Posts tagged ‘mitochondria’
This article discusses the DNA we all inherit from our mother’s mothers (our matrilineal line). This genetic material is called mitochondrial DNA or mtDNA for short.
Mothers pass mtDNA to all their children but only their daughters pass it on – largely unchanged – to the next generations. Your mtDNA was only inherited from your mother and she inherited it from her mother – and so on – back through the generations. Everyone (male and female) can test mtDNA and compare our mtDNA with others. Those who match us share a direct maternal line ancestor.
Family historians often find it difficult tracing female ancestors because women traditionally changed their surnames with marriage. As DNA does not concern itself with surnames, genealogists can use mtDNA testing as another tool to find maternal ancestors, in conjunction with other more traditional family history research methods.
Some questions that might be answered by a mtDNA test include:
- From what (general) region did my maternal line come?
- My great grandfather married twice. Am I descended from his first or second wife?
- Can I find other people also descended from the same direct maternal line to help my search for my female ancestors?
How does it work?
Other DNA tests examine the 23 pairs of chromosomes that we inherited from each of our parents. Such chromosomes are found on the double helix shaped strands inside the cell’s nucleus. Mitochondria are quite different– they lie outside the nucleus, are approximately ring-shaped and also contain DNA.
Mitochondria also carry variations caused by copying errors that occurred in the past when cells copied mtDNA to pass on to the next generation. Such mitochondrial mutations occur only very rarely so large groups of population share much of their mtDNA.
The first commercial mtDNA tests only examined small areas of the ring (HVR1 and HVR2) which together make up the D-loop area of mtDNA. Matching someone on HVR1 and HVR2 does imply a shared maternal ancestor but that ancestor possibly lived hundreds or even thousands of years ago.
In recent years Family Tree DNA has offered a Full Mitochondrial Sequence test (FMS) of the entire mitochondrial ring – including the Coding Region. Exact matches in a full sequence test share a maternal ancestor who probably lived within a genealogical timeframe – that is, in recent enough generations that we may be able to identify her in our family trees.
What do mtDNA test results look like?
A group of scientists examined and sequenced all the mtDNA of one individual and published those results in 1981. They named those results the Cambridge Reference Sequence (or CRS). A corrected version (sometimes referred to as the revised Cambridge Reference Sequence or rCRS) was published in 1999.
When I have my mtDNA tested, companies report to me where my mitochondrial DNA differs from the standard (rCRS). For each of the areas HVR1, HVR2 and Coding Region I am told the small number of differences that my mtDNA has from the rCRS. I can assume that for any locations not reported, I have the same result as the standard.
My results also include my mitochondrial haplogroup, which provides a summary statement of my DNA and mutations, indicating where I fit into the mitochondrial genetic tree of all humans.
Commercial mtDNA tests also report to me other people who have tested with the same company whose DNA closely matches my own. A ‘genetic distance of 0’ indicates that in the areas we have each tested (HVR1, HVR2 and maybe Coding Region) my mtDNA is exactly the same as that of my match. A ‘genetic distance of 1’ means that there is 1 difference between us – and so on.
Which company to use?
Family Tree DNA is the only company that currently offers a full sequence test of mtDNA for genealogical purposes. They call that test mtFull Sequence or FMS. Family Tree DNA often has sales when significantly discounted prices are available. (The company’s Facebook page is one way to learn about such sales.)
Family Tree DNA also host projects based on mtDNA geographic origins and also mtDNA haplogroups. Projects are managed by knowledgeable volunteers who analyse the similar DNA of large groups of people in order to draw further conclusions.
Joining such projects is free and you can usually join any that might be of interest – such as for those whose maternal ancestors came from a particular geographic region. Once you discover your mtDNA haplogroup, I recommend you also join the relevant mtDNA haplogroup project – others in these projects share maternal heritage, even though they do not share surnames.
What to do with my results?
We all have mitochondria inherited only from our direct maternal line (although only females pass that mtDNA on to their own children). Family historians can look for others who share ancestors with us on that matrilineal line.
In addition, genealogists can consider any female of interest in their family tree and follow her female lines forward through her daughters’ daughters and on to (either gender) in the current generation. By testing the mtDNA of other living family members, we can also look for their mtDNA matches.
I do not try to identify shared ancestors with anyone who has only tested HVR1 or HVR2. Any shared maternal ancestors might have lived thousands of years ago. However if I match exactly with someone who (like me) has tested the full sequence of mitochondria, we have about a 50% chance that our shared maternal ancestor lived within around 150 years and a 90% chance that the shared ancestor lived within about 400 years. In other words, our shared maternal ancestor may have lived recently enough to be found in our family trees.
Because testing of the full sequence of mtDNA has only been commercially available in very recent years, far fewer people have taken that test than (for example) males have tested the DNA of their direct paternal line. So currently the chance of finding someone who shares direct maternal ancestors with us is still small. However as more people take a full sequence test of mtDNA, our chances of finding others related to us on our direct maternal line will increase. As a family historian, I look forward to anything that will help me find my female ancestors and those related to me on that line.
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’.