Fears and suspicions about paternity are not a modern phenomenon. But while men previously had only to endure the nagging concern that their children looked a bit like the milkman, DNA testing is now making hints about false paternity obsolete.

Most DNA-testing companies claim a probability of less than 1 in 100,000 of wrongly matching two unrelated individuals. This figure is probably conservative.

Paternity testing involves comparing the DNA of a child with that of the possible father. Everyone inherits two sets of genetic material — one from each parent — and the two complementary sets are present in almost all cells in the form of long strands of DNA. A blood sample, or a skin sample from the inside of the cheek, is usually taken to obtain DNA from the mother, possible father and child. DNA takes the form of a twisted ladder, with each of the “rungs” representing a single letter in our genetic code. The ladder taken in its entirety is known as the genome. The majority of the genome is the same for everyone, which means that comparing a section of DNA at random would be useless for trying to confirm fatherhood.

However, there are specific sites where the genetic code varies widely from person to person. In particular there are areas where short sequences are repeated over and over again.

The repeated unit will be the same for everyone, but the number of times it is repeated differs. By cutting the DNA at precisely these points and comparing the number of repeats, a person’s DNA can be matched to those of their parents.

Comparisons are usually made at more than ten sites to minimise the chance that a mutation at one of the sites leads to “mismatch” verdict.

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