You have 23 pairs of chromosomes. Twenty-two of those pairs are matched. The
last pair may not be. It can be two X chromosomes, or it can be one X and one
Y — and the Y is tiny. This lab asks a simple question with a long answer:
why is one pair different, and what does that change?你有 23 对染色体。其中 22 对是配对相同的，最后一对可能不是：
它可以是两条 X 染色体，也可以是一条 X 和一条 Y——而 Y 非常小。
这个实验室提出一个简单但答案很长的问题：为什么有一对不一样？这会改变什么？
Work in this order: learn the words, then open the
machines and say what you see. Everything on this page is meant to be said out loud.
按这个顺序：先学词，再打开机器，把你看到的说出来。
这一页所有内容都是用来大声说的。
What this page is about, and what it is not. This is a biology lab. It is about
chromosomes, cells and bodies. It is not about how a person feels, what they are called, or
who they are. Those are real and important questions, and this page does not answer them.
One thing you will learn here is that the biology alone is already more complicated than
most people are taught.
这一页讲什么，不讲什么。这是一堂生物课，讲的是染色体、细胞和身体，
而不是一个人的感受、称呼或身份认同。那些问题真实而重要，但本页并不回答它们。
你在这里会学到的一点是：仅就生物学本身而言，它已经比大多数人被教到的复杂得多。
🔤 The words you need
Summary
Twenty-eight terms this lab uses, each with a short definition, a longer explanation, and one sentence to say aloud. A few of them come back from the Gene Lab, because you cannot talk about the X and the Y without them. Learn these first.
摘要
本实验室使用的二十八个术语，每个都有简短定义、更详细的解释，以及一句可以大声说出的例句。其中几个来自基因实验室，因为讲 X 和 Y 离不开它们。先学会这些。
Twenty-eight words. Tap 🔊 say it to hear one, and 📖 tell me more
for the real explanation. Some of these words are also in the
Gene Lab — that is on purpose. You
need them here too.
二十八个词。点“🔊 say it”听发音，点“📖 tell me more”看真正的解释。
有几个词在基因实验室里也出现过——
这是故意的，因为这里同样需要它们。
🎤 Speaking drill
Five words at a time. Press the mic, say all five in any order, then check.
一次五个词。按下麦克风，任意顺序说出这五个词，然后检查。
listening
what you say appears here…
🎬 The whole picture — about five and a half minutes, English + 中文 subtitles
Summary
An explainer of about five and a half minutes. It runs from the size difference between the X and the Y to a turtle nest, and ends on what epigenetics does and does not claim, and every picture in it is a diagram drawn in code. The narration is synthetic.
摘要
一段约五分半钟的讲解视频，从 X 与 Y 的大小差异讲到海龟巢穴，最后讲到表观遗传学主张什么、不主张什么，其中每一张图都是用代码绘制的示意图。旁白是合成语音。
Watch this before you open the machines. It walks through all nine of them, so
the lessons below have somewhere to land.
先看这个再打开下面的机器。它会把九台机器都走一遍，这样下面的课程才有落脚点。
An explainer, about five and a half minutes. A computer reads the words.
It is not a person. Every picture in it is a diagram, drawn in code — the same
diagrams you will drive yourself further down this page, redrawn to sit still. Nothing in
it is a photograph, and no picture in it was made by an image model. A chromosome map and
a temperature curve have a right answer for where every line goes, so they are drawn
rather than generated. Subtitles in English and Chinese are in the player's own controls.
Treat the video as a summary of this page, not as a source.
约五分半钟的讲解。念词的是计算机，不是真人。其中每一张图都是用代码绘制的
示意图——正是你在本页下方会亲自操作的那些图，只是画成了静止的版本。
这里没有照片，也没有任何图像模型生成的画面：染色体图谱和温度曲线的每一条线该画在哪里都有正确答案，
所以它们是被画出来的，而不是被生成的。字幕在播放器控件里。
请把视频当作本页的摘要，而不是资料来源。
🔬 Nine machines
Drag the sliders, click the diagrams, run them again. Then say what you
see in English.
拖动滑块，点击图示，反复运行。然后用英语说出你看到的。
🧬 The 23rd pair
Summary
The X chromosome carries about eight hundred genes. The Y carries fewer than fifty that make proteins. They are not two versions of the same thing. Only the small tips of the X and the Y still match each other, and those tips are the only place where the pair can swap pieces.
摘要
X 染色体约携带八百个基因，Y 上能编码蛋白质的基因不到五十个。它们并不是同一样东西的两个版本。只有 X 和 Y 两端的小片段仍然彼此匹配，而那也是这一对唯一还能交换片段的地方。
Click any part of the picture. The chromosomes are drawn to scale.
1 pixel ≈ 2.4 million letters of DNA点击图中任意部分。图中染色体按真实比例绘制。
—MILLION LETTERS
—PROTEIN GENES
—PART THAT SWAPS
The 23rd pair:
Try this:
Click chromosome 1, then click the Y. Compare the numbers.
