Re: Need Help With DNA Puzzle #dna

Adam Turner

Once you feel you have figured out these DNA puzzles, here’s another: those of us with European ancestry have about one and a half percent Neanderthal DNA, which is about as much DNA, on average, as you would get from one fourth great grandparent, six generations back. But the Neanderthals died out around 40,000 years, or roughly 160,000 generations, ago. How do you explain this?

This one is just the result of confusing one's DNA admixture at the population level with how DNA is shared from one's immediate ancestors.

If you have European ancestry, all of your immediate ancestors' genomes have about 1.5% of their SNPs that can be traced back to Neanderthal populations. That 1.5% doesn't necessarily diminish from generation to generation - those SNPs keep getting passed down again and again as long as people keep mating within the same population, just like the total proportion of markers (SNPs) in your genome that are identifiably "Ashkenazi Jewish" didn't diminish as long as your Ashkenazi Jewish ancestors kept having children with other Ashkenazi Jews. 

(Think of it this way: if someone who is 25% Ashkenazi Jewish marries someone else who is 25% Ashkenazi Jewish, their childrens' genomes, on average, will still show a 25% Ashkenazi admixture. The same principle applies in this case, even though the population we're talking about - "Europeans" - is much much larger and much much older than the population we call "Ashkenazi": both of your parents had genomes with ~1.5% identifiably Neanderthal SNPs, so you do, too. All 4 of your grandparents had 1.5% Neanderthal SNPs, so both your parents did, too. And so on and so on for thousands of years back, for as long as people kept on mating within that same, relatively homogeneous population of Europeans.)

Adam Turner

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