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  • My wife is pregnant right now with our first child,

    そして、妻の大きなお腹を見て、 ほとんどみんな、最初にこうきく。

  • and when people see her with her big baby bump,


  • the first question people ask, almost without fail,

    私たちが、こんな質問を普通にしているのは、 自分たち人間の生態を、よく理解しているからだ。

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

    私たちは、生まれてくる子の性別は 男の子50%、女の子50%なのが、当たり前だと思っている。

  • Now, there are some assumptions behind that question


  • that we take for granted because of our familiarity with our own human biology.

    それは、人類にいたるまで進化をとげてきた 性別決定のシステムで、決まっている。

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


  • of either answer, boy or girl.

    XとYの性染色体のシステムによって、 遺伝的に決まる。

  • But why is it that way?

    哺乳動物には、1組の性染色体があって、 片方を母親から、そしてもう片方を父親から受けつぐ。

  • Well, the answer depends on the sex determination system

    両方Xのときは女の子が、 XとYが1本ずつのときは、男の子が生まれる。

  • that has evolved for our species.


  • You see, for most mammals,


  • the sex of a baby is determined genetically


  • with the XY chromosome system.

    赤ちゃんが男の子か女の子かの確率は、 それぞれ50%、50%ということになる。

  • Mammals have a pair of sex chromosomes,


  • one passed down from Mom, and one from Dad.


  • A pair of X's gives us a girl,


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


  • 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,

    だが、性別は父親によって決められるのではなく、 母親によって決定される。

  • the sex is determined by the father


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


  • This system has worked well for mammals,

    けど、この動物の場合、 Z染色体とW染色体の両方があるとき、メスが生まれる。

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


  • that have worked just as well for other animals.

    母親の卵子にある性染色体が、 ZかWかで、性別が決まる。

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

    まったく違う方法で、 遺伝的に性別を決定しているグループもある。

  • but their systems can be pretty different from ours.

    たとえば、アリの性別決定は、 一番おもしろいシステムだ。

  • Birds and some reptiles have their sex genetically determined


  • but instead of the sex being determined by Dad,


  • their sex is determined by Mom.


  • In those groups, a pair of Z sex chromosomes

    食べ物を集める働きアリもいるし、 巣を掃除したり、子どもの世話をするアリもいる。

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


  • However, in these animals,


  • one Z and one W chromosome together,


  • as a pair, produces a female.

    女王アリがこのたくわえた精子を使って、 卵子を受精させたら、

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


  • it just depends on whether Mom puts a Z or a W

    しかし、もし受精させなくても 卵子はちゃんとアリになる。

  • into her egg.


  • Certain groups have taken genetic sex determination


  • in completely other directions.

    このように、オスのアリは、 片方の遺伝子のコピーだけで生きている。

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


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


  • you do not have a father.


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


  • There are soldiers that defend the colony,


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


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

    そうなると、全ての動物の性別が、 遺伝子で決まると考えがちだ。

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


  • And this is where the system gets really interesting.


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


  • then that egg will grow up to become female.

    これらの種は、性別が胚が成長しているときの 気温で決定される。

  • However, if she lays an egg without fertilizing it,

    これらの種では、卵が生まれたときには、 まだ性別が決まっていない。

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

    胚の成長が半ばまで進んでも、まだ性別は決まらない。 決められた「そのとき」がくるまで、決まらないのだ。

  • but it will always be a male.


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

    たとえば、ニシキガメは、 ある気温よりも高い場合、メスがうまれる。

  • And male ants live their life like this,


  • with only one copy of every gene,


  • much like a walking sex cell.

    ニシキガメの場合は、温かかったら、ホットなギャルが、 涼しかったら、クールな色男が生まれるってことだ。

  • This system is called a haplodiploid system,

    熱帯魚には、オスかメスかということが、 一生の後半まで、決まらないものもいる。

  • and we see it not only in ants,


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


  • Since our own sex is determined by genes,

    こいつらは厳しい階級制度のもと、 小さい集団で生活している。

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


  • it's easy to assume that for all animals

    そして、驚くべきだが、 集団内で権力あるメスが死んだとき、

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

    体が最も大きくそして権力もあるオスが、 メスに変わって、死んだメスの地位につく。

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


  • has nothing to do with genes at all,


  • and it can depend on something like the weather.


  • These are animals like alligators and most turtles.

    シダの性別は、温度とは全く異なった 環境条件によって決まる。

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


  • is determined by the temperature.


  • 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


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


  • And during this time, the sex is completely determined


  • by temperature in the nest.


  • In painted turtles, for example,


  • warm temperatures above the critical temperature


  • will produce females within the eggs,


  • and cool temperatures will produce a male.


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


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


  • they are all hot chicks and cool dudes.


  • 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.


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


  • However, as they mature, they become female.

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

  • where only the most dominant male and female reproduce.

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

  • the largest and most dominant male will then quickly become female

  • and take her place, and all of the other males will move up one rank in the hierarchy.

  • In another very different ocean animal,

  • the Green Spoonworm,

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

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

  • happens to randomly fall on the sea floor.

  • If a larva lands on the open sea floor,

  • then it will become a female.

  • But if it lands on top of a female,

  • then it will become a male.

  • So for some species, the question of boy or girl

  • is answered by genetics. For others, it's answered by the environment.

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

  • Take whiptail lizards, for example.

  • For those desert lizards, the answer is easy.

  • It's a girl. It's always a girl.

  • They are a nearly all-female species,

  • and although they still lay eggs, these eggs hatch out female clones of themselves.

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

  • Throughout the entire animal kingdom,

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

  • For humans, that system is a genetic XY system.

  • And for me and my wife, we found out

  • it's gonna be a baby boy.



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B1 中級 日本語 TED-Ed 性別 メス オス アリ 染色

【TED-Ed】性別の決まり方:あなたが考えるよりもっと複雑 (Sex determination: More complicated than you thought - Aaron Reedy)

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    VoiceTube に公開 2016 年 08 月 15 日