Click the green tips. That is the only part of the pair that still matches.
Click the red band on the Y. One gene sits there. It is the switch.
Switch the pair to X and X. The picture becomes a normal, matched pair.
试试这些：先点1 号染色体，再点 Y，比较数字；
点两端的绿色小段——那是这一对唯一还彼此匹配的部分；
点 Y 上的红色条带，那里只有一个基因，它就是开关；
把这一对切换成 X 和 X，图就变成了普通的、匹配的一对。
They are not a big one and a small one of the same thing
This is the most common mistake. People imagine the Y is a short X.
It is not. Almost every gene on the X is missing from the Y, and a few genes on the Y
are found nowhere else in the body. The X carries genes for muscle, for blood clotting,
for colour vision, for the brain — jobs that have nothing to do with sex. The Y is
mostly two things: the switch, and a set of genes for making sperm.
So the X is a normal, busy chromosome that everyone needs. Nobody
survives without at least one X. The Y is the odd one.
这是最常见的误解：人们以为 Y 就是缩短的 X，其实不是。
X 上几乎每个基因在 Y 上都找不到，而 Y 上少数几个基因在全身别处也找不到。
X 携带着与性别无关的基因——肌肉、凝血、色觉、大脑。
Y 主要只有两样东西：那个开关，以及一组制造精子的基因。
所以 X 是一条每个人都需要的、繁忙的普通染色体，没有 X 就无法存活。
真正特别的是 Y。
Which parent gives which
An egg always carries an X, because the person who made it has two X
chromosomes and nothing else to give. A sperm carries either an X or a Y. So the 23rd
pair of a baby is decided by the sperm, and it is decided by a coin toss that happens
during meiosis.
There is a neat consequence. If you have a Y, you got it from your father, and
he got it from his father. Lesson 7 follows that line.
卵子一定携带 X，因为产生它的人只有两条 X，别无可给。
精子则携带 X 或 Y。所以婴儿的第 23 对由精子决定，
而这个决定来自减数分裂中的一次“抛硬币”。
由此有一个漂亮的推论：如果你有 Y，它来自你父亲，而他的来自他父亲。第 7 课会追踪这条线。
Why are the tips the only part that still swaps?
In meiosis, each pair of chromosomes lines up and trades pieces. That
trade is called recombination, and it needs the two partners to match, letter by
letter, so they can find each other. The X and the Y stopped matching long ago along
almost their whole length. Only the small regions at each end still match. Those regions
are called the pseudoautosomal regions — "pseudo" because they behave like a
normal chromosome pair even though they are on the sex chromosomes.
The larger one, at the top, is about 2.8 million letters long. Every meiosis, the
X and Y must cross over at least once inside it, or they separate badly and the sperm
ends up with the wrong number of chromosomes. That tiny strip is doing a lot of work.
在减数分裂中，每一对染色体会配对并交换片段，这叫重组。
交换要求两条伙伴逐个字母地匹配，才能彼此找到。X 与 Y 在几乎整条长度上早已不再匹配，
只有两端的小区域仍然匹配，它们被称为拟常染色体区——“拟”是因为
它们虽然位于性染色体上，行为却像普通染色体对。
顶端较大的那一段约 280 万个字母长。每次减数分裂，X 与 Y 必须在其中至少交换一次，
否则分离出错，精子就会带上错误数目的染色体。这条小小的区段承担了很重的工作。
🔀 The switch
Summary
For the first six weeks every human embryo builds the same organ and both sets of plumbing. One gene on the Y, called S R Y, is switched on for a few days. It turns that organ into a testis. Without it, other genes turn the same organ into an ovary. Everything else follows from which one was built.
Drag the slider from week 4 to week 12. Then run it again with the switch
turned off, and watch the same starting organ take the other road.
把滑块从第 4 周拖到第 12 周，然后关掉开关再运行一次，
看同一个起始器官走上另一条路。
The switch:
can still become eithertestis pathovary path
One gene, on for a few days, and then never again
SRY does almost nothing by itself. It is a switch: it turns on
another gene, called SOX9, and SOX9 turns on more. Once the chain is running, SRY is no
longer needed and goes quiet. A gene that is active for a few days can decide the shape
of a whole body, because of what it starts.
This is why the switch is so easy to break. If SRY is missing or damaged, an
embryo with a Y builds ovaries instead. If SRY jumps onto an X by accident — and it can,
because it sits right next to the swapping tip — an embryo with two X chromosomes builds
testes. Lesson 3 has both cases.
