Sex Determination: More Complicated Than You Thought

性别决定系统：比你想象的要更复杂

Translator: Bedirhan Cinar

翻译人员: Minji Seo 校对人员: Gena Volz

My wife is pregnant right now with our first child,

我的妻子正怀着我们第一个孩子，

and when people see her with her big baby bump,

当人们看到她挺着大肚子，

the first question people ask, almost without fail, is,

第一个问题几乎总是：

"Is it a boy or is it a girl?"

“男孩还是女孩？”

Now, there are some assumptions behind that question

那么，这个问题背后有个前提，

that we take for granted

我们之所以不以为奇，

because of our familiarity with our own human biology.

是因为我们 熟悉我们自己的人类生物学。

For human babies, we take it for granted that there's a 50/50 chance

对人类婴儿而言， 我们想当然认为有各半的机会

of either answer, boy or girl.

其答案会是男生或女生。

But why is it that way?

但这是为什么？

Well, the answer depends on the sex determination system

嗯，这答案与我们这个物种的

that has evolved for our species.

性别决定系统有关。

You see, for most mammals,

你看，对大多数哺乳类动物

the sex of a baby is determined genetically

婴儿的性别取决于遗传基因的

with the XY chromosome system.

XY染色体系统。

Mammals have a pair of sex chromosomes,

哺乳动物有一对性别染色体，

one passed down from mom, and one from dad.

一条来自母亲，另一条则来自父亲。

A pair of X's gives us a girl,

一对XX会得到女儿，

and an X and a Y together gives us a boy.

而一条Ｘ及一条Ｙ配对 则得到儿子。

Since females only have X's to pass on in their egg cells,

既然雌性只有Ｘ可以传给卵子，

and males can give either an X or a Y in their sperm cells,

而雄性可以传Ｘ或Ｙ给精子，

the sex is determined by the father

因而性别由父亲决定，

and the chance of producing a male or a female is 50/50.

而生产雄性或雌性的机会为各半。

This system has worked well for mammals,

这个系统在哺乳动物内运作良好，

but throughout the tree of life, we can see other systems

但在整个演化树上， 我们还看到其他系统

that have worked just as well for other animals.

在其它动物身上运作一样良好。

There are other groups of animals that also have genetic sex determination,

其他类动物也有遗传性别决定法，

but their systems can be pretty different from ours.

但牠们的系统与我们的大相径庭。

Birds and some reptiles have their sex genetically determined,

鸟类及某些爬行类动物 也有性别遗传决定法，

but instead of the sex being determined by dad,

但其性别不是由爸爸决定，

their sex is determined by mom.

而是由妈妈决定。

In those groups, a pair of Z sex chromosomes

在这些动物里， 一对ZZ性别染色体

produces a male, so these males only have Z's to give.

会产生雄性， 所以这些雄性只能给Ｚ。

However, in these animals, one Z and one W chromosome together,

然而，这些动物如果是 一条Ｚ及一条 Ｗ 染色体配对

as a pair, produces a female.

则产生雌性。

In this system, the chance of a male or a female is still 50/50,

在这样的系统里 产生雄性或雌性的机率仍然对半开，

it just depends on whether mom puts a Z or a W

仅在于母亲是放了一条Ｚ 还是一条 Ｗ

into her egg.

到卵子里。

Certain groups have taken genetic sex determination

但某些物种则以完全不同的方法

in completely other directions.

来决定遗传性别。

Ants, for example, have one of the most interesting systems

举个例子，蚂蚁就有一套 非常有趣的系统

for determining sex, and because of it, if you are a male ant,

来决定性别，而且正因为如此， 假如你是只雄蚁，

you do not have a father.

你是没有父亲的。

In an ant colony, there are dramatic divisions of labor.

在蚁巢里，每只蚂蚁的 工作划分非常绝对。

There are soldiers that defend the colony,

有兵蚁专门守御蚁巢，

there are workers that collect food, clean the nest and care for the young,

有工蚁收集食物， 清理巢穴并照顾幼蚁，

and there's a queen and a small group of male reproductives.

还有一只蚁后 及一小群负责生殖的雄蚁。

Now, the queen will mate and then store sperm from the males.

蚁后会与雄蚁交配 并储存精子。

And this is where the system gets really interesting.

而这就是这系统有趣的地方。

If the queen uses the stored sperm to fertilize an egg,

如果蚁后拿储存的精子 使卵子受精，

then that egg will grow up to become female.

那么这个卵会长成一只雌蚁。

However, if she lays an egg without fertilizing it,

但是，如果牠下了个未受精卵，

then that egg will still grow up to be an ant,

那这个卵子仍会长成一只蚂蚁，

but it will always be a male.

但永远只是雄蚁。

So you see, it's impossible for male ants to have fathers.

所以你看，雄蚁是不可能有父亲的。

And male ants live their life like this, with only one copy of every gene,

而且雄蚁的一生就是这样， 只有单纯的一套基因，

much like a walking sex cell.

