Plain English Genetic Genealogy

Mitochondrial (mt)DNA

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mtDNA and Y-DNA, two specific types of DNA testing results, can also be useful in genealogy but, for most of my “Plain English Genetic Genealogy” blog entries, I will focus on atDNA.  It is, however, very important to understand both mt- and Y-DNA and how it may be used to further family tree research.

If you remember your high school genetics, you know that males determine the gender of each of their children.  Men carry two distinct types of gender-determining chromosomes: one X and one Y.   Males typically give either an X chromosome or a Y chromosome to their child.  If they give a Y, the child is male; if they give the X, the child is female.  There are times when a different type of chromosome situation appears but that conversation is for blogs on genetics, not genealogy.

From that same high school biology class, you should also remember that women have two identical gender chromosomes, both of which are X.  This means that women can only pass on an X chromosome to their children.  This is why the male always determines the gender of the child, never the female.  And, what gives Henry VIII a bad rep for getting rid of wives who could not produce a male heir; it wasn’t them it was him.

Mitochondrial or mtDNA

So, women only have the X chromosome to pass down and, contained in that X is mitochondrial or mtDNA.  The simple fact is that the X your mom gave you—whether you are male or female—carries the identical mtDNA that her mom gave to her.  And, her mom’s mtDNA is identical to the mtDNA that her mom gave to her and that her mom’s mom gave and her mom’s mom’s mom gave and on and on and on.

Mitochondrial DNA mutates (changes) about once every 20-30,000 years and each of those mutations are really, really small.  So, statistically speaking, you would have to go back about 1000 great grandmas to find a different mtDNA than that which your mom gave you; and, even then it would not really be that much different.  And, unlike dad, mom is an equal-opportunity donor; she gives all her children—whether sons or daughters—an exact copy of her mtDNA.

Science determined we can trace every single human (Homo sapiens) living today back to one (Homo sapien) mom who lived about 120-150,000 years ago; this mom has been given the name, Mitochondrial Eve.  [In fact, it is recognized that there were probably more than one woman with this unique mtDNA but, for the purposes of this blog, we’re going with a single woman because it works.  So when I reference 500 women, you could think of it as 500 unique mtDNA sequences irrespective of how many women there actually were.  It is just easier to write about this as if Mitochondrial Eve was a single woman.  Besides, it’s more fun and I get to use National Geographic‘s cool photo.]

It is not like we know exactly who this woman really was but, based on the length of time mutations happen and the ability to identify the differences (mutations) in mtDNA of the DNA pool we have today, it is possible to figure out how long ago lived the woman who carried the same unique mtDNA (with varying mutations) we see in today’s humans.

It is important to keep in mind that, after that first mutation of Mitochondrial Eve’s DNA, it is not as if each subsequent mutation will be the same as the one on the other side of that first split.  Imagine one little teeny piece of DNA (using the genetic code of ATCG) in Mitochondrial Eve’s mtDNA read: ATCGGTAC and the first mutation produced ATCGATGC for one line and left the other as ATCGGTAC.  The next time each mutates, ATCGATGC changed to ATCGATCG and ATCGGTAC might have been TACGGTAC.  You can now see, with 16,500 DNA building blocks (base pairs) and 37 genes, the potential for the incredibly large number of places a single mutation can happen.  This is how my mtDNA could radically differ from yours even though we both started 120,000-plus years ago with the same ancient ancestor, Mitochondrial Eve.

I’d like to take a moment to say that “Plain English Genetic Genealogy” is not intended to be highly scientific.  The intent is to (hopefully) inform readers, in language that makes sense to their needs, what most need to know to make use of their DNA test results.  My examples and explanations are to inform genealogy, not develop scientists.

Let’s approach this concept of Mitochondrial Eve a little differently.  Though I am certain someone somewhere has attempted to determine the number of Homo sapiens living 200,000 years ago, I could not find it so let’s pick a really easy number: 1000.  Of this, approximately 50 percent would be female so 500 women.  Of these, let’s say that one-quarter did not live to bear children, which leaves us 375 women, each with unique mtDNA.  Of these 375, let’s say only 281 had daughters.  This means that from this very first point in history, the unique mtDNA of 93 or 94 of these women never went any further because they only passed down their unique mtDNA to sons who do not pass it on.

