The DNA Detective: How to solve family mysteries with ancestry testing
Prof Turi King, a geneticist on DNA Family Secrets, explains the technology she uses to reunite long-lost families.
All sorts of mysteries lurk in our family trees, from long-lost relatives to adoptees’ hidden heritages. In the new series of DNA Family Secrets, geneticist Prof Turi King sets out to solve them with the help of presenter Stacey Dooley and home genetic testing kits.
Turi is a scientist, presenter, speaker and author who is passionate about communicating science to the public. She’s been working in the field of genetic genealogy since 2000 and is perhaps best known for leading the genetic analysis for the identification of King Richard III.
Tell us about the genetic technologies you use in DNA Family Secrets.
DNA sequencing technologies have really moved on in the last 20 years. Now you can sequence entire genomes. For the medical side of things, we know a lot about genes that are involved in particular diseases, and we can tell people whether or not they have a gene that predisposes them toward something.
I got involved in the family history side of things by accident. My PhD was on the link between a surname and a Y chromosome. Surnames, like Y chromosomes, come down through the male line. So are all men with the same surname related to each other? Do they all have the same Y chromosome type?
Back then the technology was based on DNA fingerprinting. We could do genetic genealogy where we could take two people with the same surname who couldn’t find the genealogical links between them. We could take a look at their Y chromosomes and see if they’re related.
Now it’s moved on and there are direct-to-consumer testing companies. So, if somebody comes to us and says, “I don’t know who my biological father is,” we can get their DNA tested by one of these direct-to-consumer testing companies. They don’t sequence your entire genome; they look at tiny differences, about 700,000 of them, which are places on the genome where we know people tend to differ.
We can then upload their DNA to various databases around the world and look for DNA sharing, which we measure with a unit known as a ‘centimorgan’. You share half of your DNA with your parents and about half with your siblings and about a quarter with aunts and uncles. So, if somebody doesn’t know who their father is, but knows who their mum is, you start to look for genetic matches that aren’t from the mother’s side. From that, you can start to build these family trees.
So this DNA sequencing technology – this ability to look at huge amounts of DNA all in one go, and look at these genetic measurements – has absolutely changed genealogical research over the last 20 years.
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How accurate and reliable are these consumer DNA testing kits?
In terms of health stuff, I always say you should go to your GP, ideally. It’s not that the tests are unreliable, it’s just that they might not be looking at all of the genetic mutations that are involved in something. They also don’t give you the care and counselling that goes with that.
For ancestry, I always say take them with a pinch of salt. What the genetic companies do is test of all those little differences. They use a computer to cut your DNA into chunks and it goes, “Right, where’s the closest match for this chunk?” Maybe for that chunk it’s Japan. It’ll do that for all the bits of your DNA that it’s got and do it multiple times to give you a kind of average. It’ll go, “This is where your matches are, in this part of the world.”
But it all depends on who’s in the databases that the company has. You’ll get differences if you go to another company because different companies have different databases. It shouldn’t be too wildly out, but you’ll get differences.
But in terms of building family trees, it’s fantastic. It doesn’t tell you precisely how somebody is related – it’s too difficult to, based on just 200 centimorgans. But those companies are actually really good because they’ll say, “You share 200 centimorgans, so there’s X per cent chance that this person is a second cousin.” And then you have to use the genealogy to work out how that person is related to you.
It’s only in the last decade or so that it’s really taken off and it’s extraordinary what you can do with it. In the first episode of this series of DNA Family Secrets, we follow Janet’s story. Her parents have passed away now, but when Janet was young she overheard a rumour one night when they had friends around. A guest said, “So what’s this about you having had a daughter in Austria during the war?”
Apparently the guests were rapidly shooed out after this. But Janet’s always wondered if she has a half-sister somewhere. So we had her DNA tested by the testing companies and I uploaded the data to various databases. I got a match on one of them. Within seconds, I was in contact with Janet’s half-sister’s daughter.
Then we have somebody like Jackie [in the second episode]. He’s a Holocaust survivor and was brought to the UK as one of the Windermere children. Looking for relatives of people who were part of the Holocaust is difficult because so many people were killed. Ordinarily, when you upload somebody’s DNA to one of these databases, you get matches – second, third, fourth cousins, even closer than that. With Jackie, we’re weren’t getting them because so many people were killed.
Then I had a breakthrough because these testing companies don’t just look for the matches in terms of you sharing 200 centimorgans; they tell you how large a chunk of DNA you share with somebody. So you get half of your DNA from each of your parents and it comes in quite big chunks of chromosomes. When that DNA is passed down through the generations, those chunks break up into smaller chunks. So the more closely related you are, the bigger the chunks of DNA you share.
I found somebody who shared quite a large chunk of DNA with Jackie, which meant I could start to home in on that family and trace relatives. I can’t tell you the excitement when you think you might have found a relative. You go, “Oh my goodness, is she still alive? Has she got kids?” There’s a real excitement in being able to answer these questions for people. And that’s part of the DNA technology nowadays.
You mentioned earlier that the Y chromosome is very powerful in tracking ancestry, but why the Y chromosome and not the X chromosome?
So we’ve got 23 pairs of chromosomes and pairs 1 to 22 are our autosomes. One half of each pair comes from mum and the other comes from dad and our 22 autosomes are the same between men and women. Pair 23 is the interesting one as it’s sex-determining. Girls have two copies of the X chromosome, one from mum and one from dad. Boys have an X chromosome and a Y chromosome. We also have another piece of DNA known as mitochondrial DNA, and that’s a small circular piece of DNA that’s in the egg.
Autosomes get shuffled with every generation. So when you’re looking at ancestry, it only takes you back a few generations because it’s such a mixture of our ancestors. But mitochondrial DNA just comes down through the female line so you can trace it back from mother to grandmother to great grandmother all the way back through time.
It’s the same with Y chromosomes, you can do that on the male line all the way back and it’s because they don’t recombine, they just get passed down through the generations. They get little, tiny mutations, but we know how those work. So you can go, “Okay, well, this is a Y chromosome type that you tend to find in this part of the world.”
Where do you see the future of this sort of DNA testing going?
The databases are growing like mad. So from when we did the first series to the second series, something like over four million people had their genes tested. I think people are moving more and more towards having their entire genomes tested – people are interested in that kind of thing. And it’s probably to do with issues relating to health. That’s certainly the way things are moving.
So I think it’s going to be the case that these databases are going to be absolutely huge and more and more people will get their entire genomes tested.
- DNA Family Secrets will air on BBC Two and iPlayer at 9pm, 11 May 2022
- This article first appeared in issue 376 of BBC Science Focus Magazine – find out how to subscribe here
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