Here is a puzzle for you to figure out: did Genghis Khan really have 16 million direct male descendants? Note the careful wording: direct male descendants. It is a factoid that has been around since 2003 when a now famous genetic study concluded that 16 milllion men in Central Asia share a remarkably similar Y-chromosome, meaning that at some time in the past they had the same forefather. The factoid that it was Genghis Khan has been doing the rounds in the international media ever since.
How can they know such things? Well, because they know how many copying mistakes there are in each generation in bits of the genome, so by looking at how much variation there is in a population they can work out how long ago they started with the same chromosomes. With a bit of statistical trickery you can also find clusters of men who will have a later common ancestor, essentially by first divvying up a large population in more homogenous ones that have similar Y-chromosomes.
Now, it is true of all men that if you go back far enough that they will have the same forefather, but the unusual things about the 16 million in Central Asia is that their common forefather appears to have arisen around the year 1000, give or take a century (and with Genghis Khan you have to give 1.5 centuries, which is your first main clue). The year 1000 is rather late for so many related people however.
The factoid is thus that Genghis Khan, who had many sons and whose many sons had a lot of sons themselves currently has 16 million male descendants. This is 8% of the Asian men studied in the 2003 study, but more than 8% in particular areas. With a roughly tenfold general population increase in that region, Genghis would have been 1.6 million times as successful as any other man living in that era.
I encourage you in the comment thread below to say if you believe this and if so, why. If not, tell me why this factoid, which to my knowledge has been unchallenged till 2012 would in fact not be true.
I will let you know on Monday what I think about it.
As a bit of background on those who believe that because something has been unchallenged for a while that it must be true: the world is full of factoids that survive in the popular media and even in the scientific community for a while, even if on closer inspection they are not all that plausible. An obvious example of a now ‘disproven factoid’ is whether the Great Wall be seen from outer space or the moon? It seemed plausible because it is so long. However on reflection you should already have your doubts: the Wall is less broad than your average house. It is basically as visible as a very long lane with large trees of a slightly different colour to the rest. Now, if you can’t see something as thin as the average house or a lane of trees from outer space, why would you be able to see a whole string of things smaller than the house or a large tree? So with a bit of common sense you could already have known it was unlikely that you’d be able to see the Great Wall from outer space (unless with special equipment, of course, and in closer orbit you might get a glimpse). It took a few decades but ultimately, indeed, we found out you can’t see the Great Wall from the moon and that what was bandied around for decades was a myth.
Perhaps the same is true of this factoid also?
Core Economics 
Paul
Aren’t you being too literal about the Great Wall. Don’t people mean that you can see the change in the vegetation on either side or the level of urbanisation. You can see the US / Canada border from space in the Great Plains. That does not mean that you can see the fence (if there is one). I bet you can see the Murray and Darling Rivers from space. By that I mean that you can see the course of the rivers from the heavily tree cover in the flood plains on either side. I don’t literally mean you can see the water. When I say I can see your point. I don’t literally mean I understand what you are talking about — I mean I can see the rough outline of your argument, from a great distance (just kidding).
Cheers
Sam
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The original paper suggest three explanations
“(1) all populations carrying star-cluster chromosomes could have descended from a common ancestral population in which it was present at high frequency; (2) many or most Mongols at the time of the Mongol empire could have carried these chromosomes; (3) it could have been restricted to Genghis Khan and his close male-line relatives, and this specific lineage could have spread as a result of their activities.”
They argue that the 3rd is more possible, but by no means conclusive. And even it includes Khan and his close relatives which are descendent from generations prior.
Anyway, keen to see peoples numbers and assumption about generations, male children and their probability of procreating etc.
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A similar case has been made out for the 5th Century Irish king Niall of the Nine Hostages. He is the reputed ancestor of about half the surnames in NW Ireland (and of course of many among the sons and daughters of the Irish diaspora including in Aus: me among them). Note that even long after its full conversion to Christianity—down to the 1600s—Ireland was very liberal or unAugustinian about sex: divorce was common and easy & concubinage widespread. Illegitimate sons were acknowledged. So in principle there could have been ‘maintenance of the genetic surge’ (tsunami? each of many sons having many sons) down to the 1600s.
But, in both his and GK’s case, I wonder if the ‘genetic-political glamour’ can really have lasted so long? I think it would have regressed to the mean after (say) a couple of centuries. And to have secured so many descendants, GK himself would need to have launched his ‘tsunami’ with 100+ sons of his own. I can’t imagine he had that many. Ergo: it is a factoid.
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Maybe I’m missing something but it doesn’t seem on the face of it to be completely absurd. If you assume that an average generation in this period and place was about 25 years and produced at least one male who lived to have his own kids, you can easily produce enormous numbers of descendants. It’s like the wheat and chessboard problem — after 30 generations or so the compounding becomes enormous. I think families with two male heirs are quite believable and, tweaking the length of the generations, you can quite easily get billions of descendants on that basis.
Without reading the study I don’t know how they link it to Genghis specifically. That seems a bigger problem. As a high status male who visited a fairly unprecedented number of places for his day, he’s definitely an obvious candidate for Adam in this population. But I don’t see why he’s necessarily more likely to fill that role than some other randy guy forgotten to history.
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I have heard (in a casual setting) a similar (though less specific) claim about descendants of Julius Caesar.
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