Genetic testing is not a concept that most people encounter outside of popular TV shows and so when you hear about services such as 23andMe’s, you might be wondering what exactly it is you’re getting. Even after you’ve signed up and have your data back, you might not know where to start exploring or how to make sense of the wealth of information you’ve just received. Since many people are unfamiliar with genetics and 23andMe offers several unique features, we’ll be presenting a series of posts to help both current customers and the curious reader to understand what you can (and can’t) learn from genetics and from 23andMe in particular.This first post is part of the “Ancestry at 23andMe” series and is an introduction to the types of information we can tell you about your ancestry. (There will also be a “Health at 23andMe” series to cover the features found in the Health portion of our service.) It may be a review for some, but should help orient those new to genetics or 23andMe (for an even more basic primer, check out our videos in Genetics 101). Read on, and let us know if you have a question you would like to see featured in future articles on the 23andMe blog! Ancestry at 23andMe: What Can You Learn?
Each type of DNA has unique properties that are informative for ancestry. Because your mitochondrial DNA comes all in one piece from your mother — and she received it from her mother, and so on — we can say pretty definitively where that piece of your DNA came from. In fact, scientists can map all mtDNA onto a family tree of sorts, tracing back to a “mitochondrial Eve”, the common female ancestor that gave rise to all variations of mtDNA around today.This mapping is possible because DNA isn’t always copied perfectly when it’s passed from parent to child. With each successive generation, the changes spread and become the background from which new changes arise, serving both to distinguish different maternal lineages from each other and to connect lineages to each other at some point in genealogical time. So one thing that 23andMe can tell you is where that piece of DNA — from your mother’s mother’s mother’s ancestors — is from. (23andMe customers can see this information in the Maternal Line feature of their account.)Similarly, because the Y chromosome is passed down exclusively from father to son, we can use it to trace the ancestry of a small part of your DNA if you are male. The same principle of DNA variation spreading and building up through generations allows scientists to construct a tree for all the paternal lineages and allows 23andMe to map a man’s Y chromosome DNA to a particular branch in that tree. (Male 23andMe customers can see this information in the Paternal Line feature of their account.)
|Your maternal — and for some, paternal — line gives you an interesting piece of your DNA history, but it is a very small part. Your autosomal and X chromosome DNA make up almost 99% of all your DNA, and is passed down from both parents (except for the X chromosome in males, which comes only from the mother). This means that the DNA can be used to infer more distant relatedness between individuals on any branch of their family tree — 4th cousins, for example, who share common great-great-great-grandparents — based on the percentage of DNA shared. Thousands of people are already making connections with distant relatives through 23andMe’s database. (23andMe customers can explore our DNA Relatives (Relative Finder) and
Unlike maternal and paternal line analysis, autosomal and X chromosome DNA analysis cannot tell us much definitively past a certain number of generations. Because each pair of chromosomes recombines, or mixes, before being passed down from parent to child, the size of possible shared segments gets successively smaller with each generation. Too small, and it’s no longer possible to tell whether the segment truly came from a shared common ancestor, or if it’s identical simply by chance. The information we can infer, however, is much more versatile. And although scientific methods using autosomal and X chromosome DNA are not as firmly established or plentiful as those used for mtDNA and Y chromosome analyses, this should change as more research accumulates.The cool part is that all of these features we’ve just mentioned are just the tip of the iceberg. One thing that separates 23andMe’s services from most other genetic ancestry services is that we use data about hundreds of thousands of variants in the human genome — places where a single DNA letter can vary between individuals, called single nucleotide polymorphisms, or SNPs (“snips”). This offers greater resolution and versatility than more traditional methods, allowing us to provide a unique and constantly evolving set of tools for exploring your ancestry.In the next post in this series, we illustrate 23andMe’s ancestry features in more detail by following some (fictional, but based on real) customers as they explore their 23andMe data.
Other posts in this series:Introducing Sheridan – We meet Sheridan and she learns about her maternal haplogroups.Sheridan’s Global Origins – Sheridan compares herself and her friends to various populations around the world.Sheridan’s Got Relatives – Sheridan responds to a fifth cousin and discovers a predicted second cousin in the 23andMe database.Finding Connections – Sheridan and Brian bond over their fourth cousin connection.Old Roots and New Horizons – Sheridan and Brian put together the pieces to sketch out their shared family tree.