A paternity test has two possible outcomes:

 1.  The tested man is not the father.  The report will say that the tested man is not the biological father (excluded) of the tested child. The report will show a minimum of three exclusions.

 2.  The tested man is the father.  If the report says that the tested man is not excluded as the biological father of the tested child, the results will be reported as a statistic known as the Combined Paternity Index.  A Combined Paternity Index of 100 or greater is the accepted standard to establish parental rights in most States..

Allele

Child

Father

Paternity Index

CSF1PO

12, 14

11, 13

Exclusion

D3S1358

13, 15

14, 16

Exclusion

TH01

6, 10

7, 9.3

Exclusion

The Father and Child do not share genetic markers in the following systems, CSF1PO, D3S1358 and THO1. Based on these results, The Father is excluded as the biological father. The probability of paternity is 0%.

Exclusion Example: The father or the mother of the child can be confirmed or ruled out by identifying genetic markers.  These markers occur in pairs and are passed from each parent to the child.  For each pair of markers, one comes from the mother (the maternal marker or allele) and the other comes from the father (the paternal marker or allele).  The father is expected to share the paternal marker with the child for each allele tested. This rule of inheritance is very reliable, mismatch (exclusion) between the tested man and the child, typically found in three or more alleles, results in exclusion of the tested man.  In this example at the loci CSF1PO you can see that the child does not have either copy of the fatherís DNA.

Inclusion (Nonexclusion) Example: The father of the child can be confirmed by identifying genetic markers.  These markers occur in pairs and are passed from each parent to the child.  For each pair of markers, one comes from the mother (the maternal marker or allele) and the other comes from the father (the paternal marker or allele).  The true biological father is expected to share the paternal allele with the child for each allele tested. The tested man cannot be excluded as the biological father of the child in question.  In this example, you can follow the paternal allele. In this example the CPI is 3,138.1219 and the probability of paternity is 99.9681%.

Combined Paternity Index (CPI) is the biostatistic that completely evaluates the genetic information.  It is a ratio (a CPI of 100 means 100 to 1) which expresses the relative "fit" of the genetic data to the

Allele

Child

Father

Paternity Index

Parental Allele

CSF1PO

12, 13

12, 14

4.182

12

D3S1358

17, 19

14, 19

19.231

19

FGA

22, 27

24, 27

19.231

27

TH01

8, 9

7, 9

2.029

9

The alleged father cannot be excluded as the biological father of the Child since they share genetic markers.  Based on the results obtained form the analysis of the alleles, D3S1358, FGA, TH01 and CSF1PO the probability of paternity is 99.9681% (prior probability = 0.5) as compared to an untested and unrelated man.

alternate hypotheses of paternity and non-paternity.  The CPI is a simple odds ratio.  Because of the increased accuracy possible with DNA testing, the generally accepted minimum standard for an inclusionary result has risen to a CPI of 100.  The CPI is calculated by multiplying all the paternity index (Product Rule) numbers together (4.182 x 19.231 x 19.231 x 2.029=3,138.122)

The CPI is a measure of the strength of the genetic evidence. It indicates whether the evidence fits better with the hypothesis that the tested man is the father or with the hypothesis that someone else is the father.

Probability of paternity is a mathematically rigorous way expressing and understanding the significance of the genetic results.  Probability of paternity has the same meaning as more commonly experienced probabilities. A probability of rain of 99% means that, of 100 such predictions one expects that only one will be followed by clear skies.  It follows that a probability of paternity depends on an evaluation of the other evidence presented, as well as the genetic evidence.  The prior probability of paternity (p) is the strength of oneís belief that the tested man is the father based only on the non-genetic evidence. 

The other (non-genetic) evidence is summarized by p, the prior probability of paternity.  p=0 means that the other evidence indicates that paternity is impossible, while p=100% means that the other evidence indicates that paternity is certain.  Of course, we have no certainty, so that in each paternity case, p is larger than 0, but less than 100%.  For reasons of convenience, it is customary to assume a value of p=50%.  The argument for using p=50% that is that the tested man is either the true father or he is not. It is a neutral number.   In the absence of any knowledge about which was the case, it is reasonable to give these two possibilities equal prior probabilities.  In this example, the probability of paternity takes the simple form: probability of paternity = (CPI)/(CPI+1) * 100.  In this example the CPI= 3,138.122 and the CPI+1= 3,139.122 therefore, the probability of paternity is  99.9681%.

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