I’m African American. Hypothetically, a thousand or so years ago, say my ancestor was an Asian (For example) who married a Black person. Naturally, their child was 1/2 Black and 1/2 Asian. That child went on to marry a Black person. The ancestry continues on and on for many years until we’re in 2022.
Coincidentally, none of the children ever married an Asian and all of the people they married had no trace of Asian blood in their lineage, making that one ancestor the only trace of Asian in me.
But do I still have Asian in me? What fraction would that even look like? Or at some point does it get “erased” as a result of being reduced by the other races, bloodlines, etc. over the course of 1000+ years?
I hope the example I gave makes sense. I guess I’m basically asking if, at some point, race/lineage gets erased from your cells/DNA if that race hasn’t been present at all in your family for several centuries.
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An ancestor theoretically contributes to 1/(2^N ) of your genome, where N is the number of generations before you they are. Assuming 25 years per generation, an ancestor from 1,000 years ago is responsible for roughly 1/(2^(1000/25)) ≈ 1 **T**rillionth of your DNA. There’s only about 3.2 **B**illion nucleotides in your entire nuclear genome. So the chance that that ancestor contributes even *1* nucleotide (i.e. 1 A, C, G or T) is about 0.3%. So yes, it’s possible for any single ancestor’s contribution to be basically entirely wiped out over a large enough time scale.
It’s further complicated by something called “recombination”. Basically, before when Dad’s making his sperm – or when Mom is making her egg – you start off with a diploid cell, that is, one with 2 of each chromosome, 1 that theoretically came from their mom and one that theoretically came from their dad. And then that cell splits into two haploid cells, cells with only half the information needed to make a new baby.
But *before they split* the chromosomes between those two haploid cells, the chromosomes do some DNA switcheroo. The chromosomes will pair up – say, chromosome 13 with the other chromosome 13 – and decide to trade corresponding regions of DNA. So then you don’t have one chromosome 13 that’s 100% grandpa’s and one that’s 100% grandma’s – there’s one that’s e.g. 12% grandpa/88% grandma, and another that’s 88% grandpa/12% grandma.
Then consider it’s basically a crapshoot which one of that pair of mixed-up (sorry, “recombined”) chromosomes is packaged in the sperm dad made that impregnanted mom, and also repeat this for *all* the chromosome pairs, not just 13, and then again repeat the whole thing for *mom*. And you see that it becomes very complicated very fast to say who’s responsible for what percentage of your genome – that’s why I said *theoretically* it’s 1/(2^N ). Theoretically it approaches this if averaged over all ancestors from level N as N gets bigger and bigger. But at the individual level, there’s just so much randomness at play. *Maybe* that ancestor from 1000 years ago happened to survive in your genome by sheer dumb luck.
With two big exceptions.
One is the Y-chromosome. Dad has it, Mom does not. That means if you have a Y-chromosome, there’s no question about who gave it to you – you had to have gotten it from Dad. Who got it from his dad, who got it from *his* dad, who got it from *his* dad, and on and on and on. When the recombination thing is going on in his testicles, the Y-chromosome can’t recombine because there isn’t another Y-chromosome to recombine *with*. It sort of *tries* to hook up with the X-chromosome, but it doesn’t work very well – only the tips recombine. The rest of the Y-chromosome survives unscathed. Basically, ~95% of the Y-chromosome only *ever* changes through mutation.
The other is your mtDNA – your mitochrondria have DNA of their own separate from the rest of the nuclear DNA (surprise!). You always get this from your mom, because your first mitochrondia are the ones mom packaged up in the egg. So again, mtDNA can’t recombine because there’s nothing to recombine *with.* So mtDNA basically only changes through mutation.
And this is the main way your ancestry *can* survive generation after generation. Because the Y-chromosome and mtDNA are *uniquely* resistant to change over time, you can look for mutations in them – say, a T here where most of the world has a G; this single-letter replacement is called a **s**ingle **n**ucleotide **p**olymorphism (**SNP**) – and reasonably ask statistical questions about them like “what percentage of people have these specific SNPs in this region of the world”? And *that’s* the thing that services like 23AndMe look at. They figure out what SNPs you have, each of which tends to roughly correlate with a certain region of the globe.
Many such SNPs are grouped together under what’s called a *haplogroup.* If you’re a guy, you have 2 of them – your Y-chromosome belongs to a Y-haplogroup, and your mtDNA belongs to a mtDNA-haplogroup. If you’re a girl, you only have the mtDNA-haplogroup.
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