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If there’s any one technology that has become a bit of a craze in recent years, it’s DNA ancestry technology. Many different companies now offer the average person insight into their genetic profile, and the wonder of this fascinating, accessible technology has certainly swept the world. But has DNA analysis technology really progressed far enough to provide the kind of accuracy most DNA ancestry services tout? The answer is a bit more complicated than you might think.
First, let’s talk about DNA ancestry technology itself. We humans share over 99.9% genetic similarity with every other human on this planet, so how is it possible for researchers to whittle out the 0.01% that makes us–and our families–unique? Our unique 0.01% is made up of features like Single Nucleotide Polymorphisms (SNPs), which are small changes of single nucleotide “letters” in the entire “book” of our genetic code. The particular SNPs that each of us have inherited are what make us unique individuals, and it is these locations that researchers analyze when using DNA to demonstrate heredity. The extent to which our millions of SNPs are analyzed depends on the method of genetic testing, but most commercial DNA ancestry tests, in the name of maintaining a reasonable price, typically examine only a small percentage of these SNPs to puzzle out a person’s heritage. The sampled locations are referenced against other DNA samples in an organization’s database, and the similarities discovered are then used to match a person to their relatives and take a stab at their global heritage.
As of right now, there are currently three different types of genetic profile services available for use in both public and private settings: Y-chromosome DNA tests, mitochondrial DNA tests, and autosomal DNA tests. Each of these methods have their own unique strengths and weaknesses, and each one also serves a slightly different purpose than the others.
Y-chromosome DNA tests are used to analyze a person’s lineage using a Y-chromosome. Chromosomes are the core packages of DNA found in every cell in our body. Most of the chromosomes we have are known as autosomes, and they are fusions of genetic information from both of our parents. Two specific chromosomes within our genetic code, however, are known as sex chromosomes. We inherit one of these from each of our parents, and the specific combination of sex chromosomes we receive is what determines whether we are born male or female. Y-chromosomes are a type of sex chromosome that can only be passed from father to son; as such, they contain very strong genetic information about a person’s paternal line. If you are a male, you inherited the same genetic information in your Y-chromosome as your father received from his father, and so on. Excepting the occasional genetic mutation, the similarity between Y-chromosomes in the same genetic line is relatively strong and can be used to trace back for many generations. However, this method does have some limitations. For one thing, Y-chromosome testing can only be directly undertaken by someone who has a Y-chromosome; this restricts about half of the population from taking it themselves. For another thing, Y-chromosome testing only traces back one part of a person’s genetic heritage. By the time you’re six generations out, you have sixty-four ancestors, and only one of them is represented by the inherited Y-chromosome. The conclusions that can be drawn from this kind of test are limited, but some fascinating insights, particularly regarding your paternal lineage over a long span of time, can still be gained through this method.
The next form of DNA ancestry testing, mitochondrial DNA testing, also takes advantage of a particular type of DNA that can only be inherited from one parent. Mitochondria are a type of organelle that exists in every cell in our body. As you might recall if you’ve ever taken a biology class, mitochondria are the “powerhouses of the cell,” vital parts of our cells’ ability to function. Mitochondria are passed down from a person’s mother, because they are found overwhelmingly in the female reproductive cells (egg cells) that form the basis of our bodies but are typically not retained from male reproductive cells (sperm cells). Mitochondria are unique among other non-nucleus organelles in that they contain a tiny bit of DNA inside them, and that tiny bit of DNA can be used to trace a person’s maternal heritage–from their mother, to their grandmother, to their great-grandmother, and so on. Though there are less than seventeen thousand base pairs present in mitochondrial DNA, it is very resistant to mutation and, as such, can trace back quite a ways in time reliably. Both males and females have mitochondrial DNA that can be sequenced for mitochondrial DNA tests, but, much like Y-chromosome tests, these tests are of very limited scope because they can only trace back through your direct maternal ancestors.
The final, and most popular, type of DNA testing is autosomal DNA testing. As mentioned earlier, autosomes are all of the non-sex chromosomes found in our cells. Autosomes account for a majority of our genetic information and contain a fusion of DNA from both our parents. Autosomal DNA tests pore through the billions of base pairs found in these chromosomes to look for a handful of specific sites where our DNA differs especially from that of our non-relatives–the Single-Nucleotide Polymorphisms (SNPs) mentioned earlier. Autosomal DNA tests typically reference about one million SNPs to make their judgements. This number sounds large, but it is dwarfed by the multi-billion nucleotide scope of a person’s entire genetic code. Even still, a lot can be learned from these one million sites–though perhaps not as much as most people think.
