The secret of the x chromosome
x染色体的奥妙
These women are identical twins, they have the same nose, the same eye color, but this one is
这两个女孩是同卵双胞胎，她们长着一样的鼻子，拥有相同颜色的瞳孔，但是其中一个女孩却是色弱，另外是正常的。
color blind for free light, and this one isn’t.how is that possible?the answer lies in their genes.for
这怎么可能呢？原因是这种情况是由她们的基因所导致的。对于人类来说，我们的身体特征是由存储在细胞核中的23对染色体上的基因信息来决定的。这些染色体由蛋白质和长而卷曲的DNA链所组成。DNA片段就叫做基因，基因会指派细胞去生产一些有特殊功能的蛋白质，从而表明细胞的身份和功能。每一对染色体都是来自父母双方的染色体各一个，23对染色体中的22对染色体都包含相同的一组基因，但是可能基因版本是不同的。这些不同是由于基因突变所导致的，这种基因序列的突变可能是你的祖先就已经有的。这种基因突变，有一些对你的身体是没有影响的，有一些是会导致疾病的，有一些会产生有利的反应。
humans ,the genetic information that determines our physical traits is stored in 23 pairs of chromosomes in the nucleus of every cell.these chromosomes are made up of proteins and long ,coiled strands of DNA.segments of DNA, called genes, tell the cell to build specific proteins, which control its identity and function.for every chromosome pair, one comes form each biological parent.in 22 of these pairs, the chromosomes contain the same set of genes, but may have different versions of those genes.the differences arrive from mutations, which are changes to the genetic sequence that may have occurred many generations ago.some of those changes have no effect, some cause diseases, and some lead to advantageous adaptations.
基因序列有两个版本的结果是，你会表现出了你父母的特征的结合。但是第23对染色体是独一无二的，这对染色体就是隐藏在色盲双胞胎后面的秘密。这对染色体，叫做x和y染色体，她们影响着生物性别。大多数的女人有两条x染色体，而大多数的男人都有一条x染色体和一条y染色体。这条y染色体包含男性生育和发育的基因。但是，x染色体还包含一些除了决定性别和生殖以外的基因，像神经系统的发育，骨骼肌肉功能的发育，眼睛中检测绿光的受体（绿光受体基因和眼盲相关）
the result of having two versions of each gene is that you display a combination of your biological parents’ traits.but the 23rd pair is unique, and that’s the secret behind the one color blind twin.this pair, called the x and y chromosomes ,influences your biological sex.most women have two x chromosomes while most men have one x and one y.the y chromosomes contains genes for male development and fertility.the x chromosome ,on the other hand, contains important genes for things other than sex determination or reproduction ,like nervous system development, skeletal muscle function ,and the receptors in the eyes that detect green light.
拥有x,y染色体的雄性仅仅能够得到x染色体的一个副本，人类已进化到了没有完全相同的两个人的程度。但是这也引发了一个问题对于拥有两个x染色体的人来说，如果两个x染色体都像正常染色体一样生产蛋白质，那么因为过量的蛋白质，胚胎的发育会完全的受损。解决的方法就是x失活，这种措施发生在发育早期，那个时候有两个x染色体的胚胎还是一个细胞球。
biological males with an x ,y chromosome pair only get one copy of all these x chromosome genesis the human body has evolved to function without duplicates.but that creates a problem for people with two x chromosomes.if both x chromosomes produced proteins ,as is normal in other chromosomes, development of the embryo would be completely impaired.the solution is x inactivation .this happens early in development when an embryo with two x chromosomes is just a ball of cells.
每一个细胞都会使一个x染色体失活，这个失活的过程是有一定程度的随机性的，这个细胞让一个亲本的x染色体失活，另一个可以让另一个亲本的染色体失活。这些失活的染色体最后会演变成一个称为巴尔体的团体，然后消失。
each cell inactivates one x chromosomes.there’s certain degree of randomness to this process.one cell may inactivate the x chromosome from one parent, and another the chromosome from the other parent.the inactive x shivers into a clump called a Barr body and goes silent.
在她们的基因序列中几乎没有一个成功的制造出蛋白质。当这些早期细胞分裂的时候，每一个都会经历x失活。因此一些细胞簇表现母体x染色体，另外一些表现父体x染色体。
almost none of its genes order proteins to be made.when these early cells divide, each passes on its x inactivation.so some clusters of cells express the maternal x chromosome, while others express the paternal x.
如果这些染色体携带不同的特征，那么这种差异就会在细胞中体现出来。这也就是为什么calico猫有斑点，一个x染色体有橙色皮毛的基因，一个x染色体有黑色皮毛的基因。皮毛的颜色显示了哪一个染色体是处于激活的状态。现在我们来解释一下上面提到的色盲双胞胎。两个姐妹都继承了一个绿光受体突变基因和一个正常的基因。胚胎是在x染色体灭活之前分裂成双胞胎，所以每一个双胞胎都有不同的灭活模式。色盲女孩,带有正常基因的x染色体在最终变成眼睛的细胞中被关闭。没有这样的基因指令，现在她也不能感觉到绿光并且是色盲
if these chromosomes carry different traits ,those differences will show up in the cells. This is why calico cats have patches.one x had a gene for orange fur and the other had a gene for black fur. the pattern of the coat reveals which one stayed active where.now we can explain our color blind twin .both sisters inherited one mutant copy of the green receptor gene and one normally functioning copy.the embryo split into twins before x inactivation ,so each twin ended up with a different inactivation pattern .in one ,the x chromosome with the normal gene was turned off in the cells that eventually became eyes.without those genetic instructions ,she now can’t sense green light and is color blind.
一些和x染色体基因突变相关的疾病，像色盲，血友病，对于有两条x染色体的个体来说相对而言是没有那么严重的，是因为有一个正常和一个突变基因，她们的细胞中仅仅有一部分会被突变基因所影响到。
disorders that are associated with mutations of x chromosome genes like color blindness, or hemophilia ,are often less severe in individuals with two x chromosomes.thats because in someone with one normal and one mutant copy of the gene, only some of their cells would be affected by the mutation.
这种疾病的严重程度取决于哪个x染色体被关闭，而且被关闭的这些细胞又处于身体的什么位置。从另外一方面来讲，那些只有一个x染色体的人只能表现这个继承来的突变染色体。
this severity of the disorder depends on which x got turned off and where those cells were.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.
仍然有一些关于x染色体失活的没有解决的问题，像x染色体上面的一些基因是如何逃脱失活的。为什么有些失活不是随机的。我们所知道的是基因诸多奥妙中的一个。
there are still unresolved questions about x inactivation ,like how some genes on the x chromosome escape inactivation and why inactivation isn’t always random.what we do know is that this mechanism is one of the many ways that genes alone don’t tell our whole story.