Two sisters take the same DNA test.

兩個姐妹進行相同的 DNA 檢測。

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

結果顯示，一個有 10% 的法國血統，另一個卻是 0%。

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

兩個姐妹有一樣的父母，因此祖先也相同。

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

那麼，為什麼其中一位會比另一位多出 10% 的法國血統？

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.

像這樣的檢測需要依賴我們的 DNA 來解答關於我們祖先的問題，但我們的 DNA 實際上並不能告訴我們是誰或來自哪裡。

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.

DNA 檢測在解答某些問題上很有優勢，比如你的父母是誰，但在其他問題上可能會有令人困惑的結果，比如你的祖先是否來自某個地區。

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

如果想了解背後的故事，首先要先了解我們的 DNA 來自哪裡。

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

每個人的 DNA 由大約 60 億個鹼基對組成，而儲存在 23 對染色體中——共 46 條。

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

這似乎是一個令人眼花撩亂的資訊量，但我們 99% 的基因組是全人類共享的。

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

剩下的 1% 包含了關於一個人的祖先來源所有獨特的特徵。

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

商業 DNA 檢測利用了這 1% 的不到 1%。

One chromosome in each pair comes from each parent.

每對染色體中的一條染色體各別來自於父母。

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

這兩條染色體在受孕時結合：當一個只有 23 條染色體的精子和卵子結合時。

The story of our ancestry becomes muddled before conception.

我們祖先來源的故事在受孕前就變得很混亂。

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

這是因為精子或卵子中的 23 條染色體和身體其他細胞的染色體並不一樣。

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

當它們從一個有 46 條染色體的細胞到一個只有 23 條染色體的性細胞時，每對細胞內的染色體都會有一些部分進行交換。

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.

這個過程被稱為重組，它意味著每個精子或卵子都含有單條染色體，是每對染色體的獨特混搭。

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

重組在每個性細胞中進行的方式都不太一樣——導致兩姊妹的染色體不但和父母的不同，也和彼此不同。

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

重組會在受孕前進行，所以你會從父母各自得到一半的 DNA，但是如果要更詳細探討的話，事情會變得更複雜。

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.

如果沒有進行重組的話，你會從祖父母各自得到 1/4 的 DNA ，從曾祖父母各自得到 1/8 ，以此類推，但因為每一代都會進行重組，導致這些數字會有所不同。

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

越久以前的祖先就越不可能被顯示在你的 DNA 中。

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

舉例來說，如果沒有進行重組的話，你的 DNA 只有 1/64 會來自六代以前的祖先。

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

重組導致那個數字有可能更高，雖然我們也不確定可以多高，但是也有可能低至 0。

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

結論是，其中一位並不是因為有較多法國的祖先才顯示較多法國血統。

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

事實上，法國的祖先只是在她的 DNA 中顯示較多。

But the story doesn't end there.

但故事並沒有在此結束。

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.

這些檢測並不是追蹤這對姊妹真正法國祖先的 DNA — 我們無法取得前幾代死者的基因組。

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

事實上，這些結果是來自現今生活在法國的人 DNA 的比較。

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

這些檢測會尋找遺傳標記，或遺傳標記的組合。

These markers are short sequences that appear in specific places.

這些標記是出現在特定位置的短序列。

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

被認為有「更多法國血統」的那一位與目前生活在法國的人有相同的遺傳標記。

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

我們預設的狀況是這些共同標記代表著來自同個地方的祖先：法國。

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，結果是基於已經進行基因組定序的人，而其中 80-90% 的人是歐洲人的後裔。

Many indigenous peoples are barely represented, if at all.

許多原住民的基因組並沒有定序過。

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

這個檢測並不會顯示資料庫內沒有數據的祖先來源，也不能拿來證明種族或文化認同。

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

而隨著越來越多人被定序，你的檢測結果有可能會改變。

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

如果再更詳細探討的話，你有可能會得到像 2% 尼安德特人這樣的檢測結果。

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.

雖然尼安德特人是和人類不同的物種，那 2% 並不是來自於我們和人類共享的 99% 基因組，而是會變動的那 1%。

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

這是因為大約 4 萬年前，某些人類和尼安德特人進行雜交，這意味著現今活著的人有尼安德特人的祖先。

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

事實上，尼安德特人的祖先很多：4 萬年間有太多代的人，導致一個尼安德特人的基因貢獻是無法被追溯到的。

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.

你可以是 100% 的法國人，也可以是 2% 的尼安德特人 — 即便兩者都來自於使我們不同的 1% DNA，但他們所佔的比例是不同的。

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

在我們的 DNA 中尋找祖先的蹤跡，事情很快就會變得很複雜。

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

無論是遺傳 DNA 的方式或是可以檢測的資訊，都讓我們很難百分之百地確認某些事情。

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.

當你想到 DNA 結構的發現時，你可能會想起兩個人的名字：沃森和克里克，但有第三位科學家的名字你應該要知道。

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

看下一支影片來了解發現雙螺旋的女性背後的故事。