Sex Determination: More Complicated Than You Thought

4,495,907 views ・ 2012-04-23

TED-Ed


Please double-click on the English subtitles below to play the video.

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Translator: Bedirhan Cinar
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My wife is pregnant right now with our first child,
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and when people see her with her big baby bump,
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the first question people ask, almost without fail, is,
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"Is it a boy or is it a girl?"
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Now, there are some assumptions behind that question
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that we take for granted
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because of our familiarity with our own human biology.
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For human babies, we take it for granted that there's a 50/50 chance
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of either answer, boy or girl.
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But why is it that way?
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Well, the answer depends on the sex determination system
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that has evolved for our species.
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You see, for most mammals,
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the sex of a baby is determined genetically
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with the XY chromosome system.
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Mammals have a pair of sex chromosomes,
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one passed down from mom, and one from dad.
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A pair of X's gives us a girl,
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and an X and a Y together gives us a boy.
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Since females only have X's to pass on in their egg cells,
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and males can give either an X or a Y in their sperm cells,
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the sex is determined by the father
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and the chance of producing a male or a female is 50/50.
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This system has worked well for mammals,
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but throughout the tree of life, we can see other systems
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that have worked just as well for other animals.
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There are other groups of animals that also have genetic sex determination,
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but their systems can be pretty different from ours.
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Birds and some reptiles have their sex genetically determined,
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but instead of the sex being determined by dad,
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their sex is determined by mom.
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In those groups, a pair of Z sex chromosomes
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produces a male, so these males only have Z's to give.
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However, in these animals, one Z and one W chromosome together,
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as a pair, produces a female.
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In this system, the chance of a male or a female is still 50/50,
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it just depends on whether mom puts a Z or a W
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into her egg.
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Certain groups have taken genetic sex determination
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in completely other directions.
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Ants, for example, have one of the most interesting systems
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for determining sex, and because of it, if you are a male ant,
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you do not have a father.
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In an ant colony, there are dramatic divisions of labor.
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There are soldiers that defend the colony,
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there are workers that collect food, clean the nest and care for the young,
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and there's a queen and a small group of male reproductives.
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Now, the queen will mate and then store sperm from the males.
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And this is where the system gets really interesting.
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If the queen uses the stored sperm to fertilize an egg,
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then that egg will grow up to become female.
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However, if she lays an egg without fertilizing it,
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then that egg will still grow up to be an ant,
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but it will always be a male.
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So you see, it's impossible for male ants to have fathers.
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And male ants live their life like this, with only one copy of every gene,
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much like a walking sex cell.
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This system is called a haplodiploid system,
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and we see it not only in ants,
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but also in other highly social insects like bees and wasps.
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Since our own sex is determined by genes,
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and we do know of these other animals that have their sex determined by genes,
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it's easy to assume that for all animals
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the sex of their babies still must be determined by genetics.
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However, for some animals, the question of whether it will be a boy or a girl
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has nothing to do with genes at all,
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and it can depend on something like the weather.
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These are animals like alligators and most turtles.
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In these animals, the sex of an embryo in a developing egg
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is determined by the temperature.
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In these species, the sex of the baby
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is not yet determined when the egg is laid,
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and it remains undetermined until sometime in the middle
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of the overall development period, when a critical time is reached.
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And during this time, the sex is completely determined
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by temperature in the nest.
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In painted turtles, for example,
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warm temperatures above the critical temperature
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will produce females within the eggs,
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and cool temperatures will produce a male.
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I'm not sure who came up with this mnemonic,
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but you can remember that when it comes to painted turtles,
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they are all hot chicks and cool dudes.
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For some tropical fish, the question of will it be a boy or will it be a girl
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isn't settled until even later in life.
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You see, clownfish all start out their lives as males,
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However, as they mature, they become female.
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They also spend their lives in small groups with a strict dominance hierarchy
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where only the most dominant male and female reproduce.
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And amazingly, if the dominant female in the group dies,
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the largest and most dominant male will then quickly become female
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and take her place, and all of the other males
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will move up one rank in the hierarchy.
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In another very different ocean animal,
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the green spoonworm,
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the sex of the babies is determined
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by a completely different aspect of the environment.
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For this species, it is simply a matter of where a larva
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happens to randomly fall on the sea floor.
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If a larva lands on the open sea floor, then it will become a female.
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But if it lands on top of a female, then it will become a male.
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So for some species, the question of boy or girl
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is answered by genetics.
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For others, it's answered by the environment.
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And for others still, they don't even bother with the question at all.
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Take whiptail lizards, for example.
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For those desert lizards, the answer is easy.
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It's a girl. It's always a girl.
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They are a nearly all-female species, and although they still lay eggs,
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these eggs hatch out female clones of themselves.
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So will it be a girl or will it be a boy?
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Throughout the entire animal kingdom,
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it does really all depend on the system of sex determination.
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For humans, that system is a genetic XY system.
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And for me and my wife, we found out
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it's going to be a baby boy.
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(Kiss)
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