Secrets of the X chromosome - Robin Ball

譯者: Helen Chang 審譯者: S Sung

The secrets of the X chromosome.

X 染色體的秘密

These women are identical twins.

這兩位是同卵雙胞胎女子

They have the same nose,

她們的鼻型相同

the same hair color,

髮色相同

the same eye color.

眼睛顏色也相同

But this one is color blind for green light,

但這一位綠色色盲

and this one isn't.

而另一位則不

How is that possible?

怎會這樣呢？

The answer lies in their genes.

答案在於她們的基因

For humans, the genetic information that determines our physical traits

決定人類身體特徵的遺傳資訊

is stored in 23 pairs of chromosomes in the nucleus of every cell.

儲存在每個細胞核的 23 對染色體裡

These chromosomes are made up of proteins and long, coiled strands of DNA.

染色體由蛋白質和 捲曲的 DNA 長鏈所構成

Segments of DNA, called genes, tell the cell to build specific proteins,

一段段被稱為基因的 DNA 指示細胞生成特定的蛋白質

which control its identity and function.

用來控制細胞的特性和功能 《神經細胞》《骨細胞》《肌肉細胞》

For every chromosome pair, one comes from each biological parent.

每對染色體由生父和生母各出一條

In 22 of these pairs, the chromosomes contain the same set of genes,

22 對染色體的兩條基因序列相同

but may have different versions of those genes.

但版本可能不同

The differences arrive from mutations,

差異來自突變

which are changes to the genetic sequence

也就是基因序列的改變

that may have occurred many generations ago.

有可能在好幾代以前已發生變異

Some of those changes have no effect,

有些變異沒影響

some cause diseases,

有些會引致疾病

and some lead to advantageous adaptations.

還有些有助於適者生存

The result of having two versions of each gene

有兩種基因版本的結果

is that you display a combination of your biological parents' traits.

是你會顯現出生身父母的綜合特性

But the 23rd pair is unique,

但是獨特的第 23 對染色體

and that's the secret behind the one color blind twin.

是色盲雙胞胎的謎底所在

This pair, called the X and Y chromosomes, influences your biological sex.

這一對決定性別的染色體 稱作 XY 染色體

Most women have two X chromosomes

大多數的女性有兩條 X 染色體

while most men have one X and one Y.

而大多數男性則 X、Y 各一條

The Y chromosome contains genes for male development and fertility.

Y 染色體含有的基因 決定男性的發育和傳宗接代

The X chromosome, on the other hand,

而 X 染色體則含有

contains important genes for things other than sex determination or reproduction,

除了性別和生育以外的重要基因

like nervous system development,

例如神經系統的發育

skeletal muscle function,

骨骼肌肉功能

and the receptors in the eyes that detect green light.

以及眼睛中的綠光感受器

Biological males with an XY chromosome pair

男性有 X 和 Y 染色體各一條

only get one copy of all these X chromosome genes,

意味他只有一組 X 染色體基因

so the human body has evolved to function without duplicates.

也就是人類已進化到 只需單組的 X 基因控制功能

But that creates a problem for people with two X chromosomes.

但有兩條 X 染色體的人就麻煩了

If both X chromosomes produced proteins, as is normal in other chromosomes,

如果和其他 22 組一樣，第 23 組的 兩條 X 染色體都控制蛋白質的生產

development of the embryo would be completely impaired.

就會徹底妨害胚胎的發育

The solution is X inactivation.

解決之道是關閉一組 X 染色體

This happens early in development when an embryo with two X chromosomes

關閉的過程發生在胚胎發育的初期

is just a ball of cells.

當時的胚胎只是一團細胞

Each cell inactivates one X chromosome.

細胞關閉 X 染色體的過程

There's a certain degree of randomness to this process.

相當隨機

One cell may inactivate the X chromosome from one parent,

很可能某個細胞關閉 來自父親的 X 染色體

and another the chromosome from the other parent.

而另一細胞細胞關閉 來自母親的 X 染色體

The inactive X shrivels into a clump called a Barr body and goes silent.

被關閉的 X 染色體萎縮失能 稱為「巴爾小體」

Almost none of its genes order proteins to be made.

它的基因對生產蛋白質 幾乎無能為力

When these early cells divide, each passes on its X inactivation.

這些早期細胞分裂時 關閉的 X 染色體也被傳了下去

So some clusters of cells express the maternal X chromosome,

因此一些細胞 顯現母系的 X 染色體

while others express the paternal X.

而另一些則是父系的 X 染色體

If these chromosomes carry different traits,

如果這些染色體攜帶不同的特質

those differences will show up in the cells.

細胞就會顯現出差異

This is why calico cats have patches.

這是為什麼雜色貓身上斑塊不同色

One X had a gene for orange fur and the other had a gene for black fur.

一條 X 基因是橙色毛 而另一條是黑色毛

The pattern of the coat reveals which one stayed active where.

毛色分佈顯現出活性基因所在的位置

Now we can explain our color blind twin.

現在來解釋色盲雙胞胎的原因

Both sisters inherited one mutant copy of the green receptor gene

兩姊妹都繼承一個變異的

and one normally functioning copy.

以及另一個正常的綠光受體基因

The embryo split into twins before X inactivation,

胚胎早在關閉 X 染色體前 就已分裂成雙胞胎

so each twin ended up with a different inactivation pattern.

因此二人各自關閉不同的 X 染色體

In one, the X chromosome with the normal gene was turned off

其中一人關閉了即將要發育成 眼睛細胞的正常綠光受體基因

in the cells that eventually became eyes.

Without those genetic instructions,

沒有正常基因的指令

she now can't sense green light and is color blind.

她感受不到綠色光，成了色盲

Disorders that are associated with mutations of X chromosome genes,

X 染色體基因變異會引致失調病症

like color blindness,

像是色盲或血友病

or hemophilia,

are often less severe in individuals with two X chromosomes.

如果個體有兩條 X 染色體 通常發病的情況會比較和緩

That's because in someone with one normal and one mutant copy of the gene,

原因是同時擁有正常和變異基因的人

only some of their cells would be affected by the mutation.

只有部分細胞會受到變異的影響

This severity of the disorder depends on which X got turned off

症狀的輕重程度取決於 關閉的是哪一條 X 染色體

and where those cells were.

以及受影響細胞的所在位置

On the other hand, all the cells in someone with only one X chromosome

然而那些只有一條 X 染色體的人

can only express the mutant copy of the gene if that's what they inherited.

若遺傳到變異的基因 所有的細胞都會有失調的症狀

There are still unresolved questions about X inactivation,

目前仍存著一些 關閉 X 染色體的未解題

like how some genes on the X chromosome escape inactivation

例如：某些 X 的基因不會被關閉

and why inactivation isn't always random.

以及關閉的選擇並不總是隨機

What we do know is that this mechanism

我們確定的是

這是遺傳機制不能單單 用基因來說明的一個例子

is one of the many ways that genes alone don't tell our whole story.