What is the difference between autosomal, Y-DNA and mitochondrial DNA tests?

Debbie Kennett explains how DNA testing can help with your family history research and the difference between autosomal, Y-DNA and mtDNA tests

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DNA testing has taken off in a big way in the past two years.

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Somewhere in the region of 26 million people have now been tested, with thousands added to the databases every day.

Even if you haven’t yet taken the plunge, you will find that many of your relatives are already in the databases.

It’s also worth keeping an eye out for special offers that will help you save money on your test.

However, a DNA test on its own is not very informative. It cannot tell you who you are or where you came from.

DNA testing is best used as a tool for your family history research in combination with documentary records. The power of genetic genealogy lies in the comparison process. You need to work with your matches in the databases, and compare family trees in order to identify a common ancestor.

DNA testing is also very useful for verifying your family history research. It can be very satisfying to have DNA evidence as additional confirmation of relationships.

However, you need to be prepared for a few surprises along the way. For this reason, when testing your relatives it’s important to ensure that you gain their consent and warn about unexpected results. The Genetic Genealogy Standards Committee provide some useful guidance that you can download as a free PDF.

There are three different types of test – autosomal DNA, Y-chromosome DNA and mitochondrial DNA – all of which have specific applications. The choices of test will depend on the questions that you want to answer.

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A grandmother, mother and daughter in 1940s Devon (Credit: Keystone/Getty Images)

Autosomal DNA testing

The autosomal DNA test is now the most popular of the three tests and tests the autosomes from both parents.

These are the chromosomes other than the two sex chromosomes (X and Y) that contain most of our DNA sequences and genes.

We receive one set of 22 autosomes from our mother and one set of 22 autosomes from our father.

Before the DNA is passed on it is shuffled up in a process known as recombination.

The DNA we inherit from our parents is therefore a patchwork of DNA from all four of our grandparents.
Y-DNA

Y-chromosome DNA (Y-DNA) is passed on by a father to his sons. The transmission usually follows the inheritance of surnames.

This test can therefore only be taken by males. Females will need to find a suitable candidate for testing such as their father, brother, uncle or cousin.

For genealogical purposes markers known as short tandem repeats (STRs) are tested. These are repeating sequences of DNA letters.

The number of repeats is added up, and for each marker you are given a number. Then your numbers are compared with the other people in the testing company’s database.

You are given a list of the Y-DNA matches whose numbers most closely match yours. The 37-marker test is the standard entry-level test, but tests are also available with 67 and 111 markers.

Y-DNA testing can be used to verify existing research, to explore which different surname lineages are related and to investigate variant spellings.

Y-DNA test results are best co-ordinated within a structured surname project, where the results can be clustered into ‘genetic families’.

Mitochondrial DNA

Mitochondrial DNA (mtDNA) is passed on by a mother to both her sons and daughters. Only females can pass on their mtDNA to the next generation.

An mtDNA test can therefore be taken by both males and females, and explores the direct maternal line, or ‘matriline’.

However, it is not so easy to use mtDNA testing for genealogy because the surname changes with each generation.

Also, mtDNA mutates very slowly. Even if you have a very close match, the common ancestor might have lived 1,000 years ago – or more.

Nevertheless, mtDNA can be helpful when used with autosomal DNA testing to rule matches in or out on the matriline.

An attendee uses a swap to test DNA at the MyHeritage UK Ltd. booth on the exhibition floor of the 2017 RootsTech Conference in Salt Lake City, Utah, U.S., on Thursday, Feb. 9, 2017. The four-day conference is a genealogy event focused on discovering and sharing family connections across generations through technology. Photographer: George Frey/Bloomberg via Getty Images
An attendee uses a swap to test DNA at the MyHeritage UK Ltd. booth on the exhibition floor of the 2017 RootsTech Conference in Salt Lake City, Utah, U.S., on Thursday, Feb. 9, 2017. The four-day conference is a genealogy event focused on discovering and sharing family connections across generations through technology. Photographer: George Frey/Bloomberg via Getty Images
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Debbie Kennett is the author of DNA and Social Networking