PATERNITY INCLUSION
When half of the DNA of non-maternal origin matches half of the putative father's DNA (see figure 1), a second phase begins.
We now need to check whether the observed match occurred by chance, or it is actually due to a biological paternity relationship.
A statistical calculation is therefore made on the basis of the frequency with which the DNA fragments shared by the child and the putative father appear in the reference population (Italian, French, German, etc., or Caucasian, Asian, African…). The final figure obtained is the Paternity Index (PI) which is a way of measuring the strength of the match between putative father and child: the higher the value, the stronger the match. This value can be converted into a probability which is known as "Probability of Paternity (W – Wahrscheinlichkeit, from the German term that means Probability). For example, a probability of paternity of 99% reflects a 99% probability that the hypothesis of paternity is correct and 1% that it is not.
A cumulative result of 100% probability is impossible, but highly significant values of 99,99% or more can be obtained. 100% mathematical certainty is not required to establish paternity and it would not be possible. Italian jurisprudence deems paternity to be "practically proven" when the probability value exceeds 99,8%.
This figure is not the same in all countries. Minimum Probability of Paternity value requirements differ between States in the USA, ranging from 99,0% to 99,5% up to much higher values. Nowadays, in non-complex cases, values of 99,99% or even much higher are normally obtained.
Sometimes, in particular cases, such as when the genetic profile of a deceased putative father has to be reconstructed or when unusual sample-types are involved (teeth, paraffin-embedded tissues, bones, etc.), the Probability of Paternity can be lower compared to cases in which fresh material is used.
A high value of probability depends on the number of genetic loci that can be analysed, and this depends on the state of preservation of the DNA: degraded or limited DNA reduces the chances of a successful outcome, since probability values are lower.
PATERNITY EXCLUSION
When there is no match between the putative father and the child, the Paternity Index is 0,00, the
Probability of Paternity is 0,00% and the alleged father is excluded from being the biological father of the child
(see figure 2).
If there is only a single difference between the DNA profile analysed in the child and that of the putative father, this may be due to a phenomenon called genetic mutation. This is a fairly rare event; current genetic indicators suggest an average probability of 3 out of 1.000.
The likelihood of 2 DNA mutations in the same pairing of putative father-child is calculated, with current systems, at 3/1.000 x 3/1.000 = c. 1/100.000.
The likelihood that 3 genetic inconsistencies are due to 3 different mutations is therefore calculated at c. 1/37.000.000. If 3 or more inconsistencies are observed, therefore, paternity is definitely excluded.
"99.99% (or more) or exclusion.
This is the result of the paternity test""
