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  • [(They) always are asking me the vexed question, 'where do we come from?'"- Joseph Dalton Hooker, in a letter to Charles Darwin.]

  • Two sisters take the same DNA test.

  • The results show that one sister is 10 percent French, the other 0 percent.

  • Both sisters share the same two parents, and therefore the same set of ancestors.

  • So how can one be 10 percent more French than the other?

  • Tests like these rely on our DNA to answer questions about our ancestry, but our DNA actually can't tell us everything about who we are or where we're from.

  • DNA tests are great at answering some questions, like who your parents are, but can provide baffling results to others, like whether you have ancestors from a particular region.

  • To understand why, it helps to know where our DNA comes from in the first place.

  • Each person's DNA consists of about 6 billion base pairs stored in 23 pairs of chromosomes -- 46 (in) total.

  • That may seem like a dizzying amount of information, but 99 percent of our genome is shared among all humans.

  • The remaining 1 percent contains everything distinct about an individual's ancestry.

  • Commercial DNA tests utilize less than 1 percent of that 1 percent.

  • One chromosome in each pair comes from each parent.

  • These halves join at conception: when a sperm and egg, each with only 23 chromosomes, combine.

  • The story of our ancestry becomes muddled before conception.

  • That's because the 23 chromosomes in a sperm or egg aren't identical to the chromosomes of every other cell in the body.

  • As they go from a cell with 46 chromosomes to a sex cell with only 23, the chromosomes within each pair swap some sections.

  • This process is called recombination, and it means that every sperm or egg contains single chromosomes that are a unique mash up of each pair.

  • Recombination occurs uniquely in each sex cellmaking two sisters' chromosomes different not only from their parents', but from each other's.

  • Recombination happens before conception, so you get exactly half of your DNA from each parent, but going further back things get more complicated.

  • Without recombination, you would get 1/4 from each grandparent, 1/8 from each great-grandparent, and so on, but because recombination happens every generation, those numbers vary.

  • The more generations removed an ancestor is, the more likely they won't be represented in your DNA at all.

  • For example, without recombination, just 1/64 of your DNA would come from each ancestor six generations back.

  • Because of recombination, that number can be higher, though we don't know for sure how highor it can be as low as 0.

  • So one sister isn't more French in the sense of having more ancestors from France.

  • Instead, the French ancestors are simply more represented in her DNA.

  • But the story doesn't end there.

  • Tests don't trace the DNA of the sisters' actual French ancestorswe don't have access to the genomes of deceased individuals from previous generations.

  • Instead, these results are based on a comparison to the DNA of people living in France today.

  • The tests look for genetic markers, or combinations of genetic markers.

  • These markers are short sequences that appear in specific places.

  • The sister deemed "more French" shares genetic markers with people currently living in France.

  • The assumption is that these shared markers indicate ancestors from the same place: France.

  • It's important to note that results are based on people who've had their genomes sequenced — 80-90% of which are of European descent.

  • Many indigenous peoples are barely represented, if at all.

  • The test won't reveal heritage from people not represented in the database, and shouldn't be used to prove race or ethnicity.

  • And as more people get sequenced, your results might change.

  • Looking further back, you may get a result like 2% Neanderthal.

  • Though Neanderthals were a separate species from humans, that 2% doesn't come out of the 99% of our genome shared among all humans, but the 1% that varies.

  • That's because about 40,000 years ago, certain human populations interbred with Neanderthals, meaning some people alive today have Neanderthal ancestors.

  • Many Neanderthal ancestors, in fact: there are so many generations in 40,000 years that a single Neanderthal's genetic contribution would be untraceable.

  • You can be both 100% French and 2% Neanderthalthough both come from the 1% of DNA that makes us different, they're accounting for different things.

  • Looking for traces of our ancestry in our DNA gets complicated very quickly.

  • Both the way we inherit DNA and the information available for testing makes it difficult to say certain things with 100% certainty.

  • When you think of the discovery of DNA, you probably think of two names: Watson and Crick, but there is a third unsung scientist whose name you should know.

  • Get the story of the woman behind the double helix with this video.

[(They) always are asking me the vexed question, 'where do we come from?'"- Joseph Dalton Hooker, in a letter to Charles Darwin.]

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What Can DNA Tests Really Tell Us About Our Ancestry? - Prosanta Chakrabarty

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    Celine Chien posted on 2020/08/24
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