SRY 本身几乎什么都不做，它是一个开关：它打开另一个叫 SOX9 的基因，
SOX9 再打开更多基因。链条一旦启动，SRY 就不再需要，随即沉默。
一个只活跃几天的基因之所以能决定整个身体的形态，靠的是它启动了什么。
这也正是这个开关容易出错的原因。如果 SRY 缺失或损坏，带 Y 的胚胎会转而造出卵巢；
如果 SRY 意外跳到 X 上——它就紧挨着交换端，确实会发生——
那么带两条 X 的胚胎会造出睾丸。第 3 课两种情况都会讲。
The other road is not "nothing happens"
For a long time, textbooks said the ovary was the default: build
nothing, and you get one. That was wrong, and it was wrong in an interesting way.
Building an ovary takes its own genes — WNT4, RSPO1, FOXL2 — working just as hard. The
two programmes actively fight each other, and the winner shuts the loser down.
We know this because of FOXL2. Switch it off in an adult mouse ovary and the
ovary starts turning into a testis — in an adult, years after development
finished. So it was never a road already taken. It is a road being held.
很长时间里，教科书说卵巢是默认结果：什么都不做就会得到它。
这是错的，而且错得很有意思。造卵巢同样需要自己的基因——WNT4、RSPO1、FOXL2——同样费力。
两套程序在主动对抗，赢的一方会把输的一方关掉。
Two cell types appear in a new testis, and each sends out one signal.
Sertoli cells make a hormone that destroys something: the tubes
that would have become a uterus. That hormone is called anti-Müllerian hormone. Its
whole job is demolition.
Leydig cells make testosterone, which keeps a second set of tubes
alive and growing.
Every embryo starts with both sets of tubes. So development is not
building one and skipping the other. It is keeping one and taking the other away. That
is a strange design, and it is a clue about history: it is easier for evolution to add a
demolition signal than to redesign an embryo from scratch.
新生的睾丸中出现两类细胞，各自发出一个信号。
Chromosomes do not shape a body directly. They set a switch, the switch builds a gonad, the gonad makes hormones, and cells read those hormones with receptors. That is four steps, and each one can vary on its own. This is why the chromosomes alone do not tell you what a body will look like.
Click a step to read what it does. Then click a case below and watch which
step in the chain is different.
点击某一步，看它做什么；再点下面的案例，看链条中哪一步不同。
Real cases. Every one of
these happens in real people, and the numbers are real.
真实案例。下面每一种都真实发生在人身上，数字也是真实的。
Read the numbers carefully. "One in twenty thousand" sounds rare. Thailand has about
seventy million people, so one in twenty thousand is roughly three and a half thousand
people. Rare for one person is common for a country. None of these are diseases in the
ordinary sense, and most people who have them are healthy.
请仔细看这些数字。“两万分之一”听起来很罕见。
泰国约有七千万人口，两万分之一就意味着大约三千五百人。
对个人来说罕见的事，对一个国家来说很常见。这些都不是通常意义上的疾病，
绝大多数当事人是健康的。
Why this matters for the word "chromosome"
If you only know step one, you will predict step four wrongly. A person
with a Y chromosome and a broken androgen receptor has XY chromosomes, has testes inside
the body, makes plenty of testosterone — and develops along female lines, because not
one cell in the body can hear the hormone. The signal is sent. Nothing receives it.
A chromosome is an instruction. An instruction only matters if something reads
it, and then acts on it. That is true of every gene on every chromosome, and this is the
clearest example in the whole of human biology.
如果你只知道第一步，就会把第四步猜错。
一个人带有 Y 染色体、体内有睾丸、也分泌大量睾酮，但雄激素受体损坏，
身体便沿着女性方向发育——因为全身没有一个细胞能“听见”这个激素。信号发出了，却无人接收。
A cell with two X chromosomes switches one of them off. Each cell chooses at random, very early, and every cell it later makes keeps the same choice. So a body with two X chromosomes is not one thing. It is a patchwork of two kinds of cell, and in a cat you can see the patches.
摘要
带两条 X 的细胞会关掉其中一条。每个细胞在很早的时候随机选择，之后它产生的所有细胞都保持同一个选择。所以带两条 X 的身体不是单一的，而是两种细胞拼成的镶嵌体——在猫身上，这些色块是看得见的。
This cat's fur colour gene sits on the X. Orange comes from one X, black from
the other. Press Run it again and you get a different cat every time.
这只猫的毛色基因位于 X 上：橙色来自一条 X，黑色来自另一条。
按再运行一次，每次都会得到一只不同的猫。
—ORANGE
—BLACK
—PATCHES
Try this:
Slide it down to 4 cells. Huge patches. The choice was made early.
Slide it up to 60 cells. Small patches, a speckled cat.
Run it several times at the same setting. Never the same cat twice.
Because two copies of eight hundred genes is too much. A cell with two
working X chromosomes would make twice as much of eight hundred proteins as a cell with
one, and the amount matters. So one X is wrapped up tight and switched off. Under a
microscope you can see it, sitting against the edge of the nucleus as a dark dot. It is
called a Barr body, and it was found in 1949, twelve years before anyone knew
what it was doing.
Mary Lyon worked that out in 1961. Her evidence was mice with patchy coats — the
same patches you are generating above.
