You could have a secret twin (but not the way you think) - Kayla Mandel Sheets

譯者: Lilian Chiu 審譯者: Helen Chang

While searching for a kidney donor,

在尋找腎臟捐贈者時，

a Boston woman named Karen Keegan stumbled upon a mystery.

波士頓女子凱倫·奇根碰到了一個謎。

When her three adult sons underwent genetic testing

當她的三位成年兒子去接受基因檢測

to determine whether they were a match for kidney donation,

以判斷他們是否符合 腎臟捐贈的配對，

the test showed that two of them weren’t actually her sons.

檢測結果顯示他們當中 有兩位並非她的兒子。

Keegan knew she was her sons’ mother—

奇根知道她是她兒子的母親——

she had conceived and given birth to them.

她懷了他們、生下他們。

Figuring there must have been an error, her doctors pursued further testing,

她的醫生認為一定有哪裡 出錯了，便做了進一步檢測，

only to uncover something even more confusing:

結果卻更令人費解。

she was her children’s biological aunt.

她是她孩子的親生阿姨。

It turned out that Keegan had a second genome in some tissues and organs.

最後發現，奇根的某些組織 和器官中有第二組基因組。

In other words, some of her cells had a completely different set of genes

換言之，

她體內有些細胞的基因 和其他細胞完全不同。

from the others.

This second set of genes belonged to her twin sister—

這第二組基因屬於 她的雙胞胎姐妹——

who had never been born.

從未出生的姐妹。

This condition, where an individual has two genomes

這種狀況，

也就是一個人體內的組織中 有兩組基因組出現，

present in the tissues of their body,

is called chimerism.

叫做嵌合現象。

The name comes from Greek mythology,

這個名稱出自希臘神話，

where chimera is an amalgam of three different animals.

神話中，客邁拉是 三種動物的混合體。

Individuals with chimerism might have two-toned skin or hair,

嵌合人可能會有兩種膚色或髮色，

or two different colored eyes,

或兩眼的顏色不同，

but most are believed to have no visible signs of the condition.

但一般相信大部分嵌合人 都沒有可看見的特徵。

Chimerism can come from a twin in utero, from a tissue or organ transplant,

嵌合現象可能來自於胎內的雙胞胎，

或來自組織或器官的移植，

or happen between a fetus and a pregnant woman.

或發生在胎兒與懷孕女性之間。

So how exactly does it happen?

那麼，這到底是怎麼發生的？

In one of the most common forms,

最常見的形式之一是：

a mother and fetus swap cells in the flow of nutrients

母親和胎兒透過胎盤傳遞營養

across the placenta.

來交換細胞。

The mother can inherit fetal stem cells,

母親可能會得到胎兒的幹細胞，

undifferentiated cells that are able to develop into any specialized cell.

沒有明顯特徵的細胞， 無法發展成任何特化細胞。

The fetal cells initially go undetected

胎兒細胞一開始不會被偵測到，

because the mother’s immune system is suppressed during pregnancy.

因為母親的免疫系統 在懷孕期間會被壓制。

But in some cases, cells with the fetus’s DNA persist

但在某些情況下，

帶有胎兒 DNA 的細胞會 存留在母親體內長達數年，

in the mother’s body for years or even decades

甚至數十年，

without being destroyed by her immune system.

都沒有被她的免疫系統摧毀。

In one case, a mother's liver was failing,

有一個案例是一位 母親的肝臟即將衰竭，

but suddenly started to regenerate itself.

但突然間卻自己開始再生。

Her doctors biopsied her liver, and found DNA in the regenerated tissue

醫生做了肝臟切片檢查，

在再生的組織中找到的 DNA

from a pregnancy almost 20 years earlier.

竟是來自近二十年前的懷孕期。

The fetal stem cells had lodged in her liver and specialized as liver cells.

胎兒的幹細胞住在她的肝臟中，

被特化為肝臟細胞。

Karen Keegan, meanwhile, acquired her second genome before she was born.

至於凱倫奇根則是

在她出生之前就取得了 她的第二組基因組。

Very early in her own mother’s pregnancy with her,

在她母親剛懷有她的初期，

Keegan had a fraternal twin.

奇根有個異卵雙胞胎。

Keegan’s embryo absorbed some fetal stem cells from her twin’s embryo,

奇根的胚胎從她雙胞胎的胚胎 吸收了一些胎兒幹細胞，

which did not develop to term.

雙胞胎的胚胎沒能發展到足月。

By the time Keegan’s fetus developed an immune system,

等到奇根的胎兒發展出了免疫系統，

it had many cells with each genome,

兩個基因組都有許多細胞，

and the immune system recognized both genomes as her body’s own—

而免疫系統認為這兩組基因組 都屬於她的身體——

so it didn’t attack or destroy the cells with the second genome.

所以它沒有攻擊或摧毀 帶有第二組基因組的細胞。

We don’t know how much of her body was composed of cells with this second genome—

我們不知道她身體有多少比例是由 帶著第二組基因組的細胞構成——

that can vary from one organ to another, and even between tissues within an organ:

每個器官都不同，

甚至同一器官中的組織也不同：

some might have no cells at all with the second genome,

有些可能完全沒有帶著 第二組基因組的細胞，

while others might have many.

有些則可能有很多。

At least some of the egg-producing tissue in her ovaries

至少，她的卵巢中 產生卵子的某些組織

must have carried the second genome.

一定有帶著第二組基因組。

Each time she conceived there would be no way to predict

每次她懷孕，都無法預測

which genome would be involved—

哪組基因組會參與——

which is how two of her children ended up with the genes

這就是為什麼她有兩個孩子

最後得到的基因竟是來自 一名從來沒有出生的女子。

of a woman who had never been born.

This can also happen to fathers.

這個狀況也可能發生在父親身上。

In 2014, when ancestry testing determined that a father

2014 年，

有一位父親在血統檢測之後 發現他是他孩子的親生叔叔，

was actually his baby’s biological uncle,

researchers discovered that 10% of the father’s sperm

研究者發現這位父親的精子有 10%

carried a second genome from an embryonic twin.

帶著來自胚胎雙胞胎的 第二組基因組。

Cases like this challenge our perception of genetics.

像這樣的例子挑戰著 我們對遺傳學的觀念。

Though there are very few documented cases of chimerism from an embryonic twin,

雖然記錄當中沒有幾個嵌合的 例子是來自胚胎雙胞胎，

we’re all amalgams to some extent,

我們都是混合體，只是程度不同，

carrying around the different genetic codes of our gut bacteria

帶著腸道細菌

甚至線粒體的不同遺傳密碼。

and even our mitochondria.

And given that 1 in 8 individual births started out as twin pregnancies,

且，有八分之一出生的人 在一開始是雙胎妊娠，

there could be many more people with two genomes—

所以可能還有更多人 有著兩組基因組——

and many more lessons to learn about the genes that make us who we are.

我們也還有太多課題要向 創造出我們的基因來學習。