Mitochondrial DNA (mtDNA) is exclusively inherited from the mother... or so we think?
With the resurgence of paternal mitochondrial DNA theories, I explore whether or not the mtDNA test I just ordered was ...a big mistake.
Seth Myers, American Comedian, Host of Late Night with Seth Myers; source: GIPHY
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First, let's consider how we even arrived at paternal mtDNA being an issue in the genealogical community...
Paternal mtDNA is a hotly debated topic. Based on a contention of hypotheses of what happens to mtDNA from the father at the moment of fertilization.
DNA Testing & Genealogy
The most common DNA test for genealogy is the autosomal DNA (atDNA) test, like Ancestry or Family Finder, which can result in a large number of genetic cousins.
The atDNA tests look at shared genes across the 23 chromosomes found within the nuclei of your cells and contain inherited genes from both mother and father.
We also have another DNA source within our cells that is passed [almost?] exclusively from mothers.
Just outside the nucleus in a cell sits an organelle called the mitochondria. It's what creates energy for the cell to use... but we didn't know about its own DNA until 1963.
There's DNA in Them There Organelles
Margit M. K. Nass was working on her postdoc in 1963 at Stockholm University. Her main focus was cellular respiration (aka creating energy for the cell) until she noticed something familiar.
During her examination of mitochondria under the university's new electron microscope, she noticed the stain and treatment she used increased the density in mitochondrial fibers which indicated that she was looking at nucleic acid.
After further tests, she produced more and more evidence that supported her findings -- that mitochondria held their own DNA, separate from nuclear DNA.
Margit M. K. Nass had discovered mitochondrial DNA.
Later, mtDNA was found to pass from mothers to all of their children but only females passed on their mtDNA to their children... not males... and became THE maternal lineage tool to help identify remains and family.
Mitochondrial DNA Looks... Different
In humans, mtDNA looks nothing like the Watson and Crick double-helix structure you might be familiar with. First of all...
It's a circle!
A double stranded, closed, circular DNA structure... with 16,569 base pairs of nucleotide and 37 genes.(1)
Compare that to the 3 billion base pairs and ~20,000 genes of nuclear DNA.
[ [ [ NERD ALERT! ] ] ]
Even I had a hard time wrapping my brain around this huge difference. So, check out the visual comparison I made:
I swear on my 2nd great grandfather's grave that I entered numbers for all of those columns!!!
The number of base pairs in nDNA overwhelms mtDNA base pairs. Even though there are more mitochondria than the singular nucleus in a cell... it's still just a fraction when compared to nuclear DNA.
Guess you could say a mother's mtDNA is more efficient... as we all know mothers tend to be!
Mitochondrial Eve and Patrilineal mtDNA
So where does this leave us with our question of mtDNA being exclusively matrilineal?
If the mtDNA that is passed on to children is not exclusive to mothers, then it leads us to question everything we've learned about human origins.
The accepted theory of today holds that mtDNA is only passed on from the mother. At the moment of fertilization the egg actively decomposes the mtDNA from the sperm midpiece or excludes the sperm midpiece/tail altogether leaving no chance of paternal mtDNA to interfere with development of the embryo. Therefore, all mtDNA is received from the mother and, going back generations, we eventually reach a "Mitochondrial Eve."(2)
The other theory states that researchers have identified paternal mtDNA from the sperm midpiece in the embryo... albeit fate unknown.(3) Evidence of mixed maternal and paternal mtDNA has been found in animals, so we know that it is at least possible.
In humans though, paternal mtDNA is yet to be accepted and it will be very tough to overrule the Mitochondrial Eve theory.
So... is mtDNA Still Valuable to Your Genealogy?
YES. Yes it is. Tracing maternal kin can be a big challenge in genealogy.
Women often went unrecorded throughout history and mostly because of their status in society. Regardless, it makes our research as genealogists and family historians no easier when you get to a certain point in historical records. You'll need every available tool in your genealogical toolbox.
Knowing your matrilineal kin completes your family history, which makes it a vital component.
Remember to use a combination of DNA tests and careful evidence analysis of the records to support findings throughout your genealogical research.
Besides... the overall impact of paternal mtDNA found to-date is so small it's almost negligible in the larger scale of human origin. Not to mention what we use mtDNA results for... which is mostly comparison and finding shared ancestors.
I wouldn't count on mtDNA getting scrapped any time soon!
Carla is an Oklahoma native who has expertise in early American pioneer, Scots-Irish, and Native American research with an emphasis in mid-western American pioneer migration. Her interest in family research and evidence analysis led her to pursue a career as an independent professional genealogist.
She uses her knowledge to solve difficult family research problems and find amazing stories in ancestry. More about Carla here.
To learn more about genealogy services click here.
SOURCES:
(1) https://www.ncbi.nlm.nih.gov/pubmed/7219534
(2) Lewin R (1987), "The unmasking of mitochondrial Eve", Science, 238 (4823): 24–26, Bibcode:1987Sci...238...24L, doi:10.1126/science.3116666, PMID 3116666
(3) Ankel-Simons F, Cummins JM (November 1996). "Misconceptions about mitochondria and mammalian fertilization: Implications for theories on human evolution". Proc. Natl. Acad. Sci. U.S.A. 93 (24): 13859–63. doi:10.1073/pnas.93.24.13859. PMC 19448. PMID 8943026.