But what are they really basing that on? Identical twins have virtually identical DNA. Not necessarily, according to a recent investigation by the Canadian Broadcasting Corporation. So what accounts for these differences? They have more to do with the limitations of the science and some key assumptions companies make when analyzing DNA for ancestry.
The companies that provide ancestry testing, like Ancestry. They also market their product in a way that suggests their test reveals something deeply meaningful about you. Your results from one company can even change over time as the company signs up more users, and gathers more data.
The companies do have webpages that explain these limitations, but you have to dig a bit to find them. You could very easily purchase one of these kits without coming across it. Consumer genetic testing is growing explosively. According to the MIT Technology Review , 26 million people or more have taken a genetic ancestry test. Tech Review also found that in , the number of tests purchased surpassed sales of all previous years combined.
As the market grows, consumers need to be aware of what exactly these tests are telling them and even more so when it comes to information about health and wellness. Even though genetic ancestry tests deliver precise percentages about our heritage, the reports are best thought of estimates, based on imperfect data. There are about 3 billion base pairs — the individual letter instructions of our genetic code — that make up the human genome.
When you spit into a tube and send it off to a company like 23andMe, Ancestry. That would be overkill. All humans have about So instead, to speed up the process, the tests look out for the locations on the genome where people commonly vary from one another.
These are spots where you might have the nucleotide the molecule that forms one half of a base pair adenine and I have thymine. In all, these single-letter changes in our DNA can help explain why one person is taller than another, or why one has brown eyes and another green. Companies can analyze half a million SNPs or more in an ancestry test.
When a genetic testing company gets a tube of your saliva in the mail, it first has to extract the DNA from it. They make copies of your DNA, then break those stands up into shorter chunks. The chunks are then fed into a machine called a genotyping array. These arrays kind of — and this is an absolute simplification — work like a coin sorter, but for SNPs. Many SNPs are meaningless when it comes to our health. But they can be useful starting points for tracing ancestry.
The more SNPs we share in common with another person, the more likely we share a similar, and more recent, ancestry. Your ancestry is estimated by comparing your SNP results with a genetic database of people with known ancestries more on this in the next section. But even with that high level of accuracy, when you process 1 million places in the genome, you might get 1, errors.
Those small errors alone can help explain why one twin might have slightly different results from another. This source of error is why the health results you get back from genetic testing companies may show discrepancies too. Errors aside, the genotyping we get from each of the consumer testing companies should be just about identical to one another that is, if the companies are looking at the same set and number of SNPs.
But how companies analyze that raw data varies. Companies like 23andMe, Ancestry. The tests are looking for evidence that you have common ancestors with people in the reference group.
But the reference group each company uses can be different. And the reference groups are changing all the time. As STAT news reports , people who used these tests just a few years ago are now finding their results have changed. Nowadays, most of the big genetic testing companies pitch themselves primarily as "ancestry" services, promising both to connect long-lost relatives and to tell users what parts of the world their ancestors came from.
For more 23andme information. For more AncestryDNA information. Customers send spit samples to these companies. Then, usually about two months later, they log in to their accounts to find personalized web pages with information like their percentage of South Asian ancestry, or Neanderthal ancestry, or details about their maternal and paternal lines.
Those are the labels given to the four nucleobases of DNA, the letters with which genes are written. This string of letters would be incomprehensible to you and, on their own, just as incomprehensible to the biologists and engineers who study them.
There's no string of letters that means "Swiss" or "Nigerian," for example. But the algorithms can pull meaning out of the strings of letters, Smith said. These companies keep the details of their algorithms somewhat secret. But it's not that their computers speak some secret language. Instead, according to geneticist Mark Stoneking, group leader of the Max Planck Institute for Evolutionary Anthropology in Germany, they're really good at spotting patterns. He used a version of these methods in his pioneering work tracing the common ancestor of all living humans, a woman referred to as " Mitochondrial Eve " who lived about , years ago.
And researchers still use these methods to track the movements and intermixing of human populations from the deep past to recent history. If a genetic anthropologist has a DNA sample and a very large library of other samples to compare it against, that anthropologist can quickly figure out which groups in the library that DNA is most closely related to, Stoneking said.
Researchers can track paternal ancestry by looking at the Y chromosome, which fathers pass to their male children. Maternal ancestry, similarly, can be found in mitochondrial DNA, which mothers pass to all of their children.
The richest and most detailed ancestry information, however, comes from comparing everything else — the 22 non-sex chromosomes — against the massive libraries.
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