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Two sisters take the same DNA test. The results show that one sister is 10% French, the other 0%.
两姐妹做了相同的DNA测试。检测结果表明，其中一位带有10%的法国血统，另一位却没有。
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?
两姐妹有着相同的父母，因此具有相同的祖先。那为什么其中一位的法国血统比另一位多了10%？
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.
类似这样的测试依靠DNA来一探我们的祖先，可是DNA并不能解答所有关我们是谁和我们来自哪里的所有疑问。
DNA tests are great at answering some questions, like who your parents are,
DNA测试非常擅于回答一些问题，比如谁是我们的亲生父母，
but can provide baffling results to others, like whether you have ancestors from a particular region.
却不能清晰的解答其他的问题，比如你是否有特定地区的祖先。
To understand why, it helps to know where our DNA comes from in the first place.
为了了解其中的原因，我们需要先了解DNA从何而来。
Each person's DNA consists of about 6 billion base pairs stored in 23 pairs of chromosomes -- 46 total.
每一个人的DNA都含有大约60亿个碱基对，存储于23对，总共46条染色体中。
That may seem like a dizzying amount of information, but 99% of our genome is shared among all humans.
这些数字看似令人眼花缭乱，但99%的基因组是所有人共享的。
The remaining 1% contains everything distinct about an individual's ancestry.
那剩下的1%则包含了所有人祖先之间的差异。
Commercial DNA tests utilize less than 1% of that 1%. One chromosome in each pair comes from each parent.
市面上的DNA测试工具只能向我们揭示那1%中1%的信息。每对染色体中的其中一条来自其中一位父母。
These halves join at conception: when a sperm and egg, each with only 23 chromosomes, combine.
当母体受孕时，这两条染色体相互接触，即各自携带23条染色体的精子和卵子会彼此结合。
The story of our ancestry becomes muddled before conception.
我们祖先的故事在受孕前是难以捉摸的。
That's because the 23 chromosomes in a sperm or egg aren't identical to the chromosomes of every other cell in the body.
这是因为精子和卵子中的23条染色体与体内其余细胞的染色体并不相同。
As they go from a cell with 46 chromosomes to a sex cell with only 23, the chromosomes within each pair swap some sections.
当体内其余带有46条染色体的细胞变成只有23条染色体的性细胞时，每对染色体中的某些信息会被提取和合并。
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.
这个过程被称为重组，这代表每一个精子和卵子含带的染色体经过“混搭”而变得独一无二。
Recombination occurs uniquely in each sex cell -- making two sisters' chromosomes different not only from their parents', but from each other's.
重组也会在每个性细胞中单独进行，使两个姐妹的染色体不仅与亲本有着不同的基因组合，而且彼此也不相同。
Recombination happens before conception, so you get exactly half of your DNA from each parent, but going further back things get more complicated.
重组发生在受孕前，这使你拥有来自父母各一半的DNA，但越往前推，情况就越复杂。
Without recombination, you would get 1/4 from each grandparent, 1/8 from each great-grandparent, and so on,
假设重组并不存在，你将会得到你祖父母DNA的1/4，曾祖父母DNA的1/8，以此类推，
but because recombination happens every generation, those numbers vary.
可正因为重组会在每一代发生，这些数据每次都会有所变化。
The more generations removed an ancestor is, the more likely they won't be represented in your DNA at all.
你与你祖先相隔的辈数越多，他们DNA就越不可能出现在你的DNA当中。
For example, without recombination, just 1/64 of your DNA would come from each ancestor six generations back.
比如，我们假设重组不会发生，你的DNA中只有1/64来源于你的六代之前的每一位祖先。
Because of recombination, that number can be higher, though we don't know for sure how high -- or it can as low as 0.
但由于重组，这个数字可能会更大，我们并不确定它有多大--或者也可能小到接近于0。
So one sister isn't more French in the sense of having more ancestors from France.
所以说，两姐妹之一有更多来自法国的祖先，并不意味着她有更多法国血统。
Instead, the French ancestors are simply more represented in her DNA. But the story doesn't end there.
而是因为有更多法国祖先的DNA出现在了她的DNA中。但故事还远没有这么简单。
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.
测试并不能追踪两姐妹真正的法国祖先的基因--我们无法获得前几代已故个体的基因组。
Instead, these results are based on a comparison to the DNA of people living in France today.
相反，这些测试结果依赖于你的基因组与当代法国人基因组的差异。
The tests look for genetic markers, or combinations of genetic markers. These markers are short sequences that appear in specific places.
这些测试是基于遗传标记或遗传标记组合，而这些标记是出现于特定地区的短序列。
The sister deemed "more French" shares genetic markers with people currently living in France.
那位更“法国”的姐妹只不过是与当代住在法国的人有着更相似的基因标记。
The assumption is that these shared markers indicate ancestors from the same place: France.
这种假设阐述的是，这些共享的标记表明他们的祖先来自于同一地区：法国。
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.
需要注意的是，这些结果都是基于已经做过基因测试的人，其中80%-90%为欧洲血统。
Many indigenous peoples are barely represented, if at all.
土著人民的DNA几乎都没有被记录。
The test won't reveal heritage from people not represented in the database, and shouldn't be used to prove race or ethnicity.
因此，这种测试无法给出没有被数据库收录的人群的遗传信息，也不应该被当作一个人种族背景的证据。
And as more people get sequenced, your results might change.
你的测试结果会随着数据库的增大而变化。
Looking further back, you may get a result like 2% Neanderthal.
再往前推，你的测试结果可能表明你带有2%尼安得特人的血统。
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.
尽管尼安得特人并非与人类属于同一物种，那2%并不来源于人类共享的那99%的基因组，而是来自于那独特的1%。
That's because about 40,000 years ago, certain human populations interbred with Neanderthals, meaning some people alive today have Neanderthal ancestors.
这是因为在大约4万年前，个别人类与尼安得特人杂交，这表明有些当代人的祖先是尼安得特人，甚至很多尼安得特人。
Many Neanderthal ancestors, in fact: there are so many generations in 40,000 years that a single Neanderthal's genetic contribution would be untraceable.
他们在4万年内拥有许多世代，这导致了单个尼安得特人的基因贡献是无法被追踪到的。
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.
你可以同时拥有100%的法国血统和2%尼安得特人血统--虽然它们都来自于使我们与众不同的那1%的基因，但它们解释的却是不同的事情。
Looking for traces of our ancestry in our DNA gets complicated very quickly.
利用DNA来寻找我们祖先的踪迹很快就会变得非常复杂。
Both the way we inherit DNA and the information available for testing makes it difficult to say certain things with 100% certainty.
我们继承DNA的方式以及可供测试的信息使得我们很难笃定结论。