Autosomal DNA tests do well with matching a person to a wide range of close relatives. Because autosomal DNA is common to all of a person’s relatives, there is no restriction on who a person can match with through autosomal DNA testing. As long as a given relative’s genetic profile exists in the system’s database, it is easy to match with them regardless of their gender or direct relationship to your parents. However, the extent to which a person can match with relatives diminishes the farther back you try to look. Single Nucleotide Polymorphisms are changes to only a single nucleotide; as such, they become increasingly susceptible to being overwritten, either by mutations or the influence of other genetic lines. Though recent relatives will have enough shared SNPs to prove their relation definitively, the ability of SNPs to predict relation grows weaker as the distance in time between you and a relative increases. By the time you go back a few generations, the amount of genetic similarity between you and your great-great-great-and-so-on ancestor has diminished substantially, making it difficult to accurately predict your true relationship. Many DNA ancestry services advertise by showcasing people discovering connections to famous figures, like royal families, but the distance in time between our modern generation and historical people is often too great to be described definitively by commercially available genetic testing.
One particular area where DNA ancestry technology (or rather, the way this technology is shared with the world) doesn’t capture the whole picture is global ancestry. Part of the appeal of DNA ancestry tests is the potential to discover the place your family is originally from; however, our ability to quantify that through genetic testing is very limited as of now, at least when it comes to readily available commercial models. When a DNA ancestry test generates your global heritage, it typically does so based on a bank of genetic information from modern individuals–not historic peoples. This makes sense–after all, genetic profiles have not existed for very long, and the amount of historical data samples that have been catalogued is far outpaced by the availability of modern samples. This means that we typically use DNA ancestry technology to compare modern global peoples, not ancient ones. If you don’t know where your family is from because they arrived in your current country a long time ago, a genetic test might not be able to definitively whittle out the region you came from on account of global population shifts and accumulated genetic changes over time. The extent to which DNA ancestry tests can analyze your similarity to current global groups is also different from many people’s assumptions. Your heritage score, like 85% Italian and 15% Irish, does not represent the “slice” of your DNA that comes from those areas of the world but, rather, the similarity between the unique SNPs in your DNA and those found in samples taken from Italy and Ireland. Similarity tests are the foundation for a lot of work regarding genetic relationships, and you can use the percentages you discover through DNA ancestry tests to make an educated guess at your heritage. However, it is a misnomer to consider your similarity percentage to be the percentage of your DNA that comes from a given region. It would be like comparing your genome to that of a fruit fly and concluding that you’re 60% fruit fly, when you actually just share a 60% similarity in genetic code with a fruit fly.
Genetic analysis technology is a fascinating tool that has developed immensely in recent years. While the results you get from readily available DNA ancestry tests can't always give you the full picture, they do serve as a great introduction to the science of genetic sequencing. As more varied and intensive samples get added to popular databases, it is likely that this technology’s accuracy and usefulness will only continue to increase. For now, tempered expectations may be the best way to interact with popular DNA ancestry tests, but this technology is certainly worth keeping an eye on as it develops into the future.
References & Further Reading
This article, which is an archived version of an article published by University College London, explains key factors about genetic ancestry testing. It offers an insight into the three types of genetic testing, the accuracy of genetic ancestry tests through history, and more. The rest of the website, though worthy of some caution considering it is a public wiki, also offers a primer on other components of understanding the human genome. It may be a worthy place to begin your exploration if you're curious to know more!
This video from TED-ed offers a simple, bite-sized explanation of the successes, and failures, of commercial DNA ancestry tests. While it covers a lot of similar ground to this article, it also goes further into depth on some genetic peculiarities, like crossing over, that can further mess with the global ancestry part of DNA ancestry tests.
This article covers fascinating discoveries that subvert common expectations regarding the inheritance of mitochondrial DNA. While mitochondrial DNA is overwhelmingly inherited from our mothers--so much so that our cells typically have mechanisms for destroying any mitochondria that persist from our fathers' contribution to our first cells--it turns out that there may be niche circumstances in which paternal mitochondrial DNA may persist in our bodies. If you find DNA fascinating, this article is a great place to start learning more about humankind's many peculiarities!
This short article explores some of the species we curiously share a goodly portion of our genetics with. If the fruit fly example in our article surprised you, this is a great place to start learning more about the way we understand and use genetic similarities between species.
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