因为八百个基因各来两份太多了。
带两条工作 X 的细胞，会比只有一条的细胞多产出一倍的八百种蛋白质，而“量”是要紧的。
于是其中一条 X 被紧紧包裹并关闭。在显微镜下能看到它：紧贴细胞核边缘的一个暗点，
叫做巴氏小体。它在 1949 年就被发现，比人们弄清它在做什么早了十二年。
The cat shows its patches because a fur gene happens to sit on the X.
Almost every other X gene does its work invisibly — inside a muscle, inside a nerve,
inside the retina. The patchwork is still there. You just cannot look at it.
It has real effects. A person who carries one damaged copy of an X gene has some
cells using the good copy and some using the damaged one. If the good cells are enough,
nothing shows. If, by chance, most cells switched off the good X in that tissue, symptoms
appear. That is why carriers of the same condition can differ so much from each other.
猫之所以露出色块，只是因为恰好有个毛色基因在 X 上。
几乎所有其他 X 基因都在看不见的地方工作——肌肉里、神经里、视网膜里。
镶嵌依然存在，只是你看不到。
它有真实的后果。若某人携带一份受损的 X 基因，
他体内有些细胞用好的那份，有些用坏的那份。如果好细胞足够多，就什么也看不出来；
但如果在某个组织里，恰好大多数细胞关掉的正是好的那条 X，症状就会出现。
这正是同一种情况的携带者之间差别可以非常大的原因。
It is not a clean off switch
About fifteen out of every hundred genes on the silenced X keep
working anyway. They escape. Another ten or so escape in some people and not in others.
Those escaping genes are one reason two X chromosomes are not simply the same as one.
The genes in the swapping tips escape too, and that makes sense: those genes are
present twice in everybody, so silencing them would create the very imbalance the
system is there to fix.
被沉默的那条 X 上，约有百分之十五的基因仍在工作——它们“逃逸”了；
另有约百分之十在一些人身上逃逸、在另一些人身上不逃逸。
这些逃逸基因正是“两条 X 并不等同于一条”的原因之一。
A person with two X chromosomes has a spare copy of every X gene. A person with one X does not. So a damaged X gene shows up far more often in people with one X, and it is passed down through people with two, who often show nothing at all. Red-green colour blindness follows this rule exactly.
摘要
有两条 X 的人，每个 X 基因都有一份备用；只有一条 X 的人没有。所以一份受损的 X 基因在只有一条 X 的人身上出现得多得多，并且往往通过毫无症状的、有两条 X 的人传下去。红绿色盲完全遵循这条规则。
Set the parents, then have children. Watch which children are affected.
设定父母，然后生育后代，看哪些孩子受影响。
0CHILDREN
—SONS AFFECTED
—DAUGHTERS AFFECTED
Mother:
Father:
no copycarrier — one copy, not affectedaffected
Try this:
Mother one copy, father not affected. Have a hundred children. About
half the sons are affected. No daughter is.
Now make the father affected as well. Now daughters can be affected too.
Mother no copies, father affected. No child is affected — but every
daughter is a carrier. The condition skips a generation and comes back.
A person with one X is hemizygous for every gene on it. "Hemi"
means half. There is no second copy to compare with, no spare to fall back on, and no
dominant or recessive to argue about. Whatever that single X says, the body does.
This is the whole reason the numbers are lopsided. Red-green colour blindness
reaches about eight in a hundred men of European ancestry and about one in two
hundred women. Nothing about the gene is different between them. Only the number of
copies is.
只有一条 X 的人，对 X 上的每个基因都是半合子。“半”就是一半的意思：
没有第二份可比较，没有备份可依靠，也无所谓显性还是隐性——
那唯一一条 X 说什么，身体就做什么。
The dots below hide a letter. Press the button to run the picture
through a filter that approximates red-green colour blindness. The letter does not get
darker or blurrier. It simply stops existing, because the two colours become the same
colour.
下面的圆点里藏着一个字母。按下按钮，
让画面通过一个近似红绿色盲的滤镜。字母不会变暗，也不会变模糊，
它只是不再存在了——因为那两种颜色变成了同一种颜色。
An approximation, not a medical test. Colour vision varies, and screens vary more.
这是近似模拟，不是医学检测。色觉因人而异，屏幕差异更大。
The most famous family with an X-linked condition
Queen Victoria carried one damaged copy of a blood-clotting gene on an
X. She was not ill. Her sons could be, and one was. Her daughters carried it into the
royal families of Spain, Germany and Russia, and their sons were ill in turn. The most
famous was Alexei, the son of the last Russian tsar.
For a century, which of the two clotting genes it was stayed a guess. In 2009 a
team sequenced DNA from the family's remains and answered it: haemophilia B, the gene
called F9. A question from the nineteenth century, closed by a laboratory in the
twenty-first.
