How many genetic ancestors does the average person have?
April 10, 2017 8:46 AM   Subscribe

Because of crossing over during meiosis, our genetic ancestry is a mix of a bunch of our ancestors. But how many ancestors, on average? (Or, better, what does the number-of-genetic-ancestors curve look like?) I know that it's probably not all of our ancestors, because there are a limited number of crossovers during a given recombination event. But how does all that math work out? What are the chances that we have lineal copies of at least a chunk of a chromosome from all of our great-grandparents? What about 5 generations back? 10?
posted by clawsoon to Science & Nature (11 answers total) 6 users marked this as a favorite
What are the odds of inheriting no DNA from a great, great, great grandparent? "...the odds of getting no DNA from a distant relative over 23 different chromosomes are very small. So small that you almost certainly have at least a bit of their DNA."
posted by Mr.Know-it-some at 9:04 AM on April 10, 2017 [3 favorites]

Good info above. I'm still not clear what you're asking but may be able to help if you can clarify.

To first approximation, a decent answer to the title question is "all of them".

You have bits of DNA that were inherited from pre-human progenitors, but you also have virtually 100% of your alleles inherited from your two parents, a set of exactly two people (ignoring de novo mutations).

You could say you got a certain allele from your dad or from your great-great-grandad, and be right either way.
posted by SaltySalticid at 9:46 AM on April 10, 2017

That's the question I'm trying to answer, Mr.Know-it-some. Thanks for the link! Based on simple math (and ignoring recombination?), "Chances of no DNA from one great, great, great grandparent: 1 in 8", and if I can convince my browser to use the correct character encoding I can see the formula he's using.

However, I am still hoping to find an analysis using actual recombination data (e.g. this kind of data).
posted by clawsoon at 10:12 AM on April 10, 2017

You could say you got a certain allele from your dad or from your great-great-grandad, and be right either way.

Aye - but it could also be true that your great-great-granddad and your great-great-grandma both had the same allele, but you only got it by lineal descent from one of them. For a given chunk of DNA, assuming that no recombination happened within it, you only have copies from two of your four grandparents. To extend that to all your ancestors and all your DNA, you have to know how many chunks of non-recombined DNA there are on average in a human genome. You also have to know how random recombination is; if it always happens in the same places in the chromosomes, that'll lead to a lower effective number of chunks of non-recombined DNA over multiple generations.

From what I've read elsewhere, the number of recombination sites appears to be finite and mostly limited to certain hotspots. The analysis in Mr.Know-it-some's link does the math for each chromosome as a non-recombined chunk, giving the answer for 23 effective chunks of non-recombined DNA. To get closer to the actual answer, you'd have to replace 23 with the actual, larger answer.

The link also says that we have "DNA from very distant ancestors", but that's different from "we have DNA from ~all~ of our distant ancestors". I want to know the answer to the second one.

Does that make the question clearer?
posted by clawsoon at 10:31 AM on April 10, 2017

You may want to read about coalescent theory.
posted by grouse at 12:08 PM on April 10, 2017 [2 favorites]

You might be interested in reading about Mitochondrial Eve.
posted by gregr at 1:07 PM on April 10, 2017 [3 favorites]

Wait But Why on your Ancestor Cone.
posted by ottereroticist at 1:34 PM on April 10, 2017 [3 favorites]

I think this post from Graham Coop might answer your question. He comes at it from the other side - how likely are you to NOT get any genes at all from a grandparent. The answer is a little complicated and quite variable! He uses real recombination data and cites some papers that might be interesting/useful.
posted by congen at 5:22 PM on April 10, 2017

Oh wait! This post gets at your question much more directly.

As a rough rule of thumb the autosomes you received from (say) your mother, k generations back is broken into (22+33*(k-1)) chucks, as your genome comes in 22 chromosomes and there are on average 33 recombination events per transmitted genome. These chunks are spread across your 2^(k-1) maternal ancestors. So, for example, nine generations ago the autosomes you receive from (say) your mum are broke, on average, into 286 large chunks, and these are spread across your 256 ancestors. Thus on average each of ancestors has contributed only a single block to you, and by chance it is possibly that they contribute zero. This gets worse the further we go back in time, your genome is broken up into more and more chunks, but this does not grow as fast as your number of ancestors. This makes it increasingly likely that you inherit no autosomal material from a particular ancestor.

There are graphs and explanations!
posted by congen at 5:28 PM on April 10, 2017 [5 favorites]

That's perfect, congen, thanks!
posted by clawsoon at 7:23 AM on April 11, 2017 [1 favorite]

BTW, grouse, I notice that the author of congen's links mentions that he's using/explaining/building toward coalescent theory, so you were pointing me in exactly the right direction. I was just too lazy to do the math myself. :-)
posted by clawsoon at 8:50 AM on April 11, 2017

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