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In high school biology, we usually learn that
the sexes in humans are fixed and concrete.
Whether you’re male or female is black-and-white
and rooted in your DNA: your 23rd pair of
chromosomes is either two X chromosomes or
an X and a Y. That’s it. End of story.
And that’s essentially what scientists thought,
too. But it turns out that sex isn’t that
straightforward.
In fact, biologists today are saying sex is
a spectrum.
And the scientific community is still working
on understanding and respecting the people
who fall in the middle of that spectrum.
To get this out of the way right up front:
we’re not talking about gender or sexuality
here.
Gender refers to social and cultural attributes
and understandings of men and women and their
roles—though, not every culture has only
two categories, and it’s increasingly seen
as a spectrum.
Plus, the gender you identify as may or may
not be the same one as what you express with
things like your clothing and behavior, all
of which can also be on a spectrum.
Sexuality describes who you are attracted
to, and it can be equally complicated and
on a spectrum. And where you are on these
spectrums isn’t necessarily fixed!
But what we are talking about