Autosomal DNA

26 February, 2016 at 2:11 pm Leave a comment

Autosomal DNA is inherited by both males and females and received equally from each of our parents.

In commercial genetic genealogy tests our autosomal DNA is compared against a large database of others, looking for identical segments of inherited DNA that may indicate we have shared ancestors.

Some questions that might be answered by an autosomal test include:

  • Can I find previously unknown cousins, solely from my DNA?
  • Can I look for relatives on all branches of my pedigree (not just my father’s patrilineal or my mother’s matrilineal lines)?
  • Can I test the accuracy of the family tree I have constructed?

How does it work?

Autosomal DNA tests examine 22 of the 23 pairs of chromosomes that we inherited from each of our parents.  The remaining pair (called the ‘sex chromosomes’) determines gender. (X-chromosome analysis is often also reported in autosomal tests.)

We inherit about half our autosomal DNA (atDNA) from each of our parents and so about one quarter from each of our grandparents (and an eighth from each of our great grandparents…) – eventually we may not have enough DNA from a particular ancestor to be recognisable.

Close relatives share large segments of atDNA that each has inherited from a recent ancestor in common. More distant relatives may carry smaller sections of identical DNA. Commercial DNA testing companies predict the approximate relationship between genetic cousins based on the size and number of shared identical segments of autosomal DNA. (Very small pieces of atDNA in common are more likely to be coincidental rather than inherited.)

Because of randomness each time autosomal DNA is passed on, even siblings do not have identical autosomal DNA inherited from their parents and the amount of atDNA shared between relatives can vary greatly. As the amount inherited from a particular ancestor diminishes over the generations, eventually two distant cousins may not share enough DNA for a commercial test to identify them as genetic relatives.

While first and second cousins (and closer relationships) should be recognised in an autosomal test, and third cousins are extremely likely to be identified, only around half of fourth cousins will be found by their atDNA. By the time of fifth cousins, only about 10-15% will be recognised and by sixth cousins, only about 2-5% will be identified as genetic relatives.

Because of this, the general rule of thumb is that best results from autosomal DNA tests occur when there is up to about 5 generations back to the shared ancestor. For those hoping to prove relationships beyond third cousins, it may be necessary to test more siblings or first cousins on one side (or both) in order to find a recognisable shared segment of inherited DNA.

Because of this ‘number of generations’ limitation, in autosomal tests it is better to test the oldest living family member (or the one who is in the earliest generation).

What do autosomal DNA test results look like?

The commercial companies’ autosomal tests generally report your matches in the database (and their contact details) with predicted relationships based on the amount of shared autosomal DNA.

Three main companies provide autosomal tests for genetic genealogy. Family Tree DNA calls their autosomal test ‘Family Finder’. 23andMe call their atDNA test ‘Relative Finder’. AncestryDNA also provides autosomal DNA testing.

Family Tree DNA and 23andMe also provide information about the location and size of shared segments.

Currently AncestryDNA does not provide any such chromosome information, directing those who test to look at the public trees of their matches, in the hope that shared ancestors can be recognised. ‘DNA circles’ link people who match DNA and who also have the same ancestor in their family trees.

Family Tree DNA and 23andMe provide ‘chromosome browsing’ tools, showing on which chromosome/s lie any shared segments. Because the test alone cannot distinguish which chromosome was inherited from a mother and which from a father, these chromosome maps appear as images showing only one of each pair of chromosomes. For instance, in the image below the background (dark) chromosomes are mine, and the segments I share with three close relatives are shown overlaying, in different colours.

FamilyTreeDNA 3 close relatives cf to KF

FamilyTreeDNA: chromosome segments I share with 3 close relatives, compared to my own chromosomes

Simplifying to only showing each pair as a single chromosome means that it might appear as if I match two people in the same area of the same chromosome but maybe I match one on the chromosome inherited from my mother and one on the chromosome inherited from my father. The two predicted genetic relatives may each match me but not each other. I should ask each to check if the other appears in their list of matches.

When a segment is shared between three or more people (meaning at least two besides myself and where each person also matches the others at that same location) then we share the same ancestor – this is called triangulation.

From the websites of all three companies (FamilyTreeDNA, 23andMe and AncestryDNA) one can download the raw autosomal test data in order to upload it to third party sites such as GEDmatch – which provide more tools for chromosome analysis as well as finding matches with those who tested with the other companies.

Admixture predictions are also based on autosomal DNA. Genetic admixture means the interbreeding of mixed population groups represented by our ancestors. This tends to be reported in a summary such as ‘30% British, 20% Northern European, …’. In fact this analysis is based on comparisons against databases which were probably created for other purposes than genealogy and the conclusions may not be accurate.

23andMe call their admixture analysis ‘Ancestry Composition’. Family Tree DNA calls theirs ‘My Origins’. AncestryDNA calls their admixture analysis ‘Genetic Ethnicity’. While these analyses might be interesting, the conclusions are not yet completely reliable.

What to do with the results?

Include in your profile information with the companies the surnames in your ancestry and where those ancestors lived. Contact others likely to be close relatives based on their autosomal testing – starting with those whose surnames or locations you recognise – or where some clue points to the relevant part of your ancestry. You may be able to identify where previously unknown genetic relatives fit into your family tree. Then you can begin to share photographs and information in the same way as with more traditional genealogical methods.

You can also test known second or third cousins in order to identify which portions of DNA you share with them, that must have been inherited from known ancestors, and then see if those same segments are also shared with potential matches identified in the database. This might help identify to which branch of the family a new suggested genetic relative belongs.

By testing the DNA of known relatives, you can also check the family tree you have constructed, to see whether DNA confirms the expected relationships.

Autosomal DNA provides another tool that genealogists can use to find relatives and prove relationships. Its benefit is that it is not restricted to a single line (patrilineal or matrilineal) but instead relates to all branches of our ancestry. Its limitation is that it might not be able to identify relatives with a shared ancestor more than about five or six generations earlier.


Entry filed under: DNA, Research techniques. Tags: , , , , , , .

DNA from our mother’s mothers

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