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性别鉴定:远比你想的要复杂

Sex Determination: More Complicated Than You Thought

我的妻子现在怀了我们的第一个孩子
My wife is pregnant right now with our first child,
当人们看到她与她的大肚子
and when people see her with her big baby bump,
问的第一个问题,几乎没有例外
the first question people ask, almost without fail, is,
是男孩還是女孩?
"Is it a boy or is it a girl?"
现在,这个问题背后有一些假设
Now, there are some assumptions behind that question
并且我们认为是理所当然的
that we take for granted
因为我们熟悉我们自己的人类生物学。
because of our familiarity with our own human biology.
对于婴儿而言,我们理所当然地认为,有相等的机会
For human babies, we take it for granted that there’s a 50/50 chance
其答案會是,男孩或女孩
of either answer, boy or girl.
但是为什么这样呢?
But why is it that way?
答案取决于性别决定系统
Well, the answer depends on the sex determination system
性別決定系統有關
that has evolved for our species.
你看,對大多數哺乳類動物
You see, for most mammals,
嬰兒的性別取決於遺傳的
the sex of a baby is determined genetically
XY染色体的系统。
with the XY chromosome system.
哺乳动物有一对性染色体,
Mammals have a pair of sex chromosomes,
一个来自妈妈,另一个来自爸爸。
one passed down from mom, and one from dad.
两个X染色体创造一个女孩,
A pair of X’s gives us a girl,
一个X和一个Y创造一个男孩。
and an X and a Y together gives us a boy.
因为女性的卵细胞只传递X染色体,
Since females only have X’s to pass on in their egg cells,
而男性可以传递一个X或Y精子细胞,
and males can give either an X or a Y in their sperm cells,
因而性別是由父親決定
the sex is determined by the father
生育一个男性或女性的机会是相等的。
and the chance of producing a male or a female is 50/50.
这个系统很好的适用于哺乳动物,
This system has worked well for mammals,
但在生命这棵大树上,我们可以看到其他系统
but throughout the tree of life, we can see other systems
也很好地适用于别的动物
that have worked just as well for other animals.
其他动物也有基因决定性别系统
There are other groups of animals that also have genetic sex determination,
但是它们的系统和我们的大不相同
but their systems can be pretty different from ours.
鸟类和一些爬行动物有它们自己的性别基因决定系统
Birds and some reptiles have their sex genetically determined,
但是和“性别由父亲决定”不同
but instead of the sex being determined by dad,
它们的性别由母亲决定
their sex is determined by mom.
在这些群体中,一对Z染色体
In those groups, a pair of Z sex chromosomes
可以创造一个雄性,所以这些雄性只能遗传Z染色体
produces a male, so these males only have Z’s to give.
但是,对这些动物而言,一个Z和一个W染色体组合
However, in these animals, one Z and one W chromosome together,
成为一对,可以创造一个雌性
as a pair, produces a female.
在這樣的系統裡,生產雄性或雌性的機會仍是相同的
In this system, the chance of a male or a female is still 50/50,
仅仅在于母親传递了一條Z染色体還是一條 W染色体
it just depends on whether mom puts a Z or a W
通过她的卵子
into her egg.
某些动物的基因决定性别
Certain groups have taken genetic sex determination
由其他方向决定
in completely other directions.
舉個例子,螞蟻就有一套非常有趣的系統
Ants, for example, have one of the most interesting systems
來決定性別,而且正因為如此如果你是隻雄蚁
for determining sex, and because of it, if you are a male ant,
你没有父亲
you do not have a father.
在一个蚁群,戏剧性地,有劳动部门
In an ant colony, there are dramatic divisions of labor.
有保卫领土的士兵,
There are soldiers that defend the colony,
有收集食物、打扫巢穴以及照顾幼蚁的工蚁
there are workers that collect food, clean the nest and care for the young,
并且有一个蚁后和一小群雄蚁
and there’s a queen and a small group of male reproductives.
现在,蚁后将交配并从雄蚁身上收集精子
Now, the queen will mate and then store sperm from the males.
并且这就是这个系统真正的有趣之处
And this is where the system gets really interesting.
如果蚁后用储存的精子使一个卵子受精,
If the queen uses the stored sperm to fertilize an egg,
这个卵子将会长大并成为雌性
then that egg will grow up to become female.
但是,如果蚁后没有使一个卵子受精
However, if she lays an egg without fertilizing it,
這個卵子仍會長成一隻螞蟻
then that egg will still grow up to be an ant,
但會是隻雄蟻
but it will always be a male.
所以,雄蟻是不可能有父親的
So you see, it’s impossible for male ants to have fathers.
雄蚁只有一个来自每个基因的复制品,像这样生活着
And male ants live their life like this, with only one copy of every gene,
就好像一個活生生的生殖細胞
much like a walking sex cell.
這個系統稱作单倍二倍体性决定性系統
This system is called a haplodiploid system,
