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While searching for a kidney donor, a Boston woman named Karen Keegan stumbled upon a mystery.
当寻求肾器官捐献者时，一位名叫凯伦·基冈的波士顿妇女偶然发现了一个谜团。
When her three adult sons underwent genetic testing to determine whether they were a match for kidney donation,
当她的三个成年儿子接受了基因测试以确定他们的肾脏捐献是否匹配，
the test showed that two of them weren't actually her sons.
测试显示其中两个实际上不是她儿子。
Keegan knew she was her sons' mother -- she had conceived and given birth to them.
基冈知道她是她儿子的母亲--她怀孕并生下了他们。
Figuring there must have been an error, her doctors pursued further testing,
想到一定有什么错误，她的医生进行了进一步的测试，
only to uncover something even more confusing: she was her children's biological aunt.
但却发现了一些更加令人困惑的事情：她是她的孩子们生物学上的姨妈。
It turned out that Keegan had a second genome in some tissues and organs.
结果是基冈在一些组织和器官中拥有第二个基因组。
In other words, some of her cells had a completely different set of genes from the others.
换句话说，她的一些细胞具有一组完全不同的基因相较于别的细胞。
This second set of genes belonged to her twin sister -- who had never been born.
这第二组基因属于属于她的双胞胎姐妹--她从未出生过。
This condition, where an individual has two genomes present in the tissues of their body, is called chimerism.
这种情况，一个人有两个基因组存在于他们身体的组织中，被称为嵌合体。
The name comes from Greek mythology, where chimera is an amalgam of three different animals.
这个名字来自于希腊神话，其中嵌合体是一种三种不同动物的混合体。
Individuals with chimerism might have two-toned skin or hair, or two different colored eyes,
患有嵌合体的人可能有双色调的皮肤或头发，或两只不同颜色的眼睛，
but most are believed to have no visible signs of the condition.
但大多数人被认为没有明显的可视的迹象。
Chimerism can come from a twin in utero, from a tissue or organ transplant, or happen between a fetus and a pregnant woman.
嵌合体可以来自子宫内的双胞胎，来自组织或器官移植，或发生在胎儿和孕妇之间。
So how exactly does it happen? In one of the most common forms, a mother and fetus swap cells in the flow of nutrients across the placenta.
那么它究竟是如何发生的呢？其中最常见的形式之一，在营养物质跨越胎盘的流动中母亲和胎儿互换细胞。
The mother can inherit fetal stem cells, undifferentiated cells that are able to develop into any specialized cell.
母亲可以继承胎儿干细胞，未分化的细胞能够发展成任何专门的细胞。
The fetal cells initially go undetected because the mother's immune system is suppressed during pregnancy.
胎儿细胞最初未被发现因为母亲的免疫系统在怀孕期间受到抑制。
But in some cases, cells with the fetus's DNA persist in the mother's body for years or even decades without being destroyed by her immune system.
但在某些情况下，带有胎儿DNA的细胞持续存在于母亲的身体里几年甚至几十年，而不被她的免疫系统摧毁。
In one case, a mother's liver was failing, but suddenly started to regenerate itself.
在一个案例中，一位母亲的肝脏正在衰竭，但突然开始自我再生。
Her doctors biopsied her liver, and found DNA in the regenerated tissue from a pregnancy almost 20 years earlier.
她的医生对她的肝脏进行了活检，并在再生的组织中发现了DNA来自近20年前的一次怀孕。
The fetal stem cells had lodged in her liver and specialized as liver cells.
胎儿的干细胞在她的肝脏中着床，特化为肝脏细胞。
Karen Keegan, meanwhile, acquired her second genome before she was born.
与此同时，凯伦·基冈在出生前就获得了她的第二个基因组。
Very early in her own mother's pregnancy with her, Keegan had a fraternal twin.
在她的母亲怀着她的早期，基冈有一个异卵双胞胎。
Keegan's embryo absorbed some fetal stem cells from her twin's embryo, which did not develop to term.
基冈的胚胎吸收了一些她的双胞胎胚胎中的胎儿干细胞，那个胚胎没有发育到足月。
By the time Keegan's fetus developed an immune system, it had many cells with each genome,
当基冈的胎儿形成了免疫系统，它的每个基因组都有很多细胞，
and the immune system recognized both genomes as her body's own -- so it didn't attack or destroy the cells with the second genome.
而免疫系统则确认这两个基因组都是她身体自己的，所以它没有攻击或破坏具有第二基因组的细胞。
We don't know how much of her body was composed of cells with this second genome
我们不知道她身体的组成有多少是带有第二基因组的细胞，
that can vary from one organ to another, and even between tissues within an organ:
不同器官之间会有差异，甚至同一器官的不同组织间也有：
some might have no cells at all with the second genome, while others might have many.
有些可能根本没有第二个基因组的细胞，而另一些可能有很多。
At least some of the egg-producing tissue in her ovaries must have carried the second genome.
在她的卵巢中至少有一些产卵组织肯定携带了第二个基因组。
Each time she conceived there would be no way to predict which genome would be involved
每次她受孕时，都会无从预测哪些基因组会被涉及，
which is how two of her children ended up with the genes of a woman who had never been born.
这就是为什么她两个孩子最终得到的基因来自于一个从未出生的女人。
This can also happen to fathers. In 2014, when ancestry testing determined that a father was actually his baby's biological uncle,
这种情况也可能发生在父亲身上。2014年，当血统测试确定一个父亲实际上是他孩子的生物学上的叔叔，
researchers discovered that 10% of the father's sperm carried a second genome from an embryonic twin.
研究人员发现，10%的的父亲的精子携带来自胚胎期的双胞胎的第二个基因组。
Cases like this challenge our perception of genetics.
这样的案例挑战我们对遗传学的认知。
Though there are very few documented cases of chimerism from an embryonic twin,
尽管胚胎双胞胎嵌合体的案例记录极少，
we're all amalgams to some extent, carrying around the different genetic codes of our gut bacteria and even our mitochondria.
我们在某种程度上都是嵌合体，携带着肠道细菌的不同的遗传密码，甚至我们线粒体的。
And given that 1 in 8 individual births started out as twin pregnancies,
鉴于每8个新生儿中就有一个开始时是双胞胎怀孕，
there could be many more people with two genomes -- and many more lessons to learn about the genes that make us who we are.
可能有更多拥有两个基因组的人，以及关于造就我们的基因更多需要学习的课程。
