What percentage is a positive paternity test?

There are two possible outcomes in a DNA paternity test

1. Exclusion. – The person tested is NOT the biological parent. A paternity exclusion indicates that the tested man is not the true biological father of the child. A report which states a paternity exclusion will show a minimum of two exclusions (i.e. the alleles do not match) at two different genetic markers. When a paternity exclusion is indicated, the probability of paternity is 0%.

2. Inclusion. – The person tested IS ACCEPTED as the biological parent and the report says NOT EXCLUDED. If the report states that the tested man is not excluded as the biological father of the tested child, a combined paternity index (CPI) of greater than 1000 is indicated. A CPI greater than 1000 indicates that the probability of paternity is greater than 99.9%. The CPI number shows the number of individuals in the combined population that would have to be tested who may have a profile the same as the tested person.

How does testing work?

We each have 2 copies of 22 chromosomes in our body. When an embryo is made, one chromosome is passed on from the mother and the other from the father. Then there are the sex chromosomes, either XX for a girl or XY for a boy. This then gives 46 chromosomes in the embryo. To establish paternity we look for markers at specific places on these chromosomes, in the child, and then we look for matching markers in the mother and the alleged father. The markers will have a number which relates to the size of the marker. We look for matches, and call these matching alleles.

If the tested man is not the true biological father of the child, the half of the child’s genetic material which comes from the father will not match the profile of the tested man. If the profile does not match, the tested man is excluded as being the true biological father of the child. This has to happen in at least 2 of 25 markers to exclude the man as the father.

We test 25 markers and then calculate the result to see if the alleged father is excluded or not excluded as being the father. When there is a match in all 25 markers we conclude there is a very high (greater than 99.9999%) chance of the alleged father being the father. We do this by calculating a Probability of Paternity. This is the percentage likelihood that a man with the alleles of the alleged father is the biological parent of the child, as compared to an untested, unrelated man of the same race.

A Combined Paternity Index is also calculated, which is the likelihood that the man tested is the biological father of this child compared to a person chosen at random from a validated population. Paternity is usually accepted if this number is greater than 1000.

If the DNA of the alleged father is consistent (to a degree of mathematical certainty) with that of the child, then the report will conclude that the alleged father cannot be excluded as the biological father of the child. If the DNA is not consistent, it will conclude that the alleged father can be excluded as the biological father of the child. It is never possible to prove 100% that a man is definitely the biological father of a child, as there is always a chance, however remote, that another man in the population may have DNA types which match that child.

Please note that inclusion of paternity does not exclude close blood relatives as being a potential father.

Men who are not the father of the child may match in a few markers due to chance, in the same way that a man and a child can have the same eye or hair colour and not be related. If a tested man does not match in more than one marker, he cannot be the biological father.

A possible mutation was observed in myself/my child, do I need to be concerned? 

Short tandem repeats (STRs), are short tandemly repeated DNA sequences that involve a repetitive unit of 1-6 base pairs. STRs are found throughout the human genome, accounting for about 3% of the entire genome.[1] Most STRs are found in the noncoding regions, while only about 8% locate in the coding regions.[2]

The short tandem repeats (STRs) used in our genetic analysis to determine parentage are in noncoding regions of the DNA. Noncoding DNA does not provide instructions for making proteins. 

STRs are used for paternity testing as they are highly polymorphic (variable) amongst people and can be used to discriminate between unrelated individuals, however, have a higher mutation rate than other regions of DNA, such as the coding regions. In STRs, as with any genetic region, a change (or mutation) can occur in the DNA causing genetic variation. Changes (or mutations) in STRs in noncoding regions are not expected to cause disease. 

When conducting parentage testing the child’s DNA is matched to the alleged parent as the child will inherit one allele from each biological parent. A direct match between the biological parent and child should be found at each locus tested. Where a direct match is not observed, the locus is assessed for a possible mutation. Inherited changes (mutations) from a true biological parent usually show an increase or decrease of one repeat unit. In this case an assumption of a probable mutation between the alleged father and child is made and used in the calculation of the probability of paternity. In such cases a comment will be made on the report regarding the assumption made of a probable mutation.  

Counselling & Support Services

DNA testing can be a highly emotional process for everyone involved. In some cases, people will have a relationship confirmed which can help to eliminate thoughts of uncertainty surrounding relationships. In other cases, the results may show that a person who you believed was related to you is not related to you in a biological way. The results from DNA testing provide conclusive evidence of a biological relationship and may not always reflect the established cultural relationships between you and the persons being tested.

These results can directly impact the persons being tested, but may also affect other family members and friends of the individuals tested. Results may be life-changing and some people may find it difficult to understand how the family structure will change following DNA testing. We strongly recommend that all persons involved in DNA testing seek counselling both prior to testing and after receiving results.

Children involved in DNA testing should have the testing explained to them and why it is being done. Counselling is recommended before testing is done, as it can be important in maintaining an ongoing relationship with the child. Further counselling is also recommended if the results of a paternity test is unexpected, or believed to have had an emotional impact on the child. Counselling by someone outside the family is sometimes easier for the child rather than discussing the issues with the immediate family members.

Paternity cases before the Family Law Court may have counselling options available to them. For cases not before the court, counselling assistance can be accessed at the following sites:

Counselling & Support Services

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