不只在螞蟻上有此現象
and we see it not only in ants,
其他高度群居昆蟲也有,如蜜蜂及黃蜂
but also in other highly social insects like bees and wasps.
我们的性别由基因决定
Since our own sex is determined by genes,
并且我们的确知道其他动物的性别由基因决定
and we do know of these other animals that have their sex determined by genes,
很容易假设所有动物都是
it’s easy to assume that for all animals
其胎儿的性別都是由基因決定
the sex of their babies still must be determined by genetics.
但是,對某些動物而言,是男是女這個問題
However, for some animals, the question of whether it will be a boy or a girl
與基因一點關係都沒有
has nothing to do with genes at all,
并且它可以由一些别的因素决定,比如天气
and it can depend on something like the weather.
某些动物,比如鳄鱼和大多数乌龟
These are animals like alligators and most turtles.
在这些动物中,一个发育中的受精卵里的胚胎的性别
In these animals, the sex of an embryo in a developing egg
由温度决定
is determined by the temperature.
在这些物种中,胎儿的性别
In these species, the sex of the baby
不是在蛋被产下时被决定,
is not yet determined when the egg is laid,
而是直到某个胚胎发育中期才能被决定,
and it remains undetermined until sometime in the middle
在某个关键时刻来临时
of the overall development period, when a critical time is reached.
在這段期間,其性別完全取決於
And during this time, the sex is completely determined
巢內的溫度
by temperature in the nest.
对未孵化的锦龟而言,比如说
In painted turtles, for example,
当温暖的温度高于临界温度
warm temperatures above the critical temperature
蛋会孵化出雌龟
will produce females within the eggs,
而低温会孵化出雄龟
and cool temperatures will produce a male.
我不知道谁想出这个助记的办法
I’m not sure who came up with this mnemonic,
但是当谈论锦龟时你可以记住
but you can remember that when it comes to painted turtles,
它们都是火辣的少妇和很酷的小伙子
they are all hot chicks and cool dudes.
对于一些热带鱼而言,关于它们是男孩还是女孩的问题
For some tropical fish, the question of will it be a boy or will it be a girl
甚至在生下来之后都无法得出答案
isn’t settled until even later in life.
你看,小丑鱼生活之初都是雄性
You see, clownfish all start out their lives as males,
但是,当它们成熟,它们变为雌性
However, as they mature, they become female.
它们也生活在一个有严格等级序列的小群体
They also spend their lives in small groups with a strict dominance hierarchy
一个只有占统治地位的雌性和雄性才能繁殖后代的地方
where only the most dominant male and female reproduce.
而且很奇異的是,如果群体內為首的雌性死了
And amazingly, if the dominant female in the group dies,
其最大最具優勢的雄鱼,會很快地變成雌性
the largest and most dominant male will then quickly become female
并且占据它的位置,而其他所有的雄性
and take her place, and all of the other males
将提升一个等级
will move up one rank in the hierarchy.
在另一种完全不同的海洋动物
In another very different ocean animal,
绿螠虫
the green spoonworm,
幼虫的性别
the sex of the babies is determined
由一个完全不同的环境方面决定
by a completely different aspect of the environment.
对于这个物种
For this species, it is simply a matter of where a larva
它只是一件关于幼虫随机掉在海底的事
happens to randomly fall on the sea floor.
如果一只幼虫降落在海底,它将成为雌性
If a larva lands on the open sea floor, then it will become a female.
但是如果它降落在雌性的顶部,它将成为一只雄性
But if it lands on top of a female, then it will become a male.
对于一些物种,男孩或者女孩的问题
So for some species, the question of boy or girl
由遗传学决定
is answered by genetics.
对另一些物种而言,这由环境决定
For others, it’s answered by the environment.
还有一些物种,这个问题甚至不能下结论
And for others still, they don’t even bother with the question at all.
例如,鞭尾蜥蜴
Take whiptail lizards, for example.
对那些沙漠蜥蜴,答案很简单
For those desert lizards, the answer is easy.
是雌性。总是雌性
It’s a girl. It’s always a girl.
她们是几乎全部雌性的物种,尽管她们依然产卵
They are a nearly all-female species, and although they still lay eggs,
这些卵孵化出克隆它们自己的雌性
these eggs hatch out female clones of themselves.
所以它将是一个女孩还是男孩?
So will it be a girl or will it be a boy?
在整个动物王国
Throughout the entire animal kingdom,
它真的完全取决于性别决定系统
it does really all depend on the system of sex determination.
對人類而言,這套系統是遺傳的XY系統
For humans, that system is a genetic XY system.
而對我及妻子,我們後來知道
And for me and my wife, we found out
这将会是一个小男孩。
it’s going to be a baby boy.

(Kiss)

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