Genealogy tracking has become big business, with many companies charging up to $300 to trace your DNA to specific historical figures or ethnic groups in the distant past by analyzing ancestry tests.
A group of scientists now offers a public warning that these ancestry tests have little scientific backing, and are often so unreliable and inaccurate that they amount to "genetic astrology."
Though advertisements for some ancestry testing companies give the impression that your unique DNA genealogy can tell you a specific story about your ancestry, the scientists say that the same history you get could be given to thousands of other people with a similar ethnic background, and that any number of different possible interpretations could come from your DNA results.
Professors David Balding and Mark Thomas of the University College London warn in a public statement from the Sense About Science campaign group that "you cannot look at DNA and read it like a book or a map of a journey" without supporting historical evidence.
Your DNA contains an enormous amount of genetic information, but most of the information that can be gleaned from it is about the genetic history of whole population - not of individual family trees.
DNA is an assortment of genetic sequences that have been inherited from many different ancestors. You double your number of ancestors with every generation, because everyone has two parents. Going back only ten generations (between 200 and 300 years) in your genealogy, you have 1024 ancestors.
Going back far enough, each of us has more ancestors than we have sections of DNA - which means that there are many ancestors from whom we have inherited no DNA, and that ultimately there will be many sequences of DNA that most people share.
"On a long trudge through history - two parents, four great-grandparents, and so on - very soon everyone runs out of ancestors and has to share them," said Steve Jones, Emeritus Professor of Human Genetics at UCL.
If an ancestry test tells you that you are genetically related to someone who lived more than several hundred years ago, it may be true about your genealogy, but not in a meaningful way.
There are several types of ancestry test, all of which use DNA samples taken from easily accessible parts of the body, like a saliva or cheek swab sample.
The individual's genetic data is compared to the DNA of people for whom there is specific information about ethnicity and geographic location, and different ancestry tests look at different types of DNA:
- Y chromosome DNA, which is inherited along the male line and only found in men
- Mitochondrial DNA, which is inherited along the female line and found in both men and women
- Autosomal DNA, which can come from any ancestor and makes up 98% of your DNA
Each of us has just only one ancestral lineage for mtDNA, and each man has just one for Y chromosome DNA; each type of DNA is passed down through generations as an individual unit. Autosomal DNA, however, is made up of thousands of sections of DNA, each with its own history.
Since all humans share the large majority of our DNA through far removed common ancestors, each of us has very little DNA that is directly inherited from a specific ancestor - even one who lived only a few generations ago.
The more steps you take up a family tree, the more negligible the DNA connection gets compared to the enormous amount we all share, and the less an ancestry test can reliably reveal.
Even genetic connections to historical ethnic groups like "Viking" or "Zulu" are vague. People's genetics do not reflect specific groups, since the high degree of genetic mixing over centuries means that even cultures with strong cultural boundaries do not have noticeable genetic differences.
People descended from more isolated populations, like the Scottish Highlands in the United Kingdom, have minor genetic differences from the general surrounding population, but they are not significant enough to identify a "Scottish gene" in an individual's genealogy.
"As a result, almost every Briton is a descendant of Viking hordes, Roman legions, African migrants, Indian Brahmins, or anyone else they fancy," said Jones.
If an ancestry test finds a connection between a particular sequence of your DNA and a specific, isolated tribal group, the only thing that can be concluded is a mere possibility that some of your ancestors were in that group.
Human history involves an incredible amount of migration, and because each of your genes has its own ancestral history, there are thousands of possible versions of your genealogy.
So is there any reliable information about your genealogy in the distant past that can come from DNA ancestry companies?
To answer a specific question about individual ancestry, you need to supplement your mtDNA or Y chromosome genetic information with reliable historical records.
It's possible, for example, for two men who find historical records indicating a common male-line ancestor 400 years ago to have their Y chromosome DNA analyzed to test their genealogy.
Some ancestry testing companies can do this reliably.
"With advanced testing you can provide a general ancestry indication i.e. Northern European, Western Africa or Middle Eastern and in some cases even more specific," said David Nicholson, director of the DNA Worldwide group to BBC News.
"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".
In general, however, DNA genealogy is far more useful for population geneticists who are trying to learn about past human migrations than it is for individuals trying to learn their specific relation to Genghis Khan.
By analyzing the DNA variation among many individuals from different regions, scientists can test possible population history models and calculate how likely they are to explain specific DNA patterns. This can tell us about populations of people, but not much about individual genealogy.
"Genetics researchers are telling us that you are better off digging around in your loft than doing a DNA ancestry test if you want to find out about your family tree," said Tracey Brown, Director of Sense About Science to the Telegraph.
[EDIT] The original version of this article was entitled "DNA Ancestry Tests Are Meaningless for Your Genealogy Search,"which was inaccurate. The source material does not question the usefulness of DNA testing for questions about immediate biological relations, like paternity tests or adoptees looking for their biological families; its criticism was limited to DNA ancestry tests that claim to answer specific questions about ancestors in the distant past without supporting evidence from historical documents.