DNA testing: a brief introduction

DNA testing has grown immensely in popularity in recent years, not least as a result of TV programmes such as Long Lost Family. But how can DNA testing help your family history research and are there any draw backs?

First of all, you need to know why you are testing. Are you testing to find out answers about your ethnicity? Are you testing to find living relatives? Are you testing to try and break down a brick wall in your family history research? Are you testing to find out more about your health and wellbeing? DNA testing can help with all these questions and answering them first will determine the best person/people to take the test, what test to use and which company to test with.

As a genealogist and family historian my main focus for the use of DNA testing is to help you breakdown those family history brick walls and trace your family history further and/or help you to find living relatives. A word of caution must now follow: DNA never lies and can uncover deep rooted family secrets which not all family members may wish to know and may find painful, shameful and humiliating. Be mindful of other people’s feeling if and when sharing surprising information.

But also remember it is just another tool in the family history research toolbox which needs to be used alongside traditional paper record research, and just like paper records, the answers may not be obvious straight away. The number of DNA matches is dependent on other people in the database who have tested, those databases are growing all the time so if a match is not immediately made, keep checking as over time it is increasingly likely there will be one. However, it is also possible because of the way DNA is inherited that a person may test and have no genetic match but still be connected genealogically.

How is DNA inherited and what test should I take?

I am not a scientist and therefore will keep this simple. There are three types of DNA for which tests can be taken:

Y-DNA – Y-DNA passes from father to son largely unchanged and can therefore identify a male line; useful for surname studies, identifying an unknown father and your ancient origins.

mtDNA – mtDNA passes from a mother to her children (sons and daughters) largely unchanged and can therefore identify a maternal line; whilst the test can be taken by both male and females, only females pass it on unchanged.

Autosomal DNA – this is passed from parent to child in varying combinations; this is the most popular DNA test as it traces both maternal and paternal lines and identifies close matches and common ancestors within the first five to seven generations. These are the type of DNA tests I work with on behalf of clients in assisting them with the family history research. The rest of this blog therefore will concentrate on Autosomal DNA.

How is Autosomal DNA inherited?

DNA is measured or expressed in centimorgans (cM) or a percentage of shared DNA. The average or expected and range of shared DNA inherited between generations is shown in this chart:

Knowing how DNA is inherited will help you decide who is/are the best person/people to take a DNA test to solve my query?

Generally speaking, the best person/people to take the test is/are the oldest living members of that family line as they will share a higher cM or % of DNA that you, but of course this depends on their willingness and ability to provide the necessary sample, bearing in mind my earlier caution with regard to deep rooted family secrets.

How do I take a DNA test?

All the commercial DNA tests available require a saliva sample. There are a number of companies with which DNA testing can be undertaken depending on the type of DNA test you require; the mains ones however are:

  • Ancestry – the largest DNA database; tests are limited to Autosomal DNA and provides cousin matching
  • My Heritage – limited to Autosomal DNA and provides cousin matching
  • Family Tree DNA – Autosomal DNA – Family Finder provides cousin matching; Separate Y-DNA and mtDNA tests
  • Living DNA – Autosomal DNA provides cousin matching; Includes Y-DNA and mtDNA haplogroup testing
  • 23 and me – Autosomal DNA – Family Finder provides cousin matching; Includes Y-DNA and mtDNA haplogroup testing

*haplogroup testing looks at the ancient origins of either the paternal (Y-DNA) or maternal (X-DNA) lines.

Tests are purchased through the various companies websites, testing kits are sent out through the post to you, you then provide the same as per the instructions provided and return the sample in the box provided. The results are usually available in about six weeks and you will receive a notification and details of how to access them. Prices of tests vary between companies with most companies running special offers at certain times of the year which are always worth watching out for.

It is also worth noting that if you test with one company, your raw DNA data can be uploaded to the others, except for Ancestry which does not have that function. My personal preference therefore is to test with Ancestry and then upload the raw data to the other sites to get the most coverage of the DNA databases available. Most site will allow you to do this without require you to subscribe to the site. Note however that if uploading raw DNA data to Family Tree DNA, Living DNA and 23 and me will not provide haplogroup data. For that data testing must be carried out with those companies.

I have my DNA results, now what?

Interpreting your results can be confusing. The range of cM and/or % shared with a cousin match will provide a clue as to how you are related however, as will be noted from the chart above, this is within a range and therefore traditional paper research should be used to confirm the exact relationship.

For example, one of my cousin matches on the Ancestry matches page states they are a 2nd to 3rd cousin sharing 328 cM or 5% DNA. Looking at the chart above, this amount of shared DNA means they could in fact be any relationship in groups D and E. A good pictorial of possible relationships for any particular match which I and many of my colleagues us is the Shared cM Project 4.0 tool v4 which is available to use for free at https://dnapainter.com/tools/sharedcmv4.

When looking at DNA cousin matches, bear in mind it may not be the person whose account a cousin match is linked to may not be the person who has tested but an older generation of their family. The amount of shared DNA and the connection to you share depends on the generation the person testing belongs to in relation to you, i.e., they may be the same generation as you, your parent or grandparents and this is turn can depend on the age their respective parents were when they were born.

For example, when I tested by great aunt a cousin match contacted me believing I was her 3rd cousin 2x removed however of course the paper research did not match this and confused her. When I pointed out that the test was not mine but two generations earlier, she found the link. My great aunt of course was her 3rd cousin 2x removed whilst I was her 5th cousin. This was confirmed after taking my own DNA test which identified the same person between a 4th to 6th cousin match sharing 27 cM or <1% DNA.

How to work with cousin match:

  • Be methodical
  • The GOAL is to identify where the match fits into your family tree
  • Prioritise matches by size, place, common ancestors, where they fit in your family tree
  • Use the size of the match to work out the most likely relationship(s) to you – use https://dnapainter.com/tools/sharedcmv4 to help with this
  • Compare family trees if the match has a public tree
  • For any cousin match look for other matches you share this may narrow down the possible relationships to you
  • Use the tools the website you have tested with have, i.e., if you have tested with Ancestry use their ThruLines feature
  • You may need to expand your own family tree to include siblings of earlier generations to find the common ancestor
  • Build ‘dirty trees’ – a bare bones “skeleton” tree to work out direct ancestors which does not worry about source citations etc but is accurate enough to be a rough working tree to check a family connection
  • Test other relatives, they will share different segments of DNA


  • Note all cousins, particularly the more distant they get, will share DNA with you
  • Do not despair if there are no or few matches to start with the databases are growing daily
  • The bigger the amount of shared DNA the more confident you can be there will be a common ancestor
  • The smaller the match the further back in time you will need to go to find that common ancestor
  • Small matches (especially those under 15cM) can be false matches!
  • DNA alone will not provide the answers – traditional paper research will also be required!

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