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

翻译人员: Zining Chen 校对人员: Yolanda Zhang

Two sisters take the same DNA test.

两姐妹做了相同的 DNA 测试。

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

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,

两姐妹有着相同的父母，

and therefore the same set of ancestors.

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%？

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

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

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

but our DNA actually can’t tell us everything

about who we are or where we’re from.

和我们来自哪里的所有疑问。

DNA tests are great at answering some questions,

DNA 非常擅于回答一些问题，

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

比如谁是我们的亲生父母， 却不能清晰的解答其他的问题，

like whether you have ancestors from a particular region.

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

每一个人的 DNA 都含有大约 60 亿个碱基对，

stored in 23 pairs of chromosomes— 46 total.

存储于 23 对，总共 46 条染色体中。

这些数字看似令人眼花缭乱，

That may seem like a dizzying amount of information,

但 99% 的基因组是所有人共享的。

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

那剩下的 1% 则包含了 所有人祖先之间的差异。

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

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

市面上的 DNA 测试工具 只能向我们揭示那 1% 中 1% 的信息。

One chromosome in each pair comes from each parent.

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,

当母体受孕时， 这两条染色体相互接触，

each with only 23 chromosomes, combine.

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

这是因为精子和卵子中的 23 条染色体

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

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,

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，

the chromosomes within each pair swap some sections.

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

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

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

经过“混搭”而变得独一无二。

Recombination occurs uniquely in each sex cell—

重组也会在每个性细胞中单独进行，

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

使两个姐妹的染色体 不仅与亲本有着不同的基因组合，

but from each other’s.

而且彼此也不相同。

Recombination happens before conception,

重组发生在受孕前，

这使你拥有来自父母各一半的 DNA，

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

but going further back things get more complicated.

但越往前推，情况就越复杂。

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

假设重组并不存在， 你将会得到你祖父母 DNA 的 1/4，

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

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,

你与你祖先相隔的辈数越多，

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

他们 DNA 就越不可能 出现在你的 DNA 当中。

For example, without recombination,

比如，我们假设重组不会发生，

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

你的 DNA 中只有 1/64 来源于你的六代之前的每一位祖先。

但由于重组，这个数字可能会更大，

Because of recombination, that number can be higher,

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

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

所以说，两姐妹之一 有更多来自法国的祖先，

in the sense of having more ancestors from France.

并不意味着她有更多法国血统，

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

而是因为有更多法国祖先的 DNA 出现在了她的 DNA 中。

But the story doesn’t end there.

但故事还远没有这么简单。

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

测试并不能追踪两姐妹 真正的法国祖先的基因——

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

我们无法获得前几代

from previous generations.

已故个体的基因组。

Instead, these results are based on a comparison

相反，这些测试结果依赖于

to the DNA of people living in France today.

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.

这些测试是基于 遗传标记或遗传标记组合，

These markers are short sequences that appear in specific places.

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

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。

with people currently living in France.

The assumption is that these shared markers indicate ancestors

这种假设阐述的是， 这些共享的标记表明他们的祖先

from the same place: France.

来自于同一地区：法国。

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

需要注意的是， 这些结果都是基于

who’ve had their genomes sequenced—

已经做过基因测试的人，

80-90% of which are of European descent.

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,

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，

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

也不应该被当作 一个人种族背景的证据。

你的测试结果 会随着数据库的增大而变化。

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

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，

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

Though Neanderthals were a separate species from humans,

尽管尼安得特人并非 与人类属于同一物种。

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

那 2% 并不来源于 人类共享的那 99% 的基因组，

but the 1% that varies.

而是来自于那独特的 1%。

That’s because about 40,000 years ago,

这是因为在大约 4 万年前，

certain human populations interbred with Neanderthals,

个别人类与尼安得特人杂交，

meaning some people alive today have Neanderthal ancestors.

这表明有些当代人的祖先 是尼安得特人，

Many Neanderthal ancestors, in fact:

甚至很多尼安得特人。

there are so many generations in 40,000 years

他们在 4 万年内拥有许多世代，

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

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—

你可以同时拥有 100% 的法国血统 和 2% 尼安得特人血统——

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

虽然它们都来自于 使我们与众不同的那 1% 的基因，

they’re accounting for different things.

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

我们继承 DNA 的方式 以及可供测试的信息

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

使得我们很难笃定结论。