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Another One
12-19-2006, 10:04 AM
I have a bunch of PBS stuff saved on the Tivo to keep me company late at night while finishing up holiday gifts. Last night it was a Nova about tracking down the current decendents of a woman whose 2500 year old skeleton was unearthed in Southern Russia.

Anyway, I'm always hearing about how mitochondrial DNA is passed down virtually unchanged in the female line, making it very useful for identifying relationships. But if mitochondrial DNA doesn't change, wouldn't most of the current female population worldwide have the same DNA, making it useless as an identifying factor? There has to be some genetic drift . . . any idea what I'm missing here?

Katherine
12-19-2006, 10:07 AM
Because people get DNA from both parents.

PatCox
12-19-2006, 10:14 AM
Jami, the mitochondrial DNA doesn't change, but the rest of the DNA does. Think of the mitochondrion as a little alien creature that lives in your cells, its actually a bacteria. The mitochondria have their own separate DNA, while your cells have their own DNA, and its the regular old cellular DNA, not the mitochondrial DNA, that determines your traits and characteristics. The DNA inside the mitochondria does not change from generation to generation. However, the DNA in the nuclei of all the other cells does change.

Andrew Craig-Bennett
12-19-2006, 10:21 AM
The mitochondrion's DNA does mutate - faster than nuclear DNA - but it is inherited 100% from the mother, because the mitochondrion is present in the ovum.

(I think!)

katey
12-19-2006, 10:22 AM
Mitochondria are the little energy-producing organelles within our cells. Some people think that they are the decendents of a formerly free-living, separate, symbiotic organism that became incorporated into us. (Think of our gut bacteria as examples of currently free-living, separate, symbiotic organism that haven't committed yet.)

It's as if we somehow evolved to incorporate the mechanism for photosynthesis. Not a bad idea, now I think of it. I'll get right on it.

Dan always cited the fact that I am descended, strictly through women, from Anne Hutchinson as evidence that the gene for intransigence is carried in mitochondrial DNA. Closer examination of my maternal line does nothing to refute that theory.

Katey

Another One
12-19-2006, 10:25 AM
Right, Pat. But they were specifically trying to determine a matrilinear relationship by looking at Mitochondrial DNA, and they were able to establish from that information alone that a 9 year old girl has an ancestor in common with a 2500 year old skeleton. Sure, we get other DNA from paternal sources, but we're ignoring that for the present because the matrilinear mitochondrial DNA is the only one which remains virtually unchanged.

So if the mitochondrial DNA is verifiably the same after 2500 years, it implies that my mitochondrial DNA (I'm getting tired of typing that word) would be the same as that of a 64th cousin 18 times removed. I mean, we are each related to everyone else in the world, if you go back far enough.

PatCox
12-19-2006, 10:26 AM
Here's Wikipedia's explanation, ACB; it appears that all of your mitochondria come from the egg, and not the sperm:

"Mitochondrial DNA (mtDNA) is DNA that is located in mitochondria. This is in contrast to most DNA of eukaryotic organisms, which is found in the nucleus. Nuclear and mtDNA are thought to be of separate evolutionary origin, with the mtDNA being derived from bacteria that were engulfed by early precursors of eukaryotic cells. Thus in cells in current organisms, the vast majority of proteins found in the mitochondria (~1500 in mammals) are encoded by nuclear DNA: some, if not most, are thought to have been originally of bacterial origin and have since been transferred to the nucleus during evolution. In mammals, 100% of the mtDNA contribution to a zygote is inherited from the mother and this is true for most, but not all, organisms. Currently, human mtDNA is present at 100-10,000 copies per cell, with each circular molecule consisting of 16,569 base pairs with 37 genes, 13 proteins (polypeptides), 22 transfer RNA (tRNAs) and two ribosomal RNAs (rRNAs)."

Andrew Craig-Bennett
12-19-2006, 10:28 AM
Whoa, Jami!

Mitochondrial DNA is not the same; it mutates faster than nuclear DNA. But all of it is inherited from the mother, in mammals.

So it is very useful in tracing matrilineal descent.

Another One
12-19-2006, 10:29 AM
So it all goes back to where we started: mitochondrial DNA is a useful way to track a matrilinear line. But if it's so durned stable, why doesn't it track every woman back to the mother of us all? And if it did, every woman would have the same mitochondrial DNA, which would make it useless for sorting us apart.

PatCox
12-19-2006, 10:29 AM
Jami, I understand mitochondrial DNA does change, just much more slowly, so we do all have different mtdna, provided we are not distant cousins. Thats how those geneticists have come up with the theory that we are all descended from one woman (Eve) from Africa who lived some surprisingly short while ago, like 100,000 years or something like that. I think this all depends on an assumption regarding the average rate of change in mtdna.

PatCox
12-19-2006, 10:31 AM
More Wikipedia, on the "Eve hypothesis"

Mitochondrial Eve (mt-mrca) is the name given by researchers to the woman who is the matrilineal most recent common ancestor for all living humans; the mitochondrial DNA (mtDNA) in all living humans is derived from hers. Mitochondrial Eve is the female counterpart of the Y-chromosomal Adam, the patrilineal most recent common ancestor.

