Statitics

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.

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.

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 DNAtest. Accuracy has more to do with the quality measures that are taken to ensure minimizing errors.