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  • 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.

  • 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.

  • 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.

  • 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.

  • 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.

  • 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 US TED-Ed determined male female egg determination

【TED-Ed】Sex determination: More complicated than you thought - Aaron Reedy

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    VoiceTube posted on 2016/03/15
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