When carrying out a paternity test the DNA of the mother, the child and the possible father are compared using DNA markers called STRs (please see ‘How does DNA testing work‘ for more information).
Each STR marker identifies up to two ‘results’ in each person’s DNA, one inherited from their mother, and one from their father. If the same sized STR result is inherited from both parents, the individual appears to have only one result for that STR test. We carry out 24 tests in each case and each person’s DNA profile is made up of the results from a minimum of 22 separate STR tests.
To carry out a paternity test we examine all the STR results in the child’s DNA profile and check that they are all matched by results in the mother’s and the father’s DNA profiles.
If the mother is not tested, we can still compare the child’s profile with the DNA profile of the alleged father to see if there is a matching result for every test, but without the mother’s DNA we have to make some assumptions in the analysis which reduce the statistical certainty of the conclusion.