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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I take it you are referring to DNA tests like AncestryDNA and 23andme.
I understand your question. The answer is that your DNA “results” are merely a comparison of what your DNA looks like compared to modern sample groups from various regions of the world. So, for example, if a substantial enough portion of my DNA looks most similar to the average DNA results found in a sample of 5,000 Scottish people, the test will say that I am Scottish. The strength of that match will determine the percentage. Is it strong enough to suggest a 40% match or only strong enough to show a 2% match?
Take, for example, my DNA results. I am predominantly of British/Northern European *recent* ancestry, but know that I have one black ancestor who was an escaped slave that married a white woman. I am 3% “West African,” my dad is 6%, my grandpa shows about 10%, and so on. This ancestor in question was born in 1776 and was my 6th great grandfather. The DNA he passed on to me has progressively been “diluted” by the addition of all of my other grandparents since then; that is, the lines of all 256 of our 6th great grandparents (which, barring any inbreeding, all people will have (but see also: pedigree collapse over time)).
In my example, this black ancestor may have Arabic ancestors from 500 years prior to when he was born. But say 23andme had a time machine and could go back and give my black 6th great grandfather a DNA test. If we assume he had one Arabic ancestor from 500 years prior (around the time Islam was solidifying its influence in West Africa), you might find that his results show only 100% West African, assuming that all of his other ancestors had no Arabic DNA (which is unlikely, but just for the sake of our example). That is, the “Arab” in him is not “traceable” amidst the backdrop of all of the other noise in his genetic pedigree, most of which still came from West Africa in recent history.
All of this to illustrate, you may show up as mostly black in your results, but there is every chance that you have all sorts of non-African ancestors. Just as an example, you could illustrate the same concept with a Northern European. If you go back 2000 years in that person’s bloodline, it’s almost 100% certain they have Nordic ancestors, Roman ancestors, Slavic ancestors, Middle Eastern ancestors, African ancestors, etc. All it takes is one branch on your family tree to mean that you likely have millions of ancestors from other parts of the world stemming back hundreds of thousands of years.
In the end, this is why only your recent ancestry really “means” anything (and even then, the “meaning” is contrived at best). “Heritage” is a better measure of where someone “came” from, and your DNA is only part of that story.
This is why Nazi concepts of an Aryan “master race” and other race-based superiority complexes are ludicrous. It’s simple math. Who you “are” in a genetic sense is ridiculously complicated, because we are an amalgamation of many hundreds of thousands of bloodlines that are not all from one place, race, or nationality.
Basically no. There are billions of bits of data in your genome. Even after 50 generations there will still be some pieces of your DNA you could trace back to that one ancestor. Now that tiny piece of information might not encode anything you notice but it will still be there directly because of them.
What I can tell is modern DNA from almost every culture or ethnic group is not “pure”. Human migration has been going on for thousands of years. Jewelry bearing the sign for Allah has been found in viking burials. Many Native American tribes ban their members from doing these ancestry DNA tests and so the “native” DNA is generally from central or south American people and used for comparison.
I do remember reading about a ship from south Asia (Chinese I believe) shipwrecked off the coast of Africa long ago and the sailors intermarried amongst the local population
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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