From a sample of DNA, genetic testing companies report on a person's possible links to famous historical figures or groups such as the Vikings. A public guide produced this month by campaign group Sense About Science says DNA cannot be read 'like a book or a map of a journey'.
'Ancestry is complicated and very messy. Genetics is even messier', said Mark Thomas, professor of evolutionary genetics at University College London (UCL), UK and co-author of the guide. 'The idea that we can read our ancestry directly from our genes is absurd'.
Every gene in the human body has its own history. Genetic ancestry companies cannot tease the various possible histories apart without statistical tests. Researchers therefore produce genetic histories on a general, population level and it is not possible to determine an individual's history accurately.
'With every generation you (nearly) double your number of ancestors because every individual has two parents. [...] We don't have to look back very far in time before we each have more ancestors than we have sections of DNA, and this means we have ancestors from whom we have inherited no DNA', states the Sense About Science guide.
Mathematical models suggest the most recent ancestor of all people alive today lived an estimated 3,500 years ago. Professor David Balding, also of UCL and co-author of the guide, said: 'Genetic relatedness isn't very meaningful beyond a handful of generations away'.
Steve Jones, emeritus professor of genetics at UCL who was not involved in writing the guide, added: 'On a long trudge through history [...] very soon everyone runs out of ancestors and has to share them. As a result, almost every Briton is a descendant of Viking hordes, Roman legions, African migrants, Indian Brahmins, or anyone else they fancy'.
Recently, a Hungarian company attracted widespread condemnation for using genetic tests to characterise a politician's 'racial purity' (see BioNews 661).
David Nicholson, director of genetic testing company DNA Worldwide, told the BBC: 'DNA cannot tell you that your ancestors were Viking, simply that your ancestry came from a part of the world common to the Vikings based on historic facts. It's important to talk to the company who provide the testing to make sure your expectations are realistic'.