维多利亚女王在一条 X 上携带一份受损的凝血基因。她本人没有患病，
但她的儿子可能患病，其中一个确实患了病。她的女儿把它带进了西班牙、德国和俄国的王室，
那些王室的儿子又相继患病，其中最有名的是末代沙皇之子阿列克谢。
整整一个世纪里，究竟是两个凝血基因中的哪一个，一直只是猜测。
2009 年，一个团队对该家族遗骸的 DNA 进行测序，给出了答案：血友病 B，基因名 F9。
一个十九世纪的问题，被二十一世纪的实验室合上了。
📉 The shrinking Y
Summary
The X and the Y began as an ordinary matched pair, about one hundred and eighty million years ago. One of them picked up the switch. Recombination then stopped along that chromosome, section by section, and a chromosome that cannot swap cannot repair itself. It has lost roughly nine hundred and fifty of its genes since then.
摘要
大约一亿八千万年前，X 和 Y 本是普通的匹配一对。其中一条获得了那个开关，随后重组一段一段地停止，而不能交换的染色体无法自我修复。从那时起，它已经丢失了大约九百五十个基因。
Drag from 180 million years ago to today. Watch the green part — the part
that can still swap — disappear.
从一亿八千万年前拖到今天，看绿色部分——仍能交换的部分——如何消失。
—YEARS AGO
—GENES ON THE Y
still swaps every generationcannot swap any more
Why not swapping is fatal, slowly
Every chromosome collects damage. Normally that does not matter, because
in every generation the pair swaps pieces, and a good copy of a broken stretch can come
back from the partner. Damage is diluted.
The Y has no partner to copy from. So every mistake it makes is permanent, and
mistakes accumulate one on top of another. A gene that stops working can never be
repaired — only deleted. Biologists call this ratchet effect, and a ratchet only turns
one way.
每条染色体都会积累损伤。通常这没关系，
因为每一代这一对都会交换片段，损坏区段的好副本可以从伙伴那里回来，损伤被稀释了。
Y 没有可供拷贝的伙伴。所以它犯下的每一个错误都是永久的，
而错误会一个叠一个地累积。一个失效的基因永远无法被修复，只能被删除。
生物学家把这称为“棘轮效应”，而棘轮只朝一个方向转。
So will the Y disappear?
This is argued about, and both sides have evidence.
One side counts backwards. Nine hundred and fifty genes lost in a hundred
and eighty million years is about five per million years. At that rate the last ones go
in roughly ten million years.
The other side looks at the recent record. The loss was not steady. It
was fast at the start and then almost stopped. Comparing the human Y with the rhesus
monkey Y shows humans have lost no protein genes in about twenty-five million
years. What is left may be what could not be lost.
Also, the Y found a trick. Large stretches of it are palindromes — the
sequence reads the same forwards and backwards. So the chromosome can fold in half and
copy one arm against the other. It has no partner, so it became its own partner.
这一点存在争论，而且双方都有证据。
另一方看近期记录：丢失并不匀速——起初很快，随后几乎停止。
把人类的 Y 与恒河猴的 Y 相比可见，人类在约两千五百万年里没有再丢失蛋白质基因。
剩下的，可能正是丢不掉的那些。
而且 Y 找到了一个办法：它有大段序列是回文——正读反读一样。
于是这条染色体可以自我对折，用一条臂去校对另一条臂。它没有伙伴，就把自己变成了伙伴。
Two animals that already lost it
The Amami spiny rat and the transcaucasian mole vole both have no Y
chromosome at all. Every individual is XO. They did not stop making males. They moved
the switch somewhere else: in the spiny rat, a small duplicated piece of DNA near the
SOX9 gene now does the job that SRY used to do.
So the honest answer to "will the Y disappear" is that it might, and it would
not be the end of anything. What matters to a species is that a switch exists. It does
not matter which chromosome the switch happens to be sitting on.
奄美刺鼠和高加索鼹形田鼠都完全没有 Y 染色体，每个个体都是 XO。
它们并没有停止产生雄性，而是把开关搬到了别处：
在刺鼠身上，SOX9 基因附近一小段重复的 DNA 如今承担了 SRY 原来的工作。
Almost all of your DNA is shuffled every generation. Two pieces are not. The Y passes from father to son without swapping, and the small DNA inside your mitochondria passes from mother to every child. Each traces one single line back through history, and the two lines almost never meet.
摘要
你几乎所有的 DNA 每一代都会被重新洗牌，只有两段不会：Y 从父亲传给儿子且不发生交换，线粒体里那一小段 DNA 由母亲传给每一个孩子。它们各自向历史深处追溯出一条单一的线，而这两条线几乎从不相遇。
Click a person. Then switch between the two lines and watch who they share
it with.
点击一个人，然后在两条线之间切换，看他与谁共享这段 DNA。
Try this:
Follow the Y and click the grandfather. It reaches only some of his
grandchildren — and none through his daughter.
Follow the mitochondria and click the grandmother. It reaches
every grandchild through her daughter, and stops dead at her sons.
