Sex Determination: More Complicated Than You Thought

性別決定系統：比你想像得更複雜

Translator: Bedirhan Cinar

譯者: Regina Chu 審譯者: Hao-Wei Chang

（音樂）

My wife is pregnant right now with our first child,

我的妻子正懷著我們第一個孩子

and when people see her with her big baby bump,

當人們看到她挺着大肚子

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

第一個問題幾乎總是

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

是男的還是女的？

Now, there are some assumptions behind that question

那麼，這個問題背後有個前提

that we take for granted

我們之所以不以為奇，是因為我們 熟悉我們自己的人類生物學

because of our familiarity with our own human biology.

對人類嬰兒而言，我們想當然 認為有各半的機會

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

of either answer, boy or girl.

其答案會是男生或女生

But why is it that way?

但這是為什麼？

Well, the answer depends on the sex determination system

嗯，這答案與我們這個物種之

that has evolved for our species.

性別決定系統有關

You see, for most mammals,

你看，對大多數哺乳類動物

the sex of a baby is determined genetically

嬰兒的性別取決於遺傳的

with the XY chromosome system.

ＸＹ染色體系統

Mammals have a pair of sex chromosomes,

哺乳動物有一對性別染色體

一條來自母親，另一條則來自父親

one passed down from mom, and one from dad.

A pair of X's gives us a girl,

一對ＸＸ會得到女兒

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

而一條Ｘ及一條Ｙ配對 則得到兒子

既然雌性只有Ｘ可以傳給卵子

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

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

而雄性可以傳Ｘ或Ｙ給精子

the sex is determined by the father

因而性別是由父親決定

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

而生產雄性或雌性的機會為各半

This system has worked well for mammals,

這個系統在哺乳動物內運作良好

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

但在整個演化樹上 我們還看到其他系統

that have worked just as well for other animals.

在其它動物身上運作一樣良好

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

其他類動物也有遺傳性別決定法

but their systems can be pretty different from ours.

但牠們的系統與我們的大庭相逕

鳥類及某些爬行類動物 也有性別遺傳決定法

Birds and some reptiles have their sex genetically determined,

but instead of the sex being determined by dad,

但其性別不是由爸爸決定

their sex is determined by mom.

而是由媽媽決定

In those groups, a pair of Z sex chromosomes

在這些動物內 一對ＺＺ性別染色體

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

會生出雄性 所以這些雄性只能給Ｚ

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

然而，這些動物如果是

一條Ｚ及一條 Ｗ 染色體配對

as a pair, produces a female.

則產生雌性

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

在這樣的系統裡 生產雄性或雌性的機會仍是各半

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

僅在於母親是放了一條Ｚ 還是一條 Ｗ

into her egg.

到卵子裡

Certain groups have taken genetic sex determination

但某些類則以完全不同的方法

in completely other directions.

來決定遺傳性別

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

舉個例子，螞蟻就有一套 非常有趣的系統

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

來決定性別，而且正因為如此 如果你是隻雄蟻

you do not have a father.

你沒有父親

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

在蟻巢裡，每隻螞蟻的 工作劃分非常絕對

There are soldiers that defend the colony,

有兵蟻專門守禦蟻巢

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

有工蟻收集食物 清理巢穴並照顧幼蟻

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

還有一隻蟻后 及一小群負責生殖的雄蟻

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

那麼，蟻后會與雄蟻交配 並儲存精子

And this is where the system gets really interesting.

而這就是這系統有趣的地方

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

如果蟻后拿儲存的精子 使卵子受孕

then that egg will grow up to become female.

那麼這個卵會長成一隻雌蟻

However, if she lays an egg without fertilizing it,

但是，如果牠下了個未受精卵

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

那這個卵子仍會長成一隻螞蟻

but it will always be a male.

但會是隻雄蟻

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

所以你看，雄蟻是不可能有父親的

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

而且雄蟻的一生就是這樣

每一種基因都只有單個

much like a walking sex cell.

