性別決定：比你想像的更複雜：英語精讀講義

My wife is pregnant right now with our first child.

我妻子現在懷著我們的第一個孩子。

And when people see her with her big baby bump, 片語搭配

當人們看到她挺著大肚子時，

the first question people ask,

人們問的第一個問題，

almost without fail, 片語搭配

幾乎無一例外，

is,

都是：

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

「是男孩還是女孩？」

Now,

其實，

there are some assumptions behind that question that we take for granted 講解

這個問題背後有一些我們理所當然的預設，

because of our familiarity with our own human biology. 片語搭配

因為我們對自身的人類生物學非常熟悉。

For human babies,

對於人類嬰兒，

we take it for granted that there's a 50-50 chance of either answer, 用詞

我們理所當然地認為任何一種答案都有 50% 的機率，

boy or girl.

男孩或女孩。

But why is it that way?

但為什麼會這樣呢？

Well,

嗯，

the answer depends on the sex determination system 講解

答案取決於性別決定系統，

that has evolved for our species.

這是為了我們這個物種演化出來的。

You see,

你看，

for most mammals,

對大多數哺乳動物來說，

the sex of a baby is determined genetically 文法

嬰兒的性別是由基因決定的，

with the XY chromosome system. 文化背景

使用的是 XY 染色體系統。

Mammals have a pair of sex chromosomes,

哺乳動物有一對性染色體，

one passed down from mom 片語動詞

一條來自母親，

and one from dad.

一條來自父親。

A pair of X's gives us a girl

一對 X 染色體會生出女孩，

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

一條 X 染色體和一條 Y 染色體則生出男孩。

Since females only have X's to pass on in their egg cells 片語動詞

由於雌性的卵細胞中只有 X 染色體可以遺傳，

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

而雄性的精子細胞既能提供 X 也能提供 Y 染色體，

the sex is determined by the father

所以性別是由父親決定的，

and the chance of producing a male or a female is 50-50. 片語搭配

並且生男生女的機率各佔 50%。

This system has worked well for mammals,

這個系統對哺乳動物來說非常有效，

but throughout the tree of life, 慣用語

但在整個生命之樹中，

we can see other systems that have worked just as well for other animals. 片語搭配

我們可以看到其他動物也有同樣有效的系統。

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

還有其他類群的動物也是由基因決定性別的，

but their systems can be pretty different from ours. 用詞

但牠們的系統可能與我們的截然不同。

Birds and some reptiles have their sex genetically determined,

鳥類和一些爬行動物的性別是由基因決定的，

but instead of the sex being determined by dad, 文法

但牠們的性別不是由父親決定，

their sex is determined by mom.

而是由母親決定的。

In those groups,

在這些類群中，

a pair of Z sex chromosomes produces a male.

一對 Z 性染色體產生雄性。

So,

因此，

these males only have Z's to give.

這些雄性只有 Z 染色體可以提供。

However,

然而，

in these animals,

在這些動物中，

one Z and one W chromosome

一條 Z 染色體和一條 W 染色體

together as a pair produces a female.

配對在一起會產生雌性。

In this system,

在這個系統裡，

the chance of a male or a female is still 50-50.

產生雄性或雌性的機率仍然是 50%。

It just depends on whether mom puts a Z or a W into her egg. 文法

這只取決於母親在卵子中放入 Z 還是 W 染色體。

Certain groups have taken genetic sex determination in completely other directions. 慣用語

某些動物的基因決定性別系統則走向了完全不同的方向。

Ants,

螞蟻

for example,

就是一個例子，

have one of the most interesting systems for determining sex 文法

牠們有著最有趣的性別決定系統之一，

and because of it,

正因為如此，

if you are a male ant,

如果你是一隻雄蟻，

you do not have a father.

你其實是沒有父親的。

In an ant colony,

在一個蟻群中，

there are dramatic divisions of labor. 用詞

有著顯著的勞動分工。

There are soldiers that defend the colony,

有保衛蟻群的兵蟻，

there are workers that collect food,

有收集食物、

clean the nest and care for the young, 用詞

清理蟻巢並照顧幼蟲的工蟻，

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

還有一隻蟻后和一小群負責繁殖的雄蟻。

Now,

接下來，

the queen will mate and then store sperm from the males. 用詞

蟻后會交配，然後儲存來自雄蟻的精子。

And this is where the system gets really interesting. 表達脈絡

這就到了該系統真正有趣的地方。

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

如果蟻后用儲存的精子使卵子受精，

then that egg will grow up to become female. 片語搭配

那麼這個卵就會發育成雌性。

However,

然而，

if she lays an egg without fertilizing it, 講解

如果她產下一枚未受精的卵，

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

這枚卵仍會發育成一隻螞蟻，

but it will always be a male.

