每當我們看見親朋好友懷孕時總會問到：「寶寶是男生還是女生呢？」
你知道嗎，這個問題背後其實就潛藏著我們對人類性別決定機制的認知！大家都熟知，人類寶寶性別由 XY 染色體決定，而男女的機率各半；但對廣大動物界的其他生物來說，性別可比我們想像中的要更複雜！TED-Ed 講者 Aaron Reedy 與動畫團隊 Buzzco Associates, Inc. 合作製作的這支動畫，就告訴你生物界複雜的性別決定機制！
基因決定性別
你是女生？或男生？我們理所當然的將人群分為這兩種性別。人類的寶寶在懷胎十七週之後，會逐漸顯露出基因密碼所決定的生理性徵，而對大部份的哺乳類動物來說，性別都是在精子與卵子結合成受精卵的那刻就決定好的，而由一對 XY 性染色體所決定的男女性別各有一半的機率：女生為 XX，男生為 XY。由於只有男生的性染色體有 X 及 Y 兩種組成可傳遞下去，人類後代的性別其實是由父親決定的！
也就是說，在科學知識尚未普及的傳統社會中，社會上重男輕女的觀念常錯誤的將壓力施加在女性身上；但實際上，決定胎兒性別的可是父親！有些動物，例如鳥類與某些爬蟲類動物，也是由基因決定性別；不同的是，鳥類決定後代性別的是擁有 ZW 性染色體母鳥。
單倍雙倍體性決定系統（haplodiploid system）
動物界還有著各式各樣有趣的性別決定機制。螞蟻和蜜蜂就屬於「單倍雙倍體性決定系統」。螞蟻社會中有著複雜的社會分工，而一個蟻巢只會有一隻蟻后。這隻蟻后將雄蟻的精子儲存起來，若是在產卵時讓卵子受精，受精卵就會成長成雌蟻，離開巢穴、成為蟻后、建立新的王國；但若在產卵時未讓卵受精，未受精的卵則會產出雄蟻，成為螞蟻王國最堅強的工蟻陣容！
讀至此，你是否和我一樣好奇：究竟蟻后是如何決定何時讓卵受精、怎麼知道巢中的工蟻數量已經足夠、是時候使用儲存好的精子產生雌蟻呢？讓我們一起尋找答案！
環境決定性別
此外，也有許多生物的性別是由周遭環境、生長發育時間點、與族群中的性別分佈所決定的。首先，短吻鰐及錦龜就是由溫度決定性別。產下的卵還沒有所謂的性別之分，等到發育期中的某個關鍵時刻，這些卵才會因為巢中的溫度而產生雌雄之別。若溫度高於特定溫度，卵會孕育出雌龜，而較冷的溫度則會使胚胎變成雄性。也就是說，錦龜的世界中，只有火辣熱情的女孩（熱）和冷酷的男孩（冷）！
另外，若是這些生物的性別由環境溫度所決定，那麼現今全球暖化及氣後變遷的問題又將對這些生物造成什麼影響呢？值得我們思考！
許多熱帶魚，例如小丑魚，則是在生命後期才有性別之分。所有的小丑魚在出生時都是雄性，逐漸長大、成熟時，他們會變成雌性。試想一個社會中，幼稚園及小學中只有小男孩，而國高中裡的學生卻又只有女孩，是不是很神奇呢？有趣的是，魚群中只有最優勢、最大隻的雌魚，與最優勢的小雄魚能交配，若這隻雌魚死去，那麼方才那隻魚群中體型大、最具優勢的雄魚就會立刻轉而變成雌魚、取代她的位置！
除了上述這種「轉性」的有趣機制，性別也可以因為幼體所處的地點而有所不同。綠蠕蟲的性別決定於幼蟲掉落的地點：若幼蟲掉到海底上，牠便會成為雌性；若幼蟲掉落到雌性身上，牠則自動變成雄性！
性別沒有分別！
除了上述這些孕育出不同性別的機制，更有些生物的性別界線是模糊的！除了我們所熟知的蚯蚓是「雌雄同體」，還有有趣的鞭尾蜥蜴，藉由孤雌生殖與假交配，讓一個全是女生的族群也能繁衍出後代、生生不絕！
在形形色色的動物世界中，我們看見不同的性別樣貌與運作方式；而這一切都提醒著我們萬物的多樣與奧妙；當發現自己被習以為常的邏輯思維綁住時，不妨走進自然，看看這些豐富的令人讚嘆的存在吧！也許能因此突破固有的想法也不一定呢！
註：
由於 TED-Ed 官方還在進行網站與影片的中文化，目前尚未開放有中文字幕的TED-Ed影片。如果對完整中文稿有興趣，以下是我們為您準備的影片中英對照稿，敬請參考：
My wife is pregnant right now with our first child, and when people see her with her big baby bump, the first question people ask, almost without fail, is, “Is it a boy or is it a girl?” Now, there are some assumptions behind that question that we take for granted because of our familiarity with our own human biology. For human babies, we take it for granted that there’s a 50/50 chance of either answer, boy or girl. But why is it that way? Well, the answer depends on the sex determination system that has evolved for our species.
我的太太懷了我們的第一胎，而每個人看到她大腹便便的模樣時總會問到：「寶寶是男生還是女生呢？」在這問題背後，其實潛藏著我們對人類性別機制的認知。我們理所當然的認為，人類寶寶是男生、女生的機率各半；但這是為什麼呢？答案就在於人類這個物種的生物決定機制。
You see, for most mammals, the sex of a baby is determined genetically with the XY chromosome system. 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, 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, but throughout the tree of life, we can see other systems that have worked just as well for other animals. There are other groups of animals that also have genetic sex determination, but their systems can be pretty different from ours.
對大部份的哺乳類動物來說，寶寶的性別是由基因中的 XY 染色體所決定的。哺乳類動物有一對性染色體，其中一個來自母親，另一個來自父親。若染色體為 XX 的為女生、XY 則為男生。由於女生的染色體中能透過卵子傳遞給下一代只有 X，而男生則有 X 及 Y 兩種可傳遞下去，後代的性別其實是由父親決定的，而男女機率各半。這套系統適用於哺乳類動物，但在生命圈的光譜中，各種生物的性別決定機制大不相同！有些動物的性別也是由基因決定的，但運作的方式就與我們相差許多。
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 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. 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 into her egg.
鳥類和部分的爬蟲類是由基因決定性別，但是男是女不是由父親決定，而是由母親的基因決定。在這些動物中，一對 Z 染色體能產生雄性，而 ZW 則產生雌性。這套系統中，雌雄兩者的機率也是各半，而決定全落在母親傳遞了 Z 或者 W 給後代！
Certain groups have taken genetic sex determination in completely other directions. 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. 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. However, if she lays an egg without fertilizing it, then that egg will still grow up to be an ant, but it will always be a male. So you see, it’s impossible for male ants to have fathers. And male ants live their life like this, with only one copy of every gene, much like a walking sex cell. This system is called a haplodiploid system, and we see it not only in ants, but also in other highly social insects like bees and wasps.
有些物種的性別決定機制則全然不同。舉例來說，螞蟻就有著世上最有趣的性別決定機制，使得所有的公蟻都沒有父親！在一個蟻群中，勞力分工十分精密，有兵蟻守護蟻巢、工蟻搜集食物、清掃蟻窩及照顧幼蟻、以及一位蟻后和一群專門交配的雄蟻。蟻后與牠們交配，並儲存下這些雄蟻們的精子。有趣的部分來了！如果蟻后產卵時用上了這些精子，這些卵就會孕育出雌蟻；然而，若她產卵時沒有讓卵受精，這些卵仍可變成螞蟻，只不過他們的性別都會是雄性！這也就是為什麼所有的雄以都不可能有父親的緣故了！所有的雄蟻一輩子就只擁有這麼一種基因，幾乎可以說是行走的生殖細胞。這樣的機制被稱為「單倍雙倍體性決定系統」，而像螞蟻、蜜蜂、黃蜂都屬這類。
Since our own sex is determined by genes, and we do know of these other animals that have their sex determined by genes, it’s easy to assume that for all animals the sex of their babies still must be determined by genetics. 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.
由於我們人類的性別由基因決定、也有其他動物的性別由基因決定，我們很容易就認為所有動物都是如此。但是，生物界中還有許多生物的性別跟基因毫無關係，而是取決於其他因素，像是天氣！短吻鱷和絕大部份的烏龜都屬這類。
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.
這些動物卵中的胚胎性別因溫度而有所不同，寶寶的性別在卵被產下時還尚未決定，而是到了整個發育期中的某個關鍵時刻，才會因為巢中的溫度而產生分別。舉例來說，錦龜的巢比某個溫度標準還高時，卵會孕育出雌龜；而較低的溫度則會讓這些胚胎變成雄性。對此，有個不知是誰發明的簡易的記憶方式是：錦龜的世界中，只有火辣的女孩（熱）和冷酷的男孩（冷）！
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.
而某些熱帶魚的性別會到生命後期才會出現。所有的小丑魚在出生時都是雄性，但當他們逐漸成熟時，他們就會變成雌性。小丑魚的小型社會階級分明，只有最有優勢的雌魚和雄魚會生育後代。令人驚奇的是，若魚群中最優勢的雌魚死去，那麼魚群中體型大、最具優勢的雄魚就會轉而變成雌魚、取代她的位置,而其他所有的雄魚都將因此遞補至更高一層的階級！
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.
海中還有另外一種生物擁有十分不同的生物性別決定系統：綠蠕蟲。 綠蠕蟲的性別決定於幼蟲掉落的地點：若幼蟲掉到海底上，牠便會成為雌性；若幼蟲掉落到雌性身上，牠則自動變成雄性。
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.
對一些物種來說，性別由基因決定；對另一些物種來說，性別則由環境決定。然而，還有一些動物的性別根本沒有這些分別，像是鞭尾蜥蜴！對這些沙漠中的蜥蜴來說，性別之迷十分簡單：所有人都是女生！這是個幾乎都由磁性組成的物種，而當她們產卵時，卵中會孕育出他們自己的複製品！
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.
究竟是男還是女？在廣大的生物世界中，性別決定系統決定了一切！而對人類來說，基因 XY 決定了性別，而我與太太的寶寶將生下的是：一個男孩！
來源 > 撰稿及翻譯：劉耘 | 影片：Sex Determination: More Complicated Than You Thought