Why?  Because while mothers pass down their mitochondrial DNA to all their children, only those children who are daughters can pass it down to their children.  For some biological reason (for which many genealogists will forever be grateful) men do not pass on their mom’s mitochondrial DNA.  So, only the women in a long line of women will pass on their unique mtDNA.  Again, it doesn’t matter how far back you go, mitochondrial DNA follows only a single female line.

mtDNA
Slide from my Basic Genetic Genealogy presentation.

So, now we’re left with 281 unique mtDNA chromosomes.  Of those, let’s conclude that together these moms had 1124 children of which 600 lived to child-bearing years, with 300 (50 percent) being female.  No matter how we look at it, this now limits the number of unique mtDNA chromosomes to a maximum of 300 and an unknown but potentially low minimum to be passed down to the next generation of Homo sapiens (because some of those 300 would have been sisters with identical mtDNA).  Even using the same percents as with the initial example, let’s say one-quarter of these moms only have sons; this leaves 225 unique mtDNA chromosomes to be passed down through the generations of daughters.  So, within just three generations, at least 275 unique mtDNA chromosomes were not passed on to future generations. This continues on and on and on until today.

And, each of these unique mtDNA chromosomes will undergo mutations—one about every 20-30,000 years.  These mutations form new unique versions of mtDNA, which are what is present in today’s humans (Homo sapiens).  Identifying and accounting for the mutations, it is possible to (tentatively) go back far enough to determine which original mtDNA chromosome has outlasted all the rest and when that woman lived.  The (unknown) owner of that original mtDNA chromosome has been called, Mitochondrial Eve and science believes she likely lived 120-150,000 years ago; this number is fluid as science progresses.

Again.  Mom passes her mom’s mtDNA to all her children.

Only daughters will pass on their mtDNA—that they get only from their mother—to their children and only their daughters will pass it on (in its identical form) to the next generation.

I am guessing that some of you—especially those searching for information on female ancestors—already see the potential of using mtDNA in your genealogical research.  Yes.  If one of the women in a direct female line (mom, mom’s mom, her mom’s mom, etc.) exactly matches another mtDNA test, then there is a common ancestor.

Tremendous, right?  Except.  How many of your 2x great aunt’s great grandchildren will know anything about your direct family line?  It is possible, absolutely, but not all that likely unless that third cousin, is a fellow genealogist with a treasure trove of diaries.  Definitely worth checking out but I would not hold my breath.

Because, if your mom only had sons, her mtDNA stops with you (because, if she only had sons, you’d be one of them).  But, if she had a sister, then that same mtDNA would be passed down, just not through your direct family line.  If each of your mom’s sisters only had sons, then your grandma’s sisters would have passed this mtDNA down to their daughters who would pass it down.  If your grandmother had no sisters, either, then you will need to keep going back to the first great grandmother who had more than one daughter who lived to bear daughters who bore daughters.  

What if you’re looking for a birth mother?  Now, I have your attention.  An mtDNA match with another person (male or female) could very likely be a half-sibling.  If they are willing to communicate freely, you may have found the birth mom.

This works even if you are trying to find your great grandmother’s birth mother.  mtDNA changes only about once every 20-30,000 years so if you find a match to your mtDNA, then that person is likely your second or third cousin from whom you have a good chance of getting your 2x great grandmother’s name (your most recent common ancestor or MRCA) and, other than the person you match, all of those ancestors you and your match have in common with mtDNA will be women all from the same maternal line.

Mitochondrial DNA also provides something called a haplogroup.  A haplogroup identifies which ancient regional signature your DNA matches.  Haplogroup information is passed down from both parents (in moms through mtDNA; in dads through Y-DNA) and is useful in understanding a person’s early origins and the migration paths their ancient ancestors took to get where you are today.  More on this in the Y-DNA blog entry.

Haplogroup

mtDNA has long been useful in an historic perspective.  For an interesting story on how science proved Jesse James really was dead and buried in Kearney, Nebraska, check out Blaine Bettinger’s The Genetic Genealogist blog on Famous DNA Review, Part IV — Jesse James.  I remember when this happened and I have been hooked on the uses of DNA ever since; long before I got captivated with genealogy.

Mitochondrial Eve image from National Geographic: “Human Journey.”

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