What DNA testing can tell you about your family history

By Rosemary Collins, 24 April 2017 - 2:57pm

For International DNA Day, Debbie Kennett, author of DNA and Social Networking, explains why DNA testing is increasingly important in family history research.

Chromosome mapping can be used to reconstruct the genomes of your ancestors
Chromosome mapping can be used to reconstruct the genomes of your ancestors (Credit: Getty)

DNA testing is a very important tool for the family historian. It can help to verify your family tree and provide helpful clues to inform the future direction of your research. It can also sometimes help to break down those long-standing brick walls.

DNA has the power to solve previously impossible cases. Foundlings, adoptees and donor-conceived individuals now have a very real chance of finding half-siblings and other close relations in the databases, which can lead to the identification of their biological parents.

If you have a match with a second cousin, it means that you share the same great grandparents. It’s then just a question of tracing the descendants to identify a suitable candidate who was in the right place at the right time.

Success stories are reported on a daily basis in America and we’re now starting to get reports from the UK as well. As the databases grow, we can expect to see many more unknown parentage cases solved.

DNA discoveries

As with traditional genealogical research, DNA testing can provide surprises so be prepared for the unexpected. You might uncover family secrets by matching with a cousin or a half-sibling that you didn’t know existed. Conversely a relation who is expected to share DNA with you might turn out not to be a genetic relative at all. In rare cases, people discover that their parents are not their biological parents. For a good overview of the ethical implications of DNA testing see the Genetic Genealogy Standards.

However, DNA testing is not a magic bullet and it won’t provide you with an instant family tree. Contrary to popular belief, you cannot take a DNA test to discover who you are and where you come from. The value of the test lies in the comparison process, so it’s important to test with a company that has a matching database.

DNA is best used in combination with genealogical records in order to form conclusions about relationships. The test itself is completely harmless; you just need to provide a cheek swab or a saliva sample.

The choice of test will depend on the questions that you want to answer, or you can just take a test for fun to see who you match in the databases. A DNA test can be considered as an investment and the value of the test will grow as more people join the databases and you get more matches.

There are three different types of test – autosomal DNA, Y-chromosome DNA and mitochondrial DNA – all of which have specific applications. Sometimes a combination of different tests will be required to solve a particular problem. 

A scientist examines a DNA sequence using a magnifying glass
A scientist examines a DNA sequence using a magnifying glass (Credit: Getty)

Case study

In my research into my Cruwys ancestors in Devon, I hit a brick wall trying to find William George Cruwys (born 1821), the brother of my great great grandfather, Thomas Cruwys (born 1831). William disappeared from English records after the 1841 census. I found a William of the right age in Prince Edward Island, Canada, but couldn’t find any records to confirm a link, though naming patterns provided a strong clue.

Frustratingly, the 1848 marriage certificate I obtained didn’t include the parents’ names. Y-DNA tests on my dad and a descendant of the Prince Edward family showed that the two lines were related though Y-DNA cannot pinpoint the date when two people share an ancestor.

However, a year later an autosomal DNA match popped up in the Family Tree DNA Family Finder database with a cousin in Canada. His ancestors were from Prince Edward Island, and he was the great great grandson of William Cruwys through a female line.

If our family trees were correct, he would be my dad’s third cousin once removed. The amount of DNA we shared was within the expected range for the presumed relationship, thus providing confirmation that the tree was correct.

The chromosome browser shows a comparison between my dad and his third cousin once removed. The three orange shapes on chromosomes 1, 3 and 11 are the segments of DNA that they share in common through descent from their mutual ancestors William Cruwys senior (1793-1846) and Margaret Eastmond (1792-1874), the parents of Thomas and William George.


Use a chromosome browser to identify shared DNA
Use a chromosome browser to identify shared DNA (Credit: Family Tree DNA)


DNA inherited on the autosomes: the 22 chromosomes which are not sex chromosomes. Inherited from both parents.

A structure in a cell’s nucleus containing genetic material. Humans have 23 pairs of chromosomes: 22 autosome pairs and one pair of sex-chromosomes. Females have two X chromosomes. Males have an X and a Y chromosome.

A group of people who descend from the same branch of the human family tree.

DNA found in mitochondria – the power houses in our cells. mtDNA, passed from mother to child, can trace the maternal line.

A repeating pattern in a DNA sequence. The number of repeats at a specific location is reported as a marker value. Standard Y-STR tests report results for 37 markers.

Single base pair mutation in a DNA sequence. 

Y chromosome DNA is passed from father to son, and is used to trace the paternal line.

For more information, see the full International Society of Genetic Genealogy glossary here.

The full version of this article, including a comparison of the five major genealogy DNA tests, is available in this month's edition of Who Do You Think You Are? Magazine.



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