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Results interpretation

Before we begin a detailed explanation of how to interpret a DNA paternity report, it is important to define some terms we will use.


Definition of Terms

Paternity Index
For individual locus this value is a likelihood ratio comparing the chance the tested mother and alleged father produced the child to the chance the mother and a man selected at random from the population producing.

Combined Paternity Index
This value is the product of the individual Paternity Indices and is a likelihood ratio comparing the tested mother and alleged father's chance of producing the child to the mother and a man selected at random from the population producing the child. It is measure of the weight of the scientific evidence obtained from the test.

Probability of Paternity
This is the probability that the alleged father may be the biological father. The probability of paternity is calculated using the combined paternity index and a prior probability.

Prior Probability
The prior probability can be from 0 to 1. This is a measure of the non scientific evidence that is the social evidence surrounding conception. Laboratories routinely use a prior probability of 0.5, which is considered neutral. This means that the weight of the social evidence in favor of paternity is equal to the evidence against paternity.


Interpretation of Results

We routinely report testing results with a probability of paternity of 99.99% or higher on inclusions and 0.00% on exclusions. If the alleged father and child do not share all of the required markers, then the alleged father is not the father of the child. This is called an "EXCLUSION" and the probability of paternity will be 0.00%. If the child and alleged father share the required markers, then the alleged father cannot be excluded as the biological father and a probability of paternity will be calculated. The minimum probability of paternity in an inclusion will be 99.99% or greater. A DNA paternity report can never provide a 100% probability of paternity because that would require comparing the DNA of the tested man to every other man on this earth, and that is not possible.

The two diagrams below provide examples of both a Inclusion and an Exclusion.

EXAMPLE OF INCLUSION RESULT

System Mother Child Alleged Father Paternity Index
D3S1358 14 , 18 14, 15 15 , 16 4.03
vWA 14 , 17 14 , 17 14 , 17 2.72
FGA 18 , 24 18 , 22 22 , 24 2.81
D8S1179 13 13 , 14 11 , 14 1.49
D21S11 31.2 , 33.2 30 , 31.2 30 3.88
D18S51 14 14 14 , 17 2.94
D16S539 9 , 12 9 , 11 11 , 12 1.57
TH01 7 , 9.3 7 , 9.3 7 , 9.3 1.41
D2S1338 19 , 24 19 , 24 19 , 23 1.89
D19S433 13 , 14 14 , 17.2 13 , 17.2 39.00

  1. Identify the two alleles (numbers) for the child at each system. (e.g. the child has a 14, 15 at system D3S1358)
  2. Determine which of the child’s alleles came from the mother. (e.g. at system D3S1358, the mother (14,18) gives the child (14,15) a 14)
  3. Therefore the alleged father must provide the child with the other allele, a 15. (e.g. at system D3S1358, the alleged father (15,16) provides the child (14,15) with the 15)
  4. This matching between the child and alleged father at system D3S1358 is an example of an inclusion.
  5. Once the alleles are analyzed for all systems, population statistics are then calculated based upon the paternal alleles provided to the child. The result is a Paternity Index for each system.
  6. The final calculation involves the multiplication of each Paternity Index with the others to come up with a Combined Paternity Index value. For example, the Paternity Index of system D3S1358 is 4.03 and the Combined Paternity Index for the overall results is 85,426 to 1.

EXAMPLE OF EXCLUSION RESULT

System Mother Child Alleged Father Paternity Index
D3S1358 14 , 18 14 , 15 17 , 18 0.00
vWA 14 , 17 14 , 17 14 , 20 2.72
FGA 18 , 24 18 , 22 23 , 25 0.00
D8S1179 13 13 , 14 8 , 14 1.49
D21S11 31.2 , 33.2 30 , 31.2 30 , 35 3.88
D18S51 14 14 15 , 18 0.00
D16S539 9 , 12 9 , 11 11 , 15 1.57
TH01 7 , 9.3 7 , 9.3 6 , 9.3 1.41
D2S1338 19 , 24 19 , 24 19 , 20 1.89
D19S433 13 , 14 14 , 17.2 15 , 16 0.00

  1. Identify the two alleles (numbers) for the child. (e.g. the child has a 14, 15 at system D3S1358)
  2. Determine which of the child’s alleles came from the mother. (e.g. at system D3S1358, the mother (14,18) gives the child (14,15) a 14)
  3. Therefore the biological father must provide the child with the other allele, a 15. However the tested alleged father is a 17,18 and could not have provided the child with a 15.
  4. This mis-match between the child and alleged father at system D3S1358 is an example of an exclusion and the paternity index is 0.00 for this system.
  5. If the child and alleged father do match, population statistics are used to derive a paternity index for that system.
  6. When the statistical calculations are applied to the all of the paternity index results in the above case, the Combined Paternity Index is 0.00 and therefore there is a 0% probability of paternity.

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