ordered list of loci known for a particular genome is called a genetic map...
(chromosomal location or site of DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
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
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
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
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
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 a the Probability of Paternity seen on paternity test reports. Minimum Probability of Paternity value requirements for state cases differ between states but the AABB
AABB
The AABB is a United States-based professional body and standards organization that was founded in 1947 as the American Association of Blood Banks. The organization is now international with members in 80 countries and has taken on a broader scope to include all of transfusion medicine as well as...
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
Allele frequency
Allele frequency or Gene frequency is the proportion of all copies of a gene that is made up of a particular gene variant . In other words, it is the number of copies of a particular allele divided by the number of copies of all alleles at the genetic place in a population. It can be expressed for...
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.
Overview of PI Calculations
The PI is a likelihood ratio that is generated by comparing two probabilities where PI = X / Y:- Numerator (“X”) – The probabilityProbabilityProbability is ordinarily used to describe an attitude of mind towards some proposition of whose truth we arenot certain. The proposition of interest is usually of the form "Will a specific event occur?" The attitude of mind is of the form "How certain are we that the event will occur?" The...
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. - 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.
Further reading
- Brenner CH (1999) Kinship Analysis by DNA When There Are Many Possibilities, Progress in Forensic Genetics 8, Eds G Sensabaugh et al. ISBN 9780444503039