This past week I've been reading a very interesting book - Who We Are And How We Got Here: Ancient DNA and the Science of the Human Past, by David Reich. It examines the history of the spread of mankind using information not from bone morphology or artifact similarities, but from the arguably more rigorous basis of DNA sequences. Human migration is charted out of Africa (and back into it), up to Eurasia, and across to the Americas. Correlations are drawn with languages and artifacts, but it's the DNA that overturns Clovis First and other older schema. There is a lot of hard science, much of it way over my head, but it was clearly worth the browse, even just for the insight on Sally Hemings and the progeny of Genghis Khan. The final chapters discuss the question of "what is race?" - a more complex question than most people realize.
The most interesting insight for me is reflected in the diagram embedded above (via), and this paragraph from the opening chapter:
’The Bible and the chronicles of royal families record who begat whom over dozens of generations. Yet even if the genealogies are accurate, Queen Elizabeth II of England almost certainly inherited no DNA from William of Normandy, who conquered England in 1066 and who is believed to be her ancestor twenty-four generations back in time. This does not mean that Queen Elizabeth II did not inherit DNA from ancestors that far back, just that it is expected that only about 1,751 of her 16,777,216 twenty-fourth-degree genealogical ancestors contributed DNA to her. This is such a small fraction that the only way William could plausibly be her genetic ancestor is if he was her genealogical ancestor in thousands of different lineage paths, which seems unlikely even considering the high level of inbreeding in the British Royal family.’So, to oversimplify it for myself: As you go back through the generations of your ancestry, the number of ancestors you have begins to increase exponentially. For the first 6 generations back (to your great-great-great-great grandparents) there is "room" in your genome for some DNA from each of them. But once you are back a dozen generations, with 4,096 ancestors (barring consaguinity), only a minority of them will have any sequences reflected in your genome.
Interesting stuff. A hard read, but a good browse. I think the book is available fulltext online here.
Makes sense to me. It blew my mind when I read about how you might have zero DNA from one of your grandparents. It would be rare, though. You get exactly half your DNA from each parent, but since you only get half of a parent's genome, you don't know which grandparental genes would be in that egg or sperm. You could get anywhere between zero and 50% of your DNA from any one of your four grandparents. Add a few generations, and those extremes would not be so rare.
ReplyDeleteyou don't actually get exactly half your DNA from both parents. mDNA means you get slightly more from your mother.
DeleteIf you like that book, I'd recommend watching lectures by David Reich and Svante Paabo on youtube. Ancient DNA studies in past few years have completely uncovered big parts of history that were completely unknown before. But it looks as though they've just scratched the surface. If you watch Paabo's lecture at the recent Schroedinger conference in Dublin, you'll see that soon they be able to sequence DNA from sediment cores. So go to your favorite site, take a core at 1m down, 2m down, etc, and then dump them in your machine to find the DNA of the people, animals, and plants that were at that spot at whatever time in history you want. Molecular archeology is coming. It will map out huge parts of deep history the way the explorers in past centuries mapped out the globe.
ReplyDeleteTiny typo: whom begat whom should be who begat whom.
ReplyDeleteFixed. And I checked - it was printed correctly in the book. The error arose in the via. (an example of a mutation that might have been carried to future generations if not for your proofreading)
Delete:-)
This isn't quite correct in two distinct ways.
ReplyDeleteWhat this is saying is that as you go down the generations your lineage exchages out parts with the community. Yet, that also means your ancestors genes are becoming part of the community. So now when you mate with someone in that community you are getting back many of your ancestors genes into your offspring. So the genes remain in circulation. It's just that The path of your anscetors genes to you didn't necessarily follow a direct line of descendants to you.
The bottom line is that unless every one of your ancestors sought out some completely unrelated gene pool to mate with your gene pool isn't dilluted as this chart implies.
The second effect is that exchange chromosomes so you are not fully stirring the pudding. That is some genes get linked to other genes so the distribution is lumpy. Some people will be much more related to their ancestors.
Just to clarify, you are saying what "isn't quite correct" is my oversimplification, not David Reich's statement - right?
DeleteI don't think, though stated several times in various posts, that charlie is quite correct either. It is entirely predicated on an "Adam and Eve" ancestor, which just isn't how this all works, as evolution is far more busy than that, and there is interbreeding between hominid spp as well.
DeleteDavid Reich is incorrect for the reasons I listed.
DeleteI explained this in the extended post below but the same mechanism that deletes genes in your direct line also adds genes from your direct line into the community around you. Thus the genes are still present in the community or can be.
Thus your grand children may get genes that you have but you did not pass on to your children. They get these from the people your children mate with not from you. But it's the same gene you have just it came via a branch that occured before you came along.
The hypothetical adam and eve was not to say there is just some original pair. It was to show this difusion and reconvergence effect in stark clarity. If you imagine everyone was descended from adam and eve, and ignore mutations along the way, then everyone out there is carrying only genes that came from adam and eve
Thus everyone has X% of adams genes and 100-X% of eve's genes. there is no loss at all!
This isn't quite correct in two distinct ways.
ReplyDeleteWhat this is saying is that as you go down the generations your lineage exchages out parts with the community. Yet, that also means your ancestors genes are becoming part of the community. So now when you mate with someone in that community you are getting back many of your ancestors genes into your offspring. So the genes remain in circulation. It's just that The path of your anscetors genes to you didn't necessarily follow a direct line of descendants to you.
The bottom line is that unless every one of your ancestors sought out some completely unrelated gene pool to mate with your gene pool isn't dilluted as this chart implies.
The second effect is that exchange chromosomes so you are not fully stirring the pudding. That is some genes get linked to other genes so the distribution is lumpy. Some people will be much more related to their ancestors.
As a sort of proof of this, for a moment, ignore mutations. In that case Every single person has 50% of the hypothetical Adam and eve ancestors no matter how many descendants have passed. As you drop genes your grandma got from adam, they are being displaced by other genes from adam by other paths than your grandma. So you can see you are never less than about half an Adam and Half and Eve.
likewise you pick up genes that your mom's grandma had and gave to your mom but that your momma did not give you. you get those via some other path that went through your dad unrelated to your moms grandma.
So quite literally you are your own grandpa.
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ReplyDelete