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During a paternity test, a DNA profile is generated
from each sample and will ultimately help prove or disprove
the paternity of a child.
The laboratory report will generally have one of three
possible conclusions, which are determined from the
DNA results and a series of complex statistical calculations.
Following is a statement of the most common conclusions
along with a simple explanation of its meaning.
- The results indicate that the alleged father is
not the biological father of the child. This conclusion
is based on the absence of two or more obligate paternal
alleles in the alleged father, as indicated by a Paternity
Index of 0.
- This means that the tested
man is not the father.
- The results indicate that the alleged father can
not be excluded as the biological father of the child.
The reported probability of paternity, as compared
to an unrelated, untested man of the same race, is
calculated assuming a prior probability of 0.5.
- This means that the tested man is the father. While the formal report language might seem awkward and ambiguous, it is a necessity founded in statistics. Because the entire human population was not tested you cannot say “you are the father” because that implies 100% probability.
- The statistical results derived from the genetic
tests fall within the inconclusive range for paternity.
Inclusion of the mother will likely be sufficient
to yield a conclusive result.
- This means that additional testing is necessary to determine if the tested man is the father or not the father. This result occasionally occurs when the mother is not included in the analysis.
Paternity Index
Paternity index is a likelihood ratio between the chances
that the alleged father may pass the paternal gene, compared
to the chance that a random man may pass the paternal
gene to the child. If the paternity index is zero, it
is because the father does not have any matching alleles
with the child at that particular marker. If the paternity
index is greater than zero it means the father does have
a matching allele with the child.
Combined Paternity Index (CPI)
The combined paternity index is the product of all of
the individual paternity index values multiplied together
and is a measure of the strength of the genetic evidence.
The CPI can range from 0 to infinity. When the combined
paternity index is 0, it means the alleged father is not
the dad. When the combined paternity index is greater
than 100 it means the alleged father is the dad, however
it cannot be stated simply as “you are the father”.
Because the probability of paternity can never be 100%
(you will often `see 99.999%), you cannot say “you
are the father” because that implies 100% certainty.
Therefore, the results are reported as “the alleged
father can not be excluded as the biological father of
the child”. When the combined paternity index is
greater than 0, but less than 100, the results are inconclusive.
Inconclusive results are most often the cause of doing
a paternity test without the mother. In most instances,
inclusion of the mother’s genetic profile into the
statistical calculations results in a conclusive result.
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 |
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- Identify the two alleles (numbers) for the
child at each system. (e.g. the child has a
14, 15 at system D3S1358)
- 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)
- 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)
- This matching between the child and alleged
father at system D3S1358 is an example of an
inclusion.
- 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.
- 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
|
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- Identify the two alleles (numbers) for the
child. (e.g. the child has a 14, 15 at system
D3S1358)
- 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)
- 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.
- 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.
- If the child and alleged father do match,
population statistics are used to derive a paternity
index for that system.
- 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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