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Paternity Statistics
The DNA test we use
examines 16 different genetic locations. At 15 of the 16 locations,
an individual will have two numbers (one from mom and the other from
dad). The DNA test is designed to yield maximum statistical power
and exceeds the DNA tests used by most law enforcement labs.
Comparison of
Profiles
The DNA profiles of
parents and their children are compared. At each genetic system
tested, the alleged father should share a number (allele) in common
with the child to be considered the true biological father. The same
holds true for the mother.
When you try to
compare extended family, such as cousins or even two siblings, the
same rules do not apply. For instance, two full siblings that share
the same parents could be 10,12 and 11,13 at one genetic location.
See Science of
Paternity for illustrations.
Likelihood of a
Match
When an alleged
father shares a number in common with the child at all 16 genetic
systems, a calculation is made that is known as the paternity index.
This index is the likelihood ratio that indicates whether the
evidence fits better with the hypothesis that the man is the father
or with the hypothesis that someone else is the father. When each
genetic system is considered together, the result can be a very
large number (combined paternity index). The larger the value, the
more strongly the genetic evidence supports the "is the father"
hypothesis. If the tested man does not share a number in common at 3
or more genetic systems, he is excluded as the father.
Probability of
Paternity
This number is what
you typically associate with paternity on TV. When an alleged
father shares a number in common with the child at all 15 genetic
systems, the probability of paternity is typically greater than
99.99%. Less frequently, the percentage may be less than 99.90%,
but in either instance the percentage is dependent on the alleles
(numbers) that the man and child share in common, the genotype (two
numbers) that they have at each genetic system, and the frequency of
the alleles in the population. It may seem complicated but, in no
way does this number represent the accuracy of the DNA test.
Accuracy has more to do with the quality measures that are taken to
ensure minimizing errors.
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