James D Watson and Francis Crick explained their double helix over two pages of Nature published 70 years ago – 25 April 1953

To DNA or Not to DNA

More and more people are choosing to take a DNA test. This may be prompted by some popular TV programmes. ITV’s DNA Journey and Long Lost Family and BBC’s Family Secrets presented by Stacey Dooley which helps individuals to find their birth families through the use of DNA.

It also helps people to discover whether or not they have inherited a harmful genetic mutation. However, none of the programmes explains which type of test to take and how to interpret the results.

Complex Molecule

DNA (Deoxyribo Nucleic Acid) is a complex molecule found in the nucleus of a cell which contains all the genetic information. These molecules consist of four basic building blocks, known as bases and the sequence of these bases form the genetic instructions. Each human cell contains 23 pairs of DNA molecules, known as chromosomes, 22 autosomes and either an XY pair (male) or an XX pair (female).

There are in excess of three billion base pairs in the human genome but commercial autosomal DNA tests only look at about half a million of them, in the areas where variations are known to occur. The rest of the genome is the same for all humans.

DNA through generations

DNA is passed down through the generations. The more DNA we have in common with another person, the more likely we share a similar, and more recent, ancestry. Long sequences of matching DNA indicate a close relative.

Family historians are not looking at the medical DNA tests, which are usually done from blood samples and look for the specific gene(s) involved. If you are worried about a specific issue you should be approaching your doctor for advice and possible testing.

Genealogy DNA tests fall into three categories:

Mitochondrial DNA which is passed from mother to child and can show deep ancestry your matrilineal line (mother’s mother’s …mother) many thousands of years ago. These tests are seldom used now.

Y-DNA, which is passed from father to son, can in some instances help identify a male ancestor.

Autosomal DNA, which is by far the most common now, and from the testing companies’ databases can help identify cousins who share DNA with the tester.

It is usually only effective back to about five generations.

To Take a DNA test…

When you take a DNA test you either spit into a test tube or rub a tiny brush around the insides of your cheek and place that into a test tube, seal it up, complete the paperwork, put it into the post and wait for your results, usually about six weeks, in the form of an email link to your personal test results.

The results are in two parts, the ethnicity admixture and the list of people who match you in varying amounts, the more DNA they share with you the more closely you are related.

The ethnicity part of a DNA test is generally thought of as being the least reliable part of the results although often it is the part that people are most interested in. It is accurate to the continental level but less so at country level.

All the companies rely on control panels of people who have reported that all of their grandparents/great grandparents were born in the same small locality and so are thus thought to be typical of that area. The bigger the panel the better the results but it’s still a new science and the ethnicity admixtures are updated (i.e. changed) from time to time.

Find a shared match

The list of shared matches is usually presented in order of the amount shared DNA which is measured in centiMorgans or cM. The higher the number of cMs you share with someone, the closer your relationship is to them. The more people in the testing company’s database the more matches you will have.

Choosing a test company for an autosomal DNA test.

I have no affiliation to any of the testing companies, but it makes sense to test with the companies which have the biggest databases of testers, both for accuracy of ethnicity admixture and for more cousin matches.

At the time of writing the statistics are:



My Heritage



Living DNA


*Database and comparison tools only.

If you are trying to identify unknown birth parentage it is especially important to get yourself into all the databases because the person you might be most interested in matching might only be in one.

Ancestry DNA and 23&Me require a test to be processed by their own laboratories, the others allow uploads from other company’s labs, sometimes free, sometimes for a small fee to unlock the ‘tools’, although that will be less than taking a full test with them.

For this reason alone the usual advice is to start with an AncestryDNA test, upload to My Heritage, Family Tree DNA, Geneanet and GedMatch and then to test with 23&Me.

Adapted from an article by

Brenda Paternoster