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When you receive your DNA paternity profiling test report, a conclusion will provide one of two statements: 1) This result provides (extremely/moderately) strong support in favour of paternity, or 2) This result excludes the alleged father as the biological father.

These conclusions are reached by AlphaBiolabs examining 25 short tandem repeat (STR) markers in the DNA profiles of the tested individuals. Each individual has two copies of each STR marker, known as alleles: one is inherited from the father and the other from the mother. These alleles at each STR marker are compared between the tested individuals. There will need to be a match with all STR markers tested for an inclusion of paternity.

The paternity index (PI) is a way of measuring the strength of a particular match based on the relative uniqueness of the allele observed. It is a statistical calculation that compares the DNA profiles obtained to an untested random individual within the general population. For example, if most people have the same allele at a particular location, the PI might be lower. If two tested individuals match at an allele, it could indeed be because of a parent–child relationship, but it could also be because most people match with this allele at this location. Conversely, if very few people have an allele at this location it’s rarer and the PI value would thus increase.

The combined paternity index (CPI) is a calculation (the product of all paternity indices) that helps formulate the probability of paternity. If the CPI is greater than 1000, there is a match between the alleged father and child (support in favour of paternity). A CPI value greater than 1000 means that the probability of paternity is over 99%. If the CPI is zero, it is a non-match between the alleged father and child and means that the alleged father is excluded as being the biological father of the child.

The probability of paternity is calculated from the CPI and indicates the chance of paternity. In other words, it is the statistical probability of the alleged father being the child’s biological father and not any other man from the same ethnic group who may share a similar DNA profile by random chance. When the probability of paternity is 99.99% for example, this means that the alleged father is 99.99% more likely to be the biological father of the child than another man chosen at random from the same ethnic group.

More information can be found in a longer article on our Learning Centre.

If you have any queries or need further information on any of our DNA testing services, please call Customer Services on 0333 600 1300 (+44 1925 255 450) or email