What is Paternity Index?

PATERNITY INDICES
DNA (alleles) from the mother, child, and alleged father are extracted, amplified, and identified. A series of mathematical calculations are then used to either completely exonerate an accused man or provide an estimate of probability of his paternity (POP).


Genetic Markers

Half of a child's genetic material (alleles) come from the mother, while the other half is contributed by the father. A series of genetic systems (loci) are analyzed in an attempt to ascertain the biological father of a child. Each genetic system in a person has two allele, these alleles are numerically labeled. In paternity testing, the alleles from the child are compared to those of the "parents" to determine if it is possible for either or both parents to have contributed the particular alleles present in the child. For instance, assume that a child has a 10 and 11 allele for a particular genetic system and the child's mother is known to possess a 10 and a 12 allele for this system. The mother must have contributed the 10 allele and the 11 allele must be paternal. In this example, any man who does not possess an 11 allele could not be the child's father (barring the possibility of mutation that converts one allele to another - something that is unlikely but can be taken into consideration if needed). In the event that a man is not excluded, the likelihood that a randomly chosen man might also be able to provide the allele in question to the child can be determined by examining the allelic frequencies from a relevant population database.


Paternity Index

The paternity index (PI) compares the likelihood that a genetic marker (allele) that the alleged father (AF) passed to the child to the probability that a randomly selected unrelated man of similar ethnic background could pass the allele to the child. This is presented in the formula X/Y, where X is the chance that the AF could transmit the obligate allele and Y is the chance that some other man of the same race could have transmitted the allele. X is assigned the value of 1 if the AF is homozygous for the allele of interest and 0.5 if the AF is heterozygous. The potential of a randomly selected man to pass the obligate gene is determined by using a database which lists the frequency distribution of individual alleles within a given genetic system.


Combined Paternity Index

The combined paternity index (CPI) is determined by multiplying the individual PIs for each locus tested. The CPI is an odds ratio that indicates how many times more likely it is that the alleged father is the biological father than a randomly selected unrelated man of similar ethnic background. The CPI is based solely on genetic evidence.


Probability of Paternity

To convert the genetic evidence to a probability of paternity (POP) it is necessary to use the Baysian theorem. This is a formula that tests the hypothesis that the accused is the biological father of the child. For example, a POP of 99% reflects a 99% probability that the hypothesis is correct and a 1% probability that it is not. The CPI is used in the Bayes formula along with another variable called a prior probability (PP). This variable represents the social evidence. Testing labs typically use a value of 0.5 for the PP assuming this is a neutral, unbiased value. The Baysian formula is CPI / CPI + (1 - PP) x 100.