The holder of this title is believed by some to have lived about 150,000 years ago in what is now Ethiopia, Kenya or Tanzania. The time she lived is calculated based on the molecular clock technique of correlating elapsed time with observed genetic drift.

Contents [hide]
1 Matrilineal descent
2 Mitochondrial DNA
3 Academic investigation
4 Eve and the Out-of-Africa theory
5 In popular culture
6 See also
7 References
8 External links



[edit] Matrilineal descent
Wilson's naming Mitochondrial Eve after Eve of the Genesis creation story has led to some misunderstandings among the general public. A common misconception is that Mitochondrial Eve was the only living human female of her time — she was not. Had she been the only living female of her time, humanity would most likely have become extinct due to population bottleneck. Many women alive at the same time as Mitochondrial Eve have descendants alive today. Some of those women may even be ancestors to all humans alive today while others may be ancestors to only some of the humans alive today. However, only Mitochondrial Eve, and her matrilineal ancestors, have a pure matrilineal line of descent to all humans alive today. Because mitochondrial DNA is passed through matrilineal descent, all humans alive today have mitochondrial DNA that is traceable back to Mitochondrial Eve.

To find the Mitochondrial Eve of all humans living today, one can start by listing all individuals alive today. For every individual (males and females), trace a line from the individual to his/her mother. Then continue those lines from each of those mothers to their mothers, and so on, effectively tracing a family tree backward in time based purely on mitochondrial lineages. Going back through time these mitochondrial lineages will converge when two or more women have the same mother. The further back in time one goes, the fewer mitochondrial ancestors of living humans there will be, until only one is left. This is the most recent common matrilineal ancestor of all humans alive today, i.e. Mitochondrial Eve.

It is possible to draw the same matrilineal tree by starting with all contemporary human females of Mitochondrial Eve. Some of these women may have died childless. Others left only male children. For the rest who became mothers with at least one daughter, one can trace a line forward in time connecting them to their daughters. As the forward lineages progress in time, more and more lineage lines become extinct, because the last female in the line dies childless or left no female children. Eventually, only one single lineage remains which includes all mothers alive today and their male and female children.

Mitochondrial Eve is the most recent common matrilineal (female-lineage) ancestor for mtDNA, not the most recent common ancestor (MRCA) of all humans. The MRCA's offspring have led to all living humans, but Mitochondrial Eve must be traced only through female lineages, so she is estimated to have lived much longer ago than the MRCA. While Mitochondrial Eve is thought to have been living around 150,000 years ago, the MRCA is estimated to have been living only 10,000 plus years ago.


[edit] Mitochondrial DNA
We know about Eve because of mitochondrial organelles that are passed only from mother to offspring. Each mitochondrion contains Mitochondrial DNA (mtDNA). A comparison of DNA sequences from mtDNA in a population reveals a molecular phylogeny. Unlike mtDNA, which is outside the nucleus, genes in nuclear DNA become mixed because of genetic recombination, and therefore we can be statistically less certain about their origins. Diversity is magnified in mtDNA and population bottlenecks are particularly magnified (Wilson et al 1985).

Just as mitochondria are inherited matrilineally, Y-chromosomes are inherited patrilineally. Thus it is possible to apply the same principles outlined above to men. The common patrilineal ancestor of all humans alive today has been dubbed Y-chromosomal Adam. Importantly, the genetic evidence suggests that the most recent patriarch of all humanity is much more recent than the most recent matriarch, suggesting that 'Adam' and 'Eve' were not alive at the same time.

Another One
12-19-2006, 10:32 AM
I think this all depends on an assumption regarding the average rate of change in mtdna.

And the average rate is . . . ? (Just curious.)

Milo Christensen
12-19-2006, 10:49 AM
And the average rate is . . . ? (Just curious.)

The low estimate is 100 times slower than the high estimate, producing results consistent with either evolutionary time spans or biblical time spans.

John E Hardiman
12-19-2006, 10:52 AM
Jami,
Mitochondrial DNA is subject to replication transposition, just like other DNA as ACB pointed out. However, the rate of that variance is slow and fairly well estimated by linage testing of other mammals. So your mtDNA is VERY close to your mothers, and her to her mothers, and so on. However, to your 10th great g-mother, there will be some variance that should fall within statistical bounds. Variance outside those bounds, then the probability of being in the maternal line is low.

Additionally, there are several "lines" of mtDNA. These come down from the "progenator" women and have been linked to general population movements. Some stayed in africa, some migrated, so generally up until the last century, large blocks of humans had similiar mtDNA, i.e. "european" mtDNA, "asian" mtDNA, "african" mtDNA, etc.

See this set of FAQs on mtDNA for pointers to direct research

http://faculty.mdc.edu/jmcnair/Joepages/The%20Real%20Eve.htm

See here for how the military uses mtDNA to even identify civil war sailors:
http://www.jpac.pacom.mil/CIL/mtDNA.htm