但牠永遠是雄性。

So,

所以，

you see,

你看，

it's impossible for male ants to have fathers. 文法

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, 用詞

雄蟻的一生就只有每個基因的一個副本，

much like a walking sex cell. 慣用語

非常像是一個行走的生殖細胞。

This system is called a haplodiploid system

這個系統被稱為單雙倍體系統，

and we see it not only in ants, 文法

我們不僅在螞蟻中能看到它，

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

在蜜蜂和黃蜂等其他高度社會化的昆蟲中也能看到。

Since our own sex is determined by genes 表達脈絡

既然我們自身的性別是由基因決定的，

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

而且我們也知道其他一些動物的性別也是由基因決定的，

it's easy to assume that for all animals,

我們很容易就會認為對於所有動物而言，

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

嬰兒的性別一定也是由基因決定的。

However,

然而，

for some animals,

對某些動物來說，

the question of whether it will be a boy or a girl 文法

到底是男孩還是女孩這個問題

has nothing to do with genes at all 慣用語

完全與基因無關，

and it can depend on something like the weather. 片語搭配

反而可能取決於天氣等因素。

These are animals like alligators and most turtles.

這些動物就包括短吻鱷和大部分龜類。

In these animals,

在這些動物中，

the sex of an embryo in a developing egg is determined by the temperature. 用詞

發育中的卵裡胚胎的性別由溫度決定。

In these species,

在這些物種裡，

the sex of the baby is not yet determined when the egg is laid 文法

卵剛產下時嬰兒的性別尚未決定，

and it remains undetermined 文法

並且會保持未決定狀態，

until sometime in the middle of the overall development period

直到整個發育期的中間階段，

when a critical time is reached. 用詞

也就是到了一個關鍵時期。

And during this time,

在這個時期，

the sex is completely determined by temperature in the nest.

性別完全由巢內的溫度決定。

In painted turtles,

在錦龜中，

for example,

例如，

warm temperatures above the critical temperature 用詞

高於臨界溫度的溫暖環境

will produce females within the eggs

會在卵內孕育出雌性，

and cool temperatures will produce a male.

而較低的溫度則會孕育出雄性。

I'm not really sure who came up with this mnemonic, 片語動詞

我不太清楚是誰想出了這個記憶口訣，

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

但你可以記住，說到錦龜，

they are all hot chicks and cool dudes. 慣用語

牠們全是辣妹（hot chicks）和酷哥（cool dudes）。

For some tropical fish,

對某些熱帶魚來說，

the question of will it be a boy or will it be a girl

到底是男孩還是女孩的問題

isn't settled until even later in life. 用詞

直到生命更晚的階段才會塵埃落定。

You see,

你看，

clownfish all start out their lives as males. 片語動詞

小丑魚在生命之初都是雄性。

However,

然而，

as they mature, 文法

隨著牠們的成熟，

they become female.

牠們會變成雌性。

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

牠們也在小群體中度過一生，有著嚴格的等級制度，

where only the most dominant male and female reproduce.

其中只有最高等級的雄性和雌性才能繁殖。

And amazingly,

令人驚訝的是，

if the dominant female in the group dies,

如果群體中處於支配地位的雌性死亡，

the largest and most dominant male

體型最大、佔支配地位的雄性

will then quickly become female

就會迅速變成雌性

and take her place 片語搭配

並取代她的位置，

and all of the other males will move up one rank in the hierarchy. 片語動詞

而其他所有雄性的等級也會隨之上升一級。

In another very different ocean animal,

在另一種截然不同的海洋動物——

the green spoonworm,

綠叉螠（green spoonworm）身上，

the sex of the babies is determined by a completely different aspect of the environment. 用詞

後代的性別由環境中一個完全不同的方面所決定。

For this species,

對這個物種而言，

it is simply a matter of where a larva happens to randomly fall on the seafloor. 片語搭配

問題僅僅在於幼蟲碰巧隨機落在了海底的哪個位置。

If a larva lands on the open seafloor,

如果幼蟲落在了空曠的海底，

then it will become a female.

牠就會變成雌性。

But if it lands on top of a female, 片語搭配

但如果牠落在了一隻雌性的身上，

then it will become a male.

牠就會變成雄性。

So,

所以，

for some species,

對某些物種來說，

the question of boy or girl is answered by genetics.

男孩還是女孩的問題是由基因解答的。

For others,

對另一些物種，

it's answered by the environment.

則由環境來解答。

And for others still, 表達脈絡

而對還有一部分物種來說，

they don't even bother with the question at all. 慣用語

牠們甚至根本不用為這個問題費心。

Take whiptail lizards, 表達脈絡

以鞭尾蜥蜴

for example.

為例。

For those desert lizards,

對於這些沙漠蜥蜴，

the answer is easy.

答案很簡單。

It's a girl.

是個女孩。

It's always a girl.

永遠是個女孩。

They are a nearly all-female species 用詞

牠們是一個幾乎全是雌性的物種，

and although they still lay eggs, 文法

儘管牠們仍然會產卵，

these eggs hatch out female clones of themselves. 片語動詞

但這些卵孵化出的都是牠們自己的雌性複製體。

So,

所以，

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

到底是個女孩還是個男孩呢？

Throughout the entire animal kingdom, 表達脈絡

放眼整個動物界，

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

這一切確實都取決於性別決定系統。

For humans,

對於人類來說，

that system is a genetic XY system.

這個系統是基因 XY 系統。

And for me and my wife,

而對於我和我的妻子，

we found out it's going to be a baby boy. 講解

我們發現我們將迎來一個男寶寶。