DNA from our mother’s mothers

26 February, 2016 at 1:52 pm Leave a comment

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?

mitochondrial images

Human cell                                                       Ring-shaped mitochondria

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.

 

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

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