Interpreting test results

To establish paternity, up to 23 highly polymorphic (variable) genetic markers are tested.

For each one of these markers, a child will receive two different copies (alleles). One allele is inherited from the biological mother and the other from the biological father.

Paternity 'NOT EXCLUDED'

When a father matches at the markers tested, a probability of paternity is calculated that is determined by the frequency of each matched allele within the population. Our paternity testing usually results in a probability greater than or equal to 99.9%, in cases where both the mother and father are tested. The probability of paternity is also expressed as the Paternity Index (PI). The PI indicates how many times more likely the tested man will match at all markers compared to a person chosen at random from the general population. For example, if the PI is 1000, the tested man is 1000 times more likely to be the biological father than any man chosen at random.

Paternity 'EXCLUDED'

The putative father is excluded from being the biological father of the child when at least two or more of the sites tested do not match, ie. if the child has alleles which cannot be matched to either the mother or the putative father. In this case the probability of paternity would be 0% and the Paternity Index = 0. A man tested with mismatches to the child is excluded as being the biological father.

How do I interpret my DNA test results?

Genetic factors are inherited equally from both parents. This genetic information is contained in every cell in the human body. The purpose of paternity testing is to compare the genetic profiles of individuals to one another, to establish the biological (genetic) relationship.

In the examples below, we have studied the numbers at a particular section of DNA, and worked out which numbers the mother, father and child have. 

Case 1

In Case 1, the child has a 15 and an 18. The mother has a 14 and a 15 - she has therefore given a copy of her 15 to the child. The father of this child must have given him / her a copy of 18. Father A has a copy of 18 (and a 21), and therefore is a match. If this situation occurs at the other 14 sections on the DNA, we say that Father A is NOT EXCLUDED from being the father of this child, i.e. that Father A IS the biological father of this child. 

  DNA Section
Mother 14 15
Child-A 15 18
Father-A 18 21

Case 2

In Case 2, the father must give an 18 as the child and the mother share a 15. Father B has a 13 and a 21 - that is Father B does not have a copy of 18, and therefore is a mismatch. Simplistically, he cannot be the father of this child, and we say Father B is EXCLUDED from being the biological father.

  DNA Section
Mother 14 15
Child-A 15 18
Father-B 13 21

Case 3

In Case 3, the child has only one number - a 15. It would seem the child has a number missing. In actual fact, the child has two copies of 15 - one from the mother and one from the father. The child is really a 15,15 (we call this homozygous), and we say this is a match and therefore Father C is NOT EXCLUDED from being the biological father of this child. 

  DNA Section
Mother 14 15
Child-B 15  
Father-C 15 16