Two sisters take the same DNA test. The results show that one sister  is 10% French, the other 0%. Both sisters share the same two parents, and therefore the same set  of ancestors. So how can one be 10% 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 total. That may seem like a dizzying amount  of information, but 99% of our genome  is shared among all humans. The remaining 1% contains everything distinct about an individual’s ancestry. Commercial DNA tests utilize  less than 1% of that 1%. 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 cell— making 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 high— or it can 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 ancestors— we 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% Neanderthal— though 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.