What can DNA tests really tell us about our ancestry? - Prosanta Chakrabarty

譯者: Lilian Chiu 審譯者: Carol Wang

Two sisters take the same DNA test.

一對姐妹接受了 同樣的 DNA 檢測。

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

結果顯示，其中一位 有 10% 法國血統，

另一位則是 0%。

Both sisters share the same two parents,

她們的生父母都完全相同，

and therefore the same set of ancestors.

因此祖先也相同，

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

其中一位怎會多出 10% 的法國血統呢？

Tests like these rely on our DNA to answer questions about our ancestry,

這類檢測是仰賴我們的 DNA 來提供關於我們祖先的資訊，

but our DNA actually can’t tell us everything

但，關於我們是誰 或我們來自何處，

about who we are or where we’re from.

我們的 DNA 並沒有所有答案。

DNA tests are great at answering some questions,

DNA 檢測很適合回答某些問題，

like who your parents are, but can provide baffling results to others,

比如你的父母是誰，

但對其他問題可能就 無法提供很好的答案，

like whether you have ancestors from a particular region.

比如你是否有來自 某個特定區域的祖先。

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

若要了解為什麼，

需要先知道我們的 DNA 最初是打哪來的。

Each person’s DNA consists of about 6 billion base pairs

每個人的 DNA 都由 六十億個鹼基對組成，

stored in 23 pairs of chromosomes— 46 total.

儲存在 23 對共 46 個染色體當中。

That may seem like a dizzying amount of information,

聽起來是大到讓人頭昏的資訊量，

but 99% of our genome is shared among all humans.

但我們的基因組當中 有 99% 都是所有人類共有的，

The remaining 1% contains everything distinct about an individual’s ancestry.

剩下的 1% 則包含了

個人世系血統的獨有資訊。

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

商業的 DNA 檢測

只用到那 1% 資訊中的不到 1%。

One chromosome in each pair comes from each parent.

每對染色體有兩條， 分別來自父親和母親，

These halves join at conception: when a sperm and egg,

受孕時，這兩條染色體結合：

each with only 23 chromosomes, combine.

精子和卵子分別只有 23 個染色體， 它倆結合時，染色體結合成對，

The story of our ancestry becomes muddled before conception.

在受孕之前，關於我們世系 血統的資訊就已經被搞亂了。

That’s because the 23 chromosomes in a sperm or egg

因為精子或卵子中的 23 個染色體

aren’t identical to the chromosomes of every other cell in the body.

和體內所有其他細胞的染色體不同。

As they go from a cell with 46 chromosomes to a sex cell with only 23,

當它們從 46 個染色體的細胞

變為只有 23 個染色體的性細胞時，

the chromosomes within each pair swap some sections.

該對染色體會交換某些區段，

This process is called recombination, and it means that every sperm or egg

這個過程叫做重組，

意味著每個精子或卵子的單條染色體

contains single chromosomes that are a unique mash up of each pair.

都是一對染色體產出的獨特混合產物。

Recombination occurs uniquely in each sex cell—

每個性細胞中都會發生獨特的重組——

making two sisters’ chromosomes different not only from their parents’,

導致這對姐妹的染色體 不僅和父母不同，

but from each other’s.

也和彼此不同。

Recombination happens before conception,

重組發生在懷孕之前，

so you get exactly half of your DNA from each parent,

你的 DNA 來自雙親， 剛好各佔一半，

but going further back things get more complicated.

但再向前探究，情況就更複雜。

Without recombination, you would get 1/4 from each grandparent,

若沒有重組，

你的 DNA 會來自 四位祖父母，各四分之一，

1/8 from each great-grandparent, and so on,

八位曾曾祖父母， 各佔八分之一，以此類推。

but because recombination happens every generation, those numbers vary.

但，因為每一代都會發生重組， 這些數字會有所不同。

The more generations removed an ancestor is,

越多代以前的祖先，

the more likely they won’t be represented in your DNA at all.

就越有可能無法在 你的 DNA 中呈現出來。

For example, without recombination,

比如，若沒有重組，

just 1/64 of your DNA would come from each ancestor six generations back.

你的 DNA 只有六十四分之一

是來自每一位六代以前的祖先。

Because of recombination, that number can be higher,

重組可能令這個比例更高，

though we don’t know for sure how high— or it can as low as 0.

不過我們無法確定多高——

或者也有可能低到是零。

So one sister isn’t more French

所以，並不是說兩姐妹其中一位

in the sense of having more ancestors from France.

有比較多來自法國的祖先，

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

而只是她的 DNA 中呈現出 比較多法國祖先的資訊。

But the story doesn’t end there.

但，還不只如此。

Tests don’t trace the DNA of the sisters' actual French ancestors—

檢測無法追蹤這對姐妹 真正的法國祖先的 DNA——

we don’t have access to the genomes of deceased individuals

我們無法取得先前世代

已故者的基因組，

from previous generations.

Instead, these results are based on a comparison

這些結果其實是根據比較來的，

to the DNA of people living in France today.

和現今法國居民的 DNA 做比較。

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

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.

遺傳標記是指在特定位置 出現的小段序列。

The sister deemed "more French" shares genetic markers

姐妹中被認為比較「法國」的那位，

和目前法國居民有共同的遺傳標記。

with people currently living in France.

The assumption is that these shared markers indicate ancestors

前提假設是，有這些共同的遺傳標記，

就表示祖先來自同一個地方：法國。

from the same place: France.

It’s important to note that results are based on people

值得注意的是，這些結果的根據

who’ve had their genomes sequenced—

是已經做了基因組定序的人——

80-90% of which are of European descent.

這些人當中有八到九成是歐洲後裔。

Many indigenous peoples are barely represented, if at all.

這些資料當中（幾乎） 沒有原住民的資料，

The test won’t reveal heritage from people not represented in the database,

資料庫中沒有的這些人， 檢測不會顯示出他們的血脈相承，

and shouldn’t be used to prove race or ethnicity.

因此不該用來當作 種族或人種的證明。

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

當有更多人接受定序， 你的檢測結果就有可能改變。

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

再往更早追溯，你得到的結果 可能是 2% 的尼安德特人。

Though Neanderthals were a separate species from humans,

雖然尼安德特人是 和現代人不同的物種，

that 2% doesn’t come out of the 99% of our genome shared among all humans,

但那 2% 並不是來自

所有人類共有的那 99% 基因組，

but the 1% that varies.

而是人人不同的那 1%。

That’s because about 40,000 years ago,

那是因為，大約四萬年前，

certain human populations interbred with Neanderthals,

有某些人類和尼安德特人雜交，

meaning some people alive today have Neanderthal ancestors.

意味著今天活著的一些人 有尼安德特人祖先，

Many Neanderthal ancestors, in fact:

事實上，有許多尼安德特人祖先：

there are so many generations in 40,000 years

四萬年經過了相當多世代，

that a single Neanderthal’s genetic contribution would be untraceable.

不太可能追蹤到單一個 尼安德特人的基因。

You can be both 100% French and 2% Neanderthal—

你有可能是 100% 的法國人， 同時也是 2% 的尼安德特人——

though both come from the 1% of DNA that makes us different,

不過這兩個結果都是來自 大家 DNA 不同的那 1%，

they’re accounting for different things.

正是它們導致了的差異。

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

透過 DNA 來追蹤我們的世系血統，

很快就會變得很複雜。

Both the way we inherit DNA and the information available for testing

因為我們遺傳 DNA 的方式 以及能取用來做檢測的資訊，

makes it difficult to say certain things with 100% certainty.

讓我們很難百分百確定某些事。