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Interpreting 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.

To test a possible parentage relationship, up to 23 highly polymorphic (variable) genetic markers are analysed. For each one of these markers, a child will receive two alleles. One allele is inherited from the biological mother and the other from the biological father.

Paternity 'NOT EXCLUDED'

When a putative 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., In cases where both the mother and putative father are tested, our paternity testing usually results in a probability greater than or equal to 99.9%. The probability of paternity is also expressed as the Paternity Index (PI). The PI indicates how many times more likely the tested man is the source of the shared alleles 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'

A putative father is excluded from being the biological father of a child when at least three sites tested do not match, ie. when the child has an allele which can be matched with the known mother but the remaining allele does not match to the putative father. A man tested with multiple mismatches to the child is excluded as being the biological father.

How do I interpret my DNA test results?

In the examples below, we have reviewed alleles at a particular section of DNA, for the mother, father and child, represented as numbers.

Case 1

In Case 1, the child has a 15 and an 18. The mother has a 14 and a 15; she has therefore passed on a copy of her 15 to the child. The father of this child must have passed on a copy of 18. Father A has a copy of 18 (and 21), and therefore is a match. If this situation occurs for all analysed markers Father A is NOT EXCLUDED from being the 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 pass on an 18, as the child and the mother both share a 15 allele at this marker. Father B has a 13 and a 21 - this means Father B does not have a copy of 18, and therefore is a mismatch at this marker. This is not consistent with inheritance from father to child. If this were to occur for multiple markers (3 or more) he is not consistent with being the father of this child, and Father B is therefore 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 allele at this marker - a 15. This does not mean the child is missing an allele. The child most likely has two copies of 15 - one from the mother and one from the father. As Father C also has a 15 allele we would indicate this is consistent with a match between child and putative father for this marker. If all markers matched between child B and Father C this indicates that 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