就好像一个活生生的生殖细胞。

This system is called a haplodiploid system,

这个系统被称作 染色体倍性性別決定系統，

and we see it not only in ants,

而我们不只在蚂蚁上看到此现象，

but also in other highly social insects like bees and wasps.

也在其他高度群居昆虫 如蜜蜂及黄蜂上看到。

Since our own sex is determined by genes,

既然我们人类的性别取决于基因，

and we do know of these other animals that have their sex determined by genes,

而且我们的确也知道有其它动物 一样由基因决定性别，

it's easy to assume that for all animals

我们很容易假设所有的动物

the sex of their babies still must be determined by genetics.

其后代的性别都是由基因决定。

However, for some animals, the question of whether it will be a boy or a girl

但是，对某些动物而言， 是男是女这个问题

has nothing to do with genes at all,

与基因一点关系都没有，

and it can depend on something like the weather.

反而是由其它因素， 比如天气来决定。

These are animals like alligators and most turtles.

这些动物包括鳄鱼和大部份龟类。

In these animals, the sex of an embryo in a developing egg

这些动物在蛋内发育的胚胎性别

is determined by the temperature.

取决于温度。

In these species, the sex of the baby

这些物种在下蛋时

is not yet determined when the egg is laid,

蛋内幼体的性别尚未决定，

and it remains undetermined until sometime in the middle

且会维持未定状态，直到大约

of the overall development period, when a critical time is reached.

胚胎发育中期，当达到某个关键时刻时。

And during this time, the sex is completely determined

在这段期间，其性别完全取决于

by temperature in the nest.

巢内的温度。

In painted turtles, for example,

举锦龟为例，

warm temperatures above the critical temperature

如果温度超过一个特定温度时，

will produce females within the eggs,

会使蛋孵出雌性幼体，

and cool temperatures will produce a male.

比较低的温度则产生雄性。

I'm not sure who came up with this mnemonic,

我不知道谁发明了这个助记法，

but you can remember that when it comes to painted turtles,

但你可以这么记，就是说到锦龟

they are all hot chicks and cool dudes.

牠们都是火热辣妹和清凉酷男。

For some tropical fish, the question of will it be a boy or will it be a girl

对某些热带鱼而言，是男是女这个问题

isn't settled until even later in life.

一定要到生命更后期才能回答。

You see, clownfish all start out their lives as males,

你看，小丑鱼在一开始都是雄性，

However, as they mature, they become female.

但是，成熟后都变成雌性。

They also spend their lives in small groups with a strict dominance hierarchy

牠们也一辈子活在小群体内 严格遵守阶级制，

where only the most dominant male and female reproduce.

只有为首的雄鱼及雌鱼能繁殖下一代。

And amazingly, if the dominant female in the group dies,

而且很奇异的是 如果小群内为首的雌性死了，

the largest and most dominant male will then quickly become female

其最大最具优势的雄鱼 会很快地变成雌性

and take her place, and all of the other males

并且代替她，然后其他所有雄性鱼的

will move up one rank in the hierarchy.

地位都依序向上提升一级。

In another very different ocean animal,

而另一种非常不一样的海洋动物，

the green spoonworm,

绿海洋蠕虫，

the sex of the babies is determined

其幼虫的性别

by a completely different aspect of the environment.

则由完全不同的其他环境因素来决定。

For this species, it is simply a matter of where a larva

这种生物，就只看幼虫

happens to randomly fall on the sea floor.

掉在海底哪里。

If a larva lands on the open sea floor, then it will become a female.

如果幼虫直接掉在海底， 就成了雌性。

But if it lands on top of a female, then it will become a male.

如果幼虫掉在雌性上面， 幼虫就成了雄性。

So for some species, the question of boy or girl

所以对某些物种而言，生男生女

is answered by genetics.

是由遗传决定。

For others, it's answered by the environment.

对其他物种而言， 则是由环境决定的。

And for others still, they don't even bother with the question at all.

还有一些则根本不在乎这个问题。

Take whiptail lizards, for example.

举鞭尾蜥为例，

For those desert lizards, the answer is easy.

对这些沙漠蜥蜴而言， 这答案很简单。

It's a girl. It's always a girl.

女生。永远都是女生。

They are a nearly all-female species, and although they still lay eggs,

这个物种几乎全是雌性， 而且尽管牠们也下蛋，

these eggs hatch out female clones of themselves.

这些蛋 孵出来的只是复制的自己罢了

So will it be a girl or will it be a boy?

所以是男是女呢？

Throughout the entire animal kingdom,

整个动物界从上到下，

it does really all depend on the system of sex determination.

的确都有一套性别决定系统。

For humans, that system is a genetic XY system.

对人类而言，这套系统是 遗传基因的XY系统，

And for me and my wife, we found out

至于我和我太太，我们后来知道

it's going to be a baby boy.

会是个小男孩。

(Kiss)

（接吻声）