Posts tagged ‘autosomes’
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.
I started down the DNA learning path several years ago. My Dad’s father was adopted, & when I eventually found his birth certificate it contained no information about his father. An unusual middle name and circumstantial evidence suggested someone, but with no documentary evidence, DNA seemed a way to test my theory.
I found a grandson of this possible ancestor – son of a son, so a good candidate for y-chromosome DNA comparison with my father. I asked – if I paid for it, would he be willing to have his DNA tested to compare with my father’s DNA? He said yes, but unfortunately the test proved that he and my father were not related. (DNA is often better at disproving rather than proving relationships.)
I used the company Family Tree DNA, which has the largest database for testing and comparison, and now that I am registered, I am advised when others match my Dad’s DNA. I hope that one day I will find someone with the right DNA,who had an ancestor in the right place and at the right time.
Some time later, I had the opportunity to speak to Megan Smolenyak about my problem & confirm my method. I asked Megan for her advice about which company should I use to test my Mum’s DNA.
Females don’t have y-chromosomes so cannot have the y-DNA tests done. However humans have other DNA outside the cell nucleus, called mitochondrial DNA (mtDNA). Mothers pass mtDNA to all their children, but only their daughters pass it on. I wanted my Mum’s DNA to be tested now and also stored for the future, for as-yet-undeveloped tests. Forensic scientists use mitochondrial DNA now, but for genealogists mtDNA is mostly only used for deep ancestry testing, not for finding ‘recent’ ancestors (those in a genealogical timeframe).
Megan suggested that I have my mother’s DNA tested with the company 23andMe, as they were developing new tests and could offer more information about female ancestors. 23andMe tests give information about genetic health issues, in addition to genealogical ancestry matching – so I took that advice.
These 2 companies that I had used (23andMe & FamilyTreeDNA) offer very different information in their test results. Results from the FamilyTreeDNA tests are tables of numbers, indicating the DNA at specific genetic marker points. There is also a YSearch database for comparing results, and even people who have had their DNA tested with other companies can search this freely – you manually enter the numbers (alleles) at various marker locations and see if the results match anyone in the YSearch database.
The results from 23andMe gave information about genetic health risks and tendencies and general DNA groupings – it required a bit more delving to actually find the numbers that correspond to the (mitochondrial) DNA markers.
Around a year ago, both these companies announced new tests involving autosomal DNA. 23andMe call this ‘Relative Finder’ – FamilyTreeDNA call it ‘Family Finder’. Humans have 23 pairs of chromosomes in every cell nucleus – 22 pairs of autosomes and also another pair, the ‘sex chromosomes’ (XX for females, XY for males). The autosomes contain bits of DNA inherited from all your ancestors, not just from all-male or all-female lines. You share larger pieces of DNA in common with close relatives, and smaller bits of autosomal DNA with relatives less closely related.
Both males and females have this autosomal DNA, so now you can find relationships with anyone sharing any common ancestor, not just the all-paternal or all-maternal lines. This new autosomal DNA test has thrown up some new possibilities and new candidates in the search for my father’s father’s heritage. (We already have found a close relative with interesting possibilities.)
However, back to the initial subject. The company 23andMe is offering a special price for the next few days, and some Facebook friends decided to take advantage of it. I agonised whether I should join them, given that I already have tested my father’s DNA as well as my mother’s mitochondrial DNA.
In terms of autosomal DNA, although the test is new, I suspect that the company FamilyTreeDNA is likely to have a bigger database for comparisons. (For me the main value of DNA tests is looking to match with others, and so larger databases are better.)
But 23andMe gives other information – about genetic related diseases – in addition to the study of ancestry. I have decided that both companies’ tests are of interest to me. So now I too have taken advantage of the current special price, and will get my own DNA tested.
Of course there are many other testing companies, and websites with information about DNA. I give talks about ‘DNA for Genealogists’ (my handout can be found on my website). The handout contains information about various testing companies and their information pages, as well as other sites with DNA tutorials, mailing lists and even a DNA Wiki.
For now though, I have joined the ranks of those waiting for a test tube to be posted to me, so I can take the next step in this DNA journey.