就好像一個活生生的生殖細胞

This system is called a haplodiploid system,

這個系統稱作 單倍雙倍體性決定系統

and we see it not only in ants,

而我們不只在螞蟻上看到此現象

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

也在其他高度群居昆蟲 如蜜蜂及黃蜂上看到

Since our own sex is determined by genes,

既然我們人類的性別取決於基因

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

我們的確也知道有其它動物 一樣由基因決定性別

it's easy to assume that for all animals

我們很容易假設所有的動物

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

其嬰兒的性別都是由基因決定

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

但是，對某些動物而言 是男是女這個問題

has nothing to do with genes at all,

與基因一點關係都沒有

and it can depend on something like the weather.

反而被其它因素，如天氣決定

這些動物像鱷魚或大部份烏龜

These are animals like alligators and most turtles.

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

這些動物在蛋內發育的胚胎性別

is determined by the temperature.

取決於溫度

這些物種在下蛋時 蛋內幼體的性別尚未決定

In these species, the sex of the baby

is not yet determined when the egg is laid,

and it remains undetermined until sometime in the middle

且會維持未定狀態，直到大約

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

胚胎發育中期這個關鍵時刻

And during this time, the sex is completely determined

在這段期間，其性別完全取決於

by temperature in the nest.

巢內的溫度

In painted turtles, for example,

舉錦龜為例

warm temperatures above the critical temperature

如果溫度超過這個關鍵溫度

will produce females within the eggs,

會使蛋孵出雌性幼體

and cool temperatures will produce a male.

比較低的溫度則產生雄性

我不知道誰發明了這個助記法

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

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

但你可以這麼記，就是說到錦龜

they are all hot chicks and cool dudes.

牠們都是辣妹及酷哥

對某些熱帶魚而言，是男是女這個問題

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

isn't settled until even later in life.

一定要到生命更後期才能回答

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

你看，小丑魚在一開始都是雄性

但是，成熟後都變成雌性

However, as they mature, they become female.

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

牠們也一輩子活在小群體內 嚴格遵守階級制

where only the most dominant male and female reproduce.

只有為首的雄魚及雌魚能繁殖下一代

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

而且很奇異的是 如果小群內為首的雌性死了

the largest and most dominant male will then quickly become female

其最大最具優勢的雄魚 會很快地變成雌性

and take her place, and all of the other males

並且代替她，然後其他雄性的 地位都會向上提升一格

will move up one rank in the hierarchy.

而另一種非常不一樣的海洋動物

In another very different ocean animal,

the green spoonworm,

綠海洋蠕蟲

the sex of the babies is determined

則完全不同 其幼蟲的性別由環境決定

by a completely different aspect of the environment.

這種生物，就只看幼蟲

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

happens to randomly fall on the sea floor.

掉在海底哪裡

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

如果幼蟲直接掉在海底

就成了雌性

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

如果幼蟲掉在雌性上面

就變成雄性

So for some species, the question of boy or girl

所以對某些物種而言，是男是女

is answered by genetics.

由遺傳決定。對其他物種 則是由環境決定

For others, it's answered by the environment.

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

還有一些則根本不在乎這個問題

Take whiptail lizards, for example.

舉鞭尾蜥為例

For those desert lizards, the answer is easy.

對這些沙漠蜥蜴而言 這答案很簡單

女生。永遠都是女生

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

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

這個物種幾乎全是雌性

而且雖然牠們也下蛋，這些蛋 孵出來的只是複製的自己罷了

these eggs hatch out female clones of themselves.

所以是男是女呢？

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

Throughout the entire animal kingdom,

整個動物界從上到下

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

的確都有一套性別決定系統

For humans, that system is a genetic XY system.

對人類而言，這套系統是 遺傳的ＸＹ系統

And for me and my wife, we found out

而對我及內人，我們後來知道

it's going to be a baby boy.

會是個小男孩

(Kiss)