Usually, the key to a family’s past lies on paper. Genealogy enthusiasts will scan reams of records of births, deaths and marriages to establish their roots— or in some cases, to see if they can identify famous or infamous ancestors.

 But in recent years, the key to your ancestors has widened beyond dusty records and old photographs. DNA — once the province of crime scenes and paternity cases — is now being used to match relatives and establish ancestry. A simple swab test is all that is needed to unlock potential family history from your genes.

Some tests establish ethnicity, while others trace the path of the maternal or paternal line over millennia. Others zone in on a shorter historical timespan to find distant relatives of both sexes. 

But how useful are these DNA tests to your own family research? 

Matrilineal and Patrilineal Descent: Y Chromosome and mtDNA tests.

To discover maternal and paternal lines, specific chromosomes need to be considered: mtDNA and Y-chromosomes. Amounting to only 2% of our total DNA, they are passed from parent to child, with Y-chromosomes inherited exclusively by males from their fathers, while mtDNA is passed onto children of both sexes by their mothers but only perpetuated by girls.

MtDNA and Y chromosomes are, according to Britain’s DNA website, “the most informative means we have for learning about our ancient lineages.” Dr Jim Wilson of the University of Edinburgh Centre for Population Health Studies, a contributor to the site, explains that these allosomes or sex chromosomes are the most useful DNA data for genealogy purposes because their codes are not “mixed up” like other genetic material.

This is because the genetic codes of allosomes stay more or less the same over the generations. Y-chromosomes mutate every 500 generations, while mtDNA mutates at a rate of .0043 per generation. So, it is theoretically possible to establish relationships between individuals centuries apart — as well as the movement of ancestors over the years.

 “If males share surnames and have preliminary family history evidence for a past relationship, “said Professor Mark Jobling, of the University of Leicester’s Genetics department, in an interview with the author, “it might be a good idea to have a Y DNA test, which could either reject the possible relationship or provide support for it.” The same criterion applies to mtDNA.

Autosomal DNA

What about the rest of our DNA, the non-sexually dependent chromosomes or autosomes? They are not to be disregarded in the hunt for our pasts. Indeed, they widen the search as they reflect genetic inheritance from both parents.  

“Autosomal DNA gets divided up every generation-you have half from your mum and half from your dad, and a quarter from each grandparent and so on “, explained Professor Jobling.  

Professor Jobling also explained how autosomal DNA could establish ethnicity. “If, for example, someone had a hypothesis that one of their great grandparents might have been Chinese, this ought to be fairly easy to confirm or reject using a whole-genome test.”

 Different tests make different claims. National Geographical’s Genographic Program autosomal test establishes migration patterns of ancestors thousands of years ago. It also claims to establish whether an individual has Neanderthal or Denisovan ancestry. Ancestry.com’s test claims the potential to link searchers with ancestors and ethnicity as far back as a thousand years ago.  

Autosomal tests can also identify distant cousins previously unknown to you. “A ‘genome-wide’ autosomal DNA test that analysis many (up to approx. 1 million) SNP (define) variants across the genome can provide useful support for possible relationships at the level of first and possibly second cousins.” said Professor Jobling,” 

This means, in theory, you could connect with previously unknown branches of your family tree.   

DNA versus paper Records.  

But because of the random nature of the transmission of autosomal DNA, its genetic coding can vary between individuals of the same family and across time. Siblings may not even share the same autosomal DNA, with some genetically closer than others. And the further back you go, the smaller the percentage of autosomal DNA you inherit from each ancestor. You may even find you retain none of the genes of certain ancestors at all — even though you can establish a relationship through the records.

For this very reason, many experts do not believe commercial DNA tests are of any use to family history. “You are better off digging around in your loft than doing a DNA test if you want to find out about your family tree,” concluded Tracey Brown, Director of “Sense about Science“  in an article by The Telegraph.

  “Such histories are either so general as to be personally meaningless or they are just speculation from thin evidence,” said Steve Jones, Emeritus Professor of Genetics at UCL in the same piece. “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.” 

Professor Jobling is less dismissive of the value of DNA tests but does offer notes of caution — particularly about the value of evidence from the different chromosomes:

“Y DNA tests can now be extremely discriminating among different Y types, “said Professor Jobling, “but mtDNA will never be so discriminating, so there may be more ambiguity about the significance of an mtDNA match between individuals.

 As for autosomal DNA, it’s only really useful for establishing close relationships. Here, Professor Jobling agrees with Professor Jones —especially once relationships move beyond second cousins. “It gets more difficult to discern significant relationships over the background ‘noise’ of the general distant relationships and DNA sharing among all of us,” he explained.

DNA tests: Are they useful Genealogical Tools?

The value of DNA tests to genealogy is by no means clear-cut. On the one hand, a test can give a broad picture of ethnicity and lines of descent. But on its own and without any backup on paper, it seems tests prove very little.  

But the possibility of identifying previously unknown genetically related individuals is intriguing, especially if they can illuminate areas of your family tree you were once in the dark about. 

Perhaps the only way to judge the advantages of a DNA ancestry test is to try one out…..

Resources

The Mutation Rate in the Human mtDNA Control Region