Okay, I get that you send a company some DNA - hopefully not collected from a crime scene - and they sent you back a thing saying where you allegedly came from. IT’s like a huge thing these days. They even have Youtube videos, presumably trying to teach some sort of lesson, where some idiot sits down and is like “I’m totally Spanish, and I’m DEFINITELY not Dutch, I hate the Dutch” and they’re like “Ha! You fool! You’re only 37.5 Spanish, and you’re 25% Dutch! Plus 12.5% Russian, 12.5% Hungarian, and traces of Finn, Arab, and giraffe!” and the guy learns a valuable lesson.
My question is:
Assuming your DNA can assess you as having, say, Irish ancestry, doesn’t it have to only look back a certain period of time? I mean, I know I have Welsh ancestry at some point, but that’s for a certain period of time. I may have had ancestors there from, let’s say, 600 to 1600 AD. But prior to that maybe my ancestors were in Brittany or Denmark. Great Britain, to use that as an example, has been colonized by a number of waves of people. And prior to that maybe they were in the Balkans. If you go back far enough we’re all 100% “Southeastern Africa,” right?
So when they tell you you’re x% “Irish” or “Italian” what exactly does that mean? What period of time are they looking at?
What would stop a company like this from just sending out bullshit results?
There are certain DNA sequences that are found mostly or nearly exclusively among certain groups. There actually aren’t that many that are truly definitive, it’s mostly based on statistics. So… sequence XYZ is found in 90% of people the island nation of Frutututu and in only 2% of everyone else, so if you have XYZ you probably have Frutututu ancestry. More of them are more broad - sequence PDQ is found in 60% of Europeans and 40% of Asians, but almost never in sub-Saharan Africa so having PDQ indicates Eurasian ancestry. That sort of thing. It should be pointed that lack of PDQ does not rule out Eurasian ancestry.
It’s not looking back at a particular time period, it’s based on what you have now. So if you had a particular gene that due to chance and luck is passed down from 500 or 1000 years ago they can say you have ancestry from that person who introduced it to your lineage, but because of how DNA is replicated there is no time limit. That’s why we can find Neanderthal genes in people that had to have come from an ancestor 30,000 or more years ago - there’s no time limit.
One the other hand, even if you know via other evidence that you had an Italian grandmother among an otherwise Polish background, if you just didn’t happen to inherit any “Italian” genes (by which I mean genes that are most commonly found in Italians, they wouldn’t be exclusive to Italians) such a test would not reveal “Italian ancestry” even though you definitely had an Italian grandma. That’s because most of the human genome is the same from person to person, the individual variation is in only single digit percentages of DNA.
One way that you can measure how closely people are related by looking for specific genetic mutations. The mutations are hirarchal. Say at some point in the past mutation x happens. Say 100 million people have a mutation in the x family. But later, a second mutation happens to x, call it x1, and 40 million out of those 100 million people in the x family have x1. Later still, there is a third mutation x1a, which is carried by 15 million people in the x1 family, which is part of the x family. Then you have one final mutation, x1a1, which is shared by 2 million people in the x1a family, which is in the x1 family, which is in the x family.
Those are the raw numbers. Then, to determine where mutations first took place, you take as many genetic samples as possible from people all over the world. If 80 percent of the people with the plain x mutation live in Kenya, that is evidence that the x gene first appeared in Kenya. If 80 percent of the people with x1 live in Ethiopia, it is evidence that carriers of x migrated there from Kenya. (And if there is a large percentage of people to the south in Tanzania with the as-yet unmentioned x2 mutation, then that is evidence that a different group from Kenya migrated there.) You can tease out layers of migrations over time by paying attention to finer levels of mutations, such as x1b3a5b.
Useful units of DNA for tracking those mutations are mitochondria and by Y chromosomes in males and mitochondria in females (along with other genes).
Subscribing to this thread because I just submitted one of those tests a couple of weeks ago and should have results within a month. If I find out anything that adds to an answer I’ll post.
They have to at least try, since some people may already know a fair amount about their ancestry. I am certain that I am not an Australian aborigine, for example, so if they told me that I would know they were just making stuff up.
Don’t most of these companies provide the raw data?
E.g. there’s a particular variant called “rs143383”, and the testing company says you have two copies of the T allele at this location. You can look up more information about that variation, and find that the T allele frequency is 71% in East Asian populations, 63% in European populations, and 3% in sub-Saharan African populations. Already, by looking at a single variant, you have an indication of your ancestry which you can check against their explicit predictions.
Since these companies test hundreds of thousands of variants, and use much more fine-grained analysis, it’s not practical to manually review the raw data for every variant. However, many of the tools they use are open source, and rely on public genetic databases. If you’re really interested you can learn to do these analyses yourself – just be prepared to teach yourself several semester’s worth of upper-level genetics and bioinformatics courses.
Well, my results match exactly my family history - which isn’t provided as part of the process. My wife’s was a pretty good match with her family history, although it differs in some interesting ways with her brother’s report that line up with the way DNA inheritance works.
My DNA match was 98% from one area, exactly matching what we’ve verified through various other family research. It also matched me with my niece based on our DNA, even though neither of us did the input for our family connections.