Posts tagged ‘autosomal’
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.
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.
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.
DNA technology is advancing so rapidly that it is difficult to keep abreast of the advances and possibilities. Moreover rapidly falling prices make genetic testing more affordable and so more accessible. Here are some current options:
Test 1: Y-chromosome tests, for males to test DNA inherited from their father’s fathers
It is now possible for under US$200 for males to test the DNA they have inherited from their father’s father’s fathers, with sufficient accuracy to determine whether two men likely share a common ancestor ‘within a genealogical timeframe’ and how many generations ago that common ancestor probably lived.
I have used this test to discern whether two families with the same surname were actually related to each other, in situations where I have not yet found documentary proof. I have also used this particular DNA test to check (and finally refute) a theory about who might have been the biological father of an adopted male. It was necessary to find a living male descendant (down an all-male line) from the adopted male and also to find a living male descendant (down an all-male line) from the hypothesised birth father, and then compare the DNA that each inherited from their father’s fathers.
DNA is not related to surnames and so I am not restricted to testing two men with the same surname – the test is valid for any two men who might share a common male ancestor. However when I order this test, if I choose to use a commercial testing company like Family Tree DNA – which has a huge (and growing) database – I might find in their database a match with some living descendant who shares a common ancestor that I did not know about. This is especially useful for adoptees.
The above DNA test is only available to males (as only males have a Y-chromosome). Females like me need to ask a near male relative to be tested. I have asked my father and also my mother’s brother to be tested – this opens up for examination my nearest male lines.
Test 2: Mitochondrial tests, for anyone to test DNA inherited from their mother’s mothers
Useful DNA tests are no longer limited to males. We all have a different type of DNA (called mitochondria) that we inherit from our mother’s mother’s mothers. Mitochondria mutates so slowly that formerly the only conclusions we could draw from our maternal line was about ancient ancestors and their migratory patterns.
However that is no longer true. The company Family Tree DNA offers full sequence tests of all our mitochondria (DNA that is inherited from our mothers) that allow us to identify people who share an ancestor through our mother’s mother’s mothers, within about 200 years. [Thank you Bill Hurst for pointing out that while 23andMe also tests the ‘coding region’ of our mitochondria, they do not test or give results for all 16,571 locations, so theirs is not in fact a full sequence test.]
When the above matrilineal full sequence tests first became available, they cost close to $1,000. That price has dropped now to under US$300 (sometimes under $200).
Test 3: Autosomal tests, to test the DNA inherited half from each of our parents
We are not restricted to testing only the DNA of our father’s fathers or our mother’s mothers. Since 2010 it is possible to test the remaining nuclear DNA (that is, not the sex chromosomes). This DNA is called autosomal. Family Tree DNA calls their autosomal test Family Finder, while 23andMe calls a similar test Relative Finder. (Again these tests are under US$300 and sometimes under $200.)
These particular tests can check the DNA of our ancestors regardless of gender, because we inherit about half our autosomal DNA from each of our parents (and via them, from their ancestors) and this DNA can also be compared with the DNA of others. However as we inherit about one quarter of our DNA from each of our grandparents (and so about one eighth from each of our great grandparents) – eventually the inherited material from one particular ancestor becomes so small as to be difficult to identify definitively. Consequently, when comparing this autosomal DNA with someone else, our best conclusions are when the common ancestor lived no more than about 6 generations ago.
Use the tests in conjunction
While the above tests examine separate DNA, the tests can be used in conjunction. When looking at the summary of DNA results for people that 23andMe identified as likely to be my 3rd to 5th cousins (identified via the Relative Finder – or autosomal test), I noticed that one of the matches also seemed to have very similar Y-chromosome (father’s fathers) DNA to my mother’s brother. I sent an email and by swapping names of grandparents and their parents, we soon identified that this person was the son of a 3rd cousin to me (and so indeed within the range of 3rd to 5th cousins).
It is not necessary to understand how a car works in order to drive it, but it is necessary to know the functions of driving. In the same way it is unnecessary to understand much about the science of DNA in order to use it as a tool – but it is necessary to understand what sorts of questions can be answered by the different DNA tests so you know how to apply them as tools to aid your family history research.
This field is changing quickly
Because genetic tests available to the public are changing frequently (and certainly the prices are) readers need to beware of relying on conclusions written years ago or by someone who has not ‘kept up’ with tests currently available. This blog post is partly in response to an article I read this week entitled ‘The DNA dilemma’ – I do not agree with many of the conclusions in that piece.
It is no longer true to say that the only available information to be derived from maternal DNA (or mitochondria) is about ancient migrations of peoples – recent relatives can now be found by a full sequencing of the mitochondria (test available from Family Tree DNA for under $300).
It is no longer true that autosomal DNA can only make generalised indicators of race origins. (Autosomal DNA is sometimes referred to as ‘nuclear DNA’ but that is incorrect because the sex chromosomes are also inside the cell nucleus and the autosomes are the other pairs of chromosomes that are not the sex chromosomes.) Nor is it necessary to ‘test each generation in turn’. Autosomal DNA can identify that two people shared common ancestors within 6 generations (and possibly beyond, but it is less accurate beyond 6 generations). Many genealogists will not know all of their ancestors back even 6 generations, and so this DNA test can predict likely distant cousins who may not have been found by a paper trail.
There are differences between the DNA tests used in forensic law enforcement compared to commercial tests. Without going into too much scientific detail, legal forensics examine repeating groups of DNA at certain points on the autosomes whereas commercial autosomal tests examine the autosomal SNPs (something like ‘typo’ mutations). The tests are entirely different. Be wary about confusing the markers referred to in tests of the Y-chromosome (the DNA inherited father-to-son) – which are entirely different to the markers of autosomal DNA examined by forensic law enforcement agents.
Some people have suggested that male DNA studies are only relevant between males who share surnames. That is not true. There are many examples where family trees show a son with a different surname to his father – whether the name was changed by deed poll, by adoption, by remarriage of the mother – or for many other reasons. It is not the same surname that defines two people as father and son. Likewise DNA tests do not take surnames into account, so the test result is just as accurate whether two men share a surname or not.
In my opinion, the most recent DNA tests available to genealogists offer precise information which can supplement traditional genealogical methods. Family trees are still needed to identify ancestors and draw conclusions, however DNA tests can supplement other genealogical research, filling in gaps left by paper trails. With such tools we can test our conclusions and assumptions in constructed family trees as DNA can confirm or disprove reputed relationships. As databases grow, commercial DNA tests are more likely to help us find relatives that we might not have found by ‘traditional methods’.