Find a pair of cousins who share neither line, even though they share both
grandparents.
Your other chromosomes are useless for this. Chromosome 7 came from both
your parents, mixed, and their chromosome 7s were mixed too. Trace it back and it fans
out into hundreds of ancestors at once. You cannot follow a line through it.
The Y does not swap, so a son's Y is his father's Y, letter for letter, apart
from the rare new mutation. The mitochondria are not chromosomes at all — they are tiny
power stations inside your cells, with their own small circle of DNA, and a sperm's
mitochondria are destroyed after fertilisation. So they come from the egg only.
Those rare new mutations are the useful part. They are the marks that let a
laboratory sort millions of Y chromosomes into a family tree.
你其他的染色体在这件事上派不上用场。
你的 7 号染色体来自父母双方并且混合过，而他们的 7 号染色体也混合过；
向上追溯，它一下子就散成几百个祖先，根本追不出一条线。
Y 不交换，所以除了偶发的新突变，儿子的 Y 与父亲的 Y 逐字相同。
线粒体则根本不是染色体——它们是细胞里的小型发电站，带着自己那一小圈 DNA，
而精子的线粒体在受精后会被销毁，所以它们只来自卵子。
那些偶发的新突变正是有用的部分：它们是标记，
让实验室能把数百万条 Y 染色体排进一棵家族树。
What this cannot tell you
Ten generations back you have about a thousand ancestors. Your Y comes
from exactly one of them, and your mitochondria from exactly one other. That is two
people out of a thousand — about one fifth of one percent of your family.
So a test that says "your Y line came from here" is telling you something true
and something very narrow. It describes one thread, not the cloth. Any advertisement
that turns that thread into an identity is selling you more than the science says.
往上十代，你大约有一千位祖先。你的 Y 恰恰来自其中一位，
线粒体又恰恰来自另一位——一千人中的两人，约占你家族的千分之二。
所以，一份说“你的 Y 世系来自这里”的检测，讲的是真话，但也是极窄的一句话：
它描述的是一根线，而不是整块布。任何把这根线说成身份认同的广告，
卖给你的都超出了科学能支持的范围。
🌡️ Other ways to decide
Summary
X and Y is one solution, not the solution. Birds use a different pair, and there it is the mother who decides. Many turtles use the temperature of the sand. Some fish change sex as adults. Each system evolved separately, which tells you the switch matters and the machinery does not.
摘要
X 与 Y 只是一种方案，而不是唯一方案。鸟类用的是另一对染色体，而且由母亲决定；许多海龟靠沙子的温度；有些鱼在成年后改变性别。每套系统都是独立演化出来的——这说明要紧的是有一个开关，至于用什么机器并不要紧。
Click an animal.
点击一种动物。
Move the sand
temperature. This is a real curve, measured in real nests.
移动沙温。这是在真实巢穴中测得的真实曲线。
29.0°CSAND
50%FEMALE HATCHLINGS
This is happening now. In the northern Great Barrier Reef, green turtle nests are
so warm that scientists sampling young turtles found more than ninety-nine out of a
hundred were female. A population can look healthy for years while quietly running
out of one half of itself.
这正在发生。在大堡礁北部，绿海龟的巢温高到
科学家取样的幼龟中超过百分之九十九是雌性。
一个种群可以在多年里看起来很健康，却在悄悄地耗尽自己的一半。
What all of them have in common
Every system here does the same job: it makes a bipotential organ commit
to one path, early, and it makes different individuals commit differently. The
signal can be a gene, a dose, a temperature, or the death of the biggest fish on
the reef. The machinery is not the point. The switch is.
That is why these systems can be swapped out and replaced so often across the
tree of life — and it is the best reason to think a human Y chromosome disappearing
would be an event in the history of a chromosome, not in the history of a species.
这里的每一套系统做的都是同一件事：
让一个双向潜能的器官在早期确定一条路，并让不同个体确定得不一样。
信号可以是一个基因、一个剂量、一个温度，或者礁上最大那条鱼的死亡。
机器不是重点，开关才是。
这正是这些系统能在生命之树上被反复替换的原因——
也是最有力的理由，让人相信：人类 Y 染色体若真的消失，
那将是一条染色体历史上的事件，而不是一个物种历史上的事件。
🧠 What a cell remembers
Summary
Marks on top of DNA decide which genes a cell uses. You have already driven one: X-inactivation is the clearest example in human biology. Hard experience really does change these marks within one life. Whether they then travel to a grandchild is a separate question, and a much harder one, because the marks are wiped almost clean between generations.
摘要
DNA 上层的标记决定细胞使用哪些基因。你已经操作过其中一个：X 染色体失活是人类生物学中最清楚的例子。艰难的经历确实会在一个人的一生中改变这些标记。但它们是否会传到孙辈，是另一个问题，而且难得多——因为这些标记在代际之间几乎被完全擦除。
Add methyl tags to the DNA and watch the gene go quiet. Then keep an eye
on the last number: no matter how many tags you add, not one letter of the sequence
changes.给 DNA 加上甲基标记，看着基因安静下来。然后盯住最后一个数字：
无论你加多少标记，序列里没有一个字母发生改变。
0METHYL TAGS
100%GENE OUTPUT
0LETTERS CHANGED
tagged CGuntagged CGthe reader that starts the gene
Try this:
Add tags one at a time. The output does not fall evenly — almost nothing
happens, and then it falls off a cliff.
Press 🧽 Wipe it clean. That is not a trick. It is roughly what happens to
the germline between one generation and the next.
Watch letters changed the whole time. It never moves. That is the difference
between a mark and a mutation.
You will hear that trauma is stored in your DNA. The letters of
your DNA do not change. What changes is the layer of marks sitting on top of
them — chemical tags like methylation — which decide how loudly each gene is
read. That is epigenetics, and it is a real and well-measured thing.
It matters which one you say. A changed letter is permanent and copied
exactly. A mark is neither. Marks are added, removed and rewritten all the time,
which is what makes them useful — and what makes them hard to inherit.
你会听到“创伤被储存在 DNA 里”这种说法。你 DNA 的字母并没有改变，
改变的是覆盖在它们之上的标记层——像甲基化这样的化学标签——
它们决定每个基因被读取的强弱。这就是表观遗传学，是真实且已被充分测量的现象。
Go back to the silenced X. Every cell with two X chromosomes
picks one and shuts it down, and it stays shut in every cell made from it afterwards.
Nothing in the DNA sequence changed. The silenced X is wrapped up and tagged, and the
tags are copied at every cell division. That is epigenetics, and the calico cat is a
photograph of it.
Now the part that matters here. Your mother's silenced X is not the one you
silenced. The mark was wiped, and your own embryo threw the coin again, from
scratch. A cat's daughters do not inherit her pattern. The lab's own machine is a
demonstration of the thing that makes inheriting a mark so difficult.
回到被沉默的 X那一课。每个带两条 X 的细胞都会挑一条关掉，
而在此后由它产生的每个细胞中，那条 X 都保持关闭。DNA 序列没有任何改变——
被沉默的那条 X 被包裹起来并加上标签，标签在每次细胞分裂时被复制。这就是表观遗传学，
三色猫就是它的一张照片。
Hard experience changes these marks, and we can measure it. Early stress
is linked to changed methylation on genes in the stress-response system, in animals under
controlled conditions and in people. Nobody serious argues about this part.
So if the claim is "what happened to you changes how your body reads its own
genes for years afterwards" — that is supported.
艰难的经历确实会改变这些标记，而且可以被测量。
早期压力与应激反应系统相关基因上的甲基化变化有关联——在受控条件下的动物身上如此，
在人身上也是如此。这一部分没有认真的学者提出异议。
Across generations, it is not settled.
The evidence in mice is genuinely strong. In one careful experiment, male mice were taught
to fear one particular smell, and their pups and grand-pups reacted more strongly to that
same smell, having never met it. The team even used IVF, so the pups never met their
fathers either — which rules out the pups simply learning it.
In humans it is much weaker. The best-known studies followed Holocaust survivors
and their children and found differences in the marks on a stress gene. The first of those
studies had thirty-two survivors and twenty-two children in it. That is a small
number to carry a claim this large, the authors said so themselves, and larger follow-up
studies are still working through it.
跨代传递并未有定论。
小鼠中的证据确实很强：在一项严谨的实验中，雄性小鼠被训练害怕一种特定气味，
它们的子代和孙代对同一种气味反应更强烈，尽管从未接触过它。
研究团队甚至使用了体外受精，使幼鼠从未见过父亲——这排除了幼鼠只是“学来的”这一解释。
What is solidly passed down, and needs no epigenetics at all
Be careful not to let a hard question hide an easy answer. A parent who
lived through something terrible may be frightened, watchful, silent about it, or unwell.
A war destroys money, land, schooling and neighbourhoods, and those losses land on
children directly. Children learn fear by watching. All of that is real, measurable, and
passed to the next generation without any mark on any chromosome.
So when a study finds that the grandchildren of survivors are doing worse, the
first question is not which gene. It is whether anything needed to travel through
an egg or a sperm at all.
小心别让一个困难的问题遮住一个简单的答案。
一位经历过可怕事件的父母，可能会恐惧、警觉、闭口不谈，或者身体不好；
战争摧毁金钱、土地、教育和社区，这些损失直接落在孩子身上；孩子也会通过观察学会恐惧。
这一切都是真实的、可测量的，而且无需任何染色体上的标记就能传给下一代。
Why eggs and sperm make this a question about timing
The two germlines are built on completely different schedules, and that
is why the counting comes out differently depending on which parent it happened to.
Eggs are made early. All of a person's egg cells are formed before they
are born. So a pregnant woman is carrying her child and the cells that will become
her grandchild, at the same time, in the same body. One event reaches three generations
at once, and none of it is inheritance.
Sperm are made continuously, from puberty onward, which is why a father's
exposure reaches one generation fewer — and why the mouse experiments use fathers. It is
the cleaner test.
两种生殖系的建造时间表完全不同，这正是“事情发生在哪一位家长身上”会数出不同结果的原因。
Three questions, and they will sort almost anything you read.
One: did the DNA letters change, or the marks on them? Almost always the marks —
so "rewrites your DNA" is already wrong. Two: which generation was physically
present when it happened? The drop-down above does that counting. Three: how many
people were in the study?
None of that makes the field fake. It is a real field, doing hard work on a
question that matters. It means the honest headline is longer and duller than the one
you will actually be shown.
三个问题，几乎可以帮你分辨你读到的任何一篇报道。
第一：改变的是 DNA 的字母，还是它上面的标记？几乎总是标记——
所以“改写你的 DNA”本身就是错的。第二：事情发生时，哪一代人当时在场？上面的折叠说明里就在数这件事。
第三：这项研究有多少参与者？
The eight things worth carrying out of this lab.
这个实验室里最值得记住的八点。
The Y is not a small X. The X carries about eight hundred genes and everybody
needs one. The Y carries fewer than fifty protein genes, and almost none of them are
also on the X.
Y 不是缩小的 X。X 携带约八百个基因，每个人都需要一条；
Y 上的蛋白质基因不到五十个，而且几乎没有一个也出现在 X 上。
One gene, switched on for a few days, starts everything. SRY does not build a
body. It starts a chain, and then goes quiet.
一个只开启几天的基因启动了一切。SRY 并不建造身体，
它只是启动一条链条，然后沉默。
The other path is built, not skipped. Making an ovary takes its own active
genes, and switching one of them off in an adult starts turning the organ into a testis.
另一条路是被建造出来的，不是被跳过的。
造卵巢需要它自己的活跃基因；在成年个体中关掉其中一个，
器官就会开始变成睾丸。
Chromosomes are step one of four. Chromosome, gonad, hormone, receptor. Each
step can vary on its own, so the chromosomes alone do not tell you what a body will look
like.
染色体只是四步中的第一步：染色体、性腺、激素、受体。
每一步都可能单独出现差异，所以仅凭染色体并不能断定身体会是什么样子。
A body with two X chromosomes is a patchwork. Each cell switched one X off at
random, very early, and its descendants all kept that choice. A calico cat is that
patchwork made visible.
带两条 X 的身体是一块拼布：每个细胞在很早时随机关掉一条 X，
它的后代全都保留了这个选择。三色猫就是这块拼布被看见的样子。
One copy means no spare. That single fact explains why red-green colour
blindness reaches about eight in a hundred men and about one in two hundred women.
只有一份就意味着没有备份。仅这一个事实，
就解释了为什么红绿色盲在男性中约为百分之八，在女性中约为两百分之一。
A chromosome that cannot swap cannot repair itself. That is why the Y has lost
most of its genes — and why other animals have replaced the whole system more than once.
不能交换的染色体无法自我修复。
这正是 Y 丢失了大部分基因的原因，也是别的动物不止一次整套替换这一系统的原因。
Experience changes the marks on DNA, not the letters. Within one life that is
solid. Across generations it is not settled — and before believing any such story, count
who was physically there when it happened.
经历改变的是 DNA 上的标记，而不是字母。
在一个人的一生之内，这一点是确凿的；跨代传递则尚无定论——
在相信任何这类说法之前，先数一数当时有哪些世代真的在场。
Thank you for working through it —
and see you in the next lab.
感谢你完成这一课——我们下个实验室见。
🖼️ About the pictures
Every picture on this page is a diagram, drawn
by the page itself in code while you watch. Nothing here is a photograph, and nothing here
was generated by an image model. That is a deliberate choice: a chromosome map, a family
tree and a temperature curve are all specified geometry — there is a right answer for
where every line goes, and a picture that reinterprets it is simply wrong. The colours in
the colour-vision demonstration are an approximation of one common kind of colour blindness,
and screens differ, so treat it as a demonstration and not as a test.
本页上的每一张图都是示意图，由页面用代码当场绘制。
这里没有照片，也没有任何图像模型生成的内容。这是有意的选择：
染色体图谱、家族树和温度曲线都属于被规定好的几何——每一条线该画在哪里都有正确答案，
而重新“演绎”它的图片只会是错的。色觉演示中的颜色是对一种常见色盲的近似模拟，
而且不同屏幕差别很大，请把它当作演示，而不是检测。
Related labs on this site:
Gene Lab — what DNA is, how a cell reads it, and how it is
edited. Neuro Lab — how a neuron decides.
本站相关实验室：基因实验室——
DNA 是什么、细胞如何读取它、以及如何编辑它；
神经实验室——神经元如何做决定。