The secrets of the X chromosome. These women are identical twins.
X染色体的秘密。这两位女性是双胞胎。
They have the same nose, the same hair color, the same eye color.
她们有相同的鼻子，同样的发色，眼睛颜色也相同。
But this one is color blind for green light, and this one isn't.
但其中一人却是绿色色盲，另一人则不是。
How is that possible? The answer lies in their genes.
这是为什么呢？答案在于她们的基因。
For humans, the genetic information that determines our physical traits
对于人类来说，决定身体特征的遗传信息
is stored in 23 pairs of chromosomes in the nucleus of every cell.
存在于细胞核中的23对染色体。
These chromosomes are made up of proteins and long, coiled strands of DNA.
这些染色体由蛋白质和卷曲的长链DNA构成。
Segments of DNA, called genes, tell the cell to build specific proteins, which control its identity and function.
DNA由基因构成，它可以使细胞生成特定蛋白质，控制其特性和功能。
For every chromosome pair, one comes from each biological parent.
一对染色体中，一条来自父亲，另一条则来自母亲。
In 22 of these pairs, the chromosomes contain the same set of genes, but may have different versions of those genes.
22对染色体都携带一组基因，但它们表达的性状可能并不相同。
The differences arrive from mutations,
这种改变来源于变异，
which are changes to the genetic sequence that may have occurred many generations ago.
即基因序列的变化，这可能在几代人之前就已产生。
Some of those changes have no effect, some cause diseases, and some lead to advantageous adaptations.
一些变异不会造成影响，一些会导致疾病，另一些则会帮助提高适应性。
The result of having two versions of each gene is that you display a combination of your biological parents' traits.
存在这两种状态的基因会使你综合表现出父母的特性。
But the 23rd pair is unique, and that's the secret behind the one color blind twin.
但是第23对染色体非常独特，这也是色盲双胞胎之谜的所在。
This pair, called the X and Y chromosomes, influences your biological sex.
这一对X和Y染色体决定着性别。
Most women have two X chromosomes while most men have one X and one Y.
绝大多数女性拥有两条X染色体，而男性拥有X、Y染色体各一条。
The Y chromosome contains genes for male development and fertility.
Y染色体包含与男性特征和生殖系统相关的基因。
The X chromosome, on the other hand, contains important genes for things other than sex determination or reproduction,
而X染色体则包含除性征和生殖之外其他发展所需要的基因，
like nervous system development, skeletal muscle function, and the receptors in the eyes that detect green light.
例如神经系统的发育、骨骼肌肉功能以及眼睛中绿色光的感受器。
Biological males with an XY chromosome pair only get one copy of all these X chromosome genes,
男性拥有X和Y染色体意味着他只能获得一组X染色体信息，
so the human body has evolved to function without duplicates.
进化使人们能够在无基因复制的情况下也能实现基因的表达。
But that creates a problem for people with two X chromosomes.
但是这给拥有两条X染色体的人带来了问题。
If both X chromosomes produced proteins, as is normal in other chromosomes,
如果两条X染色体都如其他染色体一样表达蛋白质性状，
development of the embryo would be completely impaired.
它将彻底破坏胚胎阶段的发育。

The solution is X inactivation.
为了应对这种情况，X染色体需要失活。
This happens early in development when an embryo with two X chromosomes is just a ball of cells.
这个过程在胚胎发育初期产生，此时的胚胎只是一团细胞。
Each cell inactivates one X chromosome.
每个细胞中的一条X染色体会失活。
There's a certain degree of randomness to this process.
这个过程存在一定的随机性。
One cell may inactivate the X chromosome from one parent, and another the chromosome from the other parent.
某细胞中，来自父亲的X染色体可能失活，而另一细胞中，失活的则是来自母亲的X染色体。
The inactive X shrivels into a clump called a Barr body and goes silent.
失活的X染色体会萎缩成一团，称为巴尔小体。
Almost none of its genes order proteins to be made.
它的基因几乎无法表达蛋白质。
When these early cells divide, each passes on its X inactivation.
当这些早期细胞分裂时，每个细胞都带有失活的X染色体。
So some clusters of cells express the maternal X chromosome, while others express the paternal X.
一些细胞表达的是母系X染色体，而另一些则表达父系的X染色体。
If these chromosomes carry different traits, those differences will show up in the cells.
如果这些染色体携带不同的特质，这些差异将表现在细胞中。
This is why calico cats have patches.
这也是杂色猫身上不同毛色的原因。
One X had a gene for orange fur and the other had a gene for black fur.
一条X染色体上的基因决定着橙色毛，而另一条则会产生黑色毛。
The pattern of the coat reveals which one stayed active where.
毛色分布也可以体现出基因的位置。
Now we can explain our color blind twin.
现在我们来解释色盲双胞胎的问题。
Both sisters inherited one mutant copy of the green receptor gene and one normally functioning copy.
二人都继承了一个变异的和一个正常的绿色光感受基因。
The embryo split into twins before X inactivation, so each twin ended up with a different inactivation pattern.
早期胚胎在X染色体失活之前就已分裂成双胞胎，因此每个人拥有了不同的失活模式。
In one, the X chromosome with the normal gene was turned off in the cells that eventually became eyes.
在发育成眼睛的细胞中，一人的正常绿光感受基因失活。
Without those genetic instructions, she now can't sense green light and is color blind.
由于缺失了这个基因，她无法感受绿色光，成为色盲。
Disorders that are associated with mutations of X chromosome genes, like color blindness, or hemophilia,
X染色体基因变异引起的失调病症有很多，例如色盲或是血友病，
are often less severe in individuals with two X chromosomes.
拥有两条X染色体的个体更不容易患上这些疾病。
That's because in someone with one normal and one mutant copy of the gene,
原因在于如果人同时拥有正常和变异基因，
only some of their cells would be affected by the mutation.
只有部分细胞会受到变异的影响。
This severity of the disorder depends on which X got turned off and where those cells were.
失调的严重性取决于哪一条X染色体失活以及这些细胞的位置。
On the other hand, all the cells in someone with only one X chromosome can only express the mutant copy of the gene if that's what they inherited.
然而若某人只有一条X染色体，那么所有的细胞只能表达变异的基因。
There are still unresolved questions about X inactivation,
很多关于X染色体失活的问题仍然尚未解决，
like how some genes on the X chromosome escape inactivation and why inactivation isn't always random.
例如，为何某些X染色体上的基因并不会失活，为何失活并不总是随机的。
What we do know is that this mechanism is one of the many ways that genes alone don't tell our whole story.
我们现在只能确定基因并不是这种机制唯一的解释。