Sex Determination: More Complicated Than You Thought

From something as small and complex as a chromosome to something as seemingly simple as the weather, sex determination systems vary significantly across the animal kingdom. Biologist and teacher Aaron Reedy shows us the amazing differences between species when it comes to determination of gender.

Lesson by Aaron Reedy, animation by BuzzCo (http://buzzzco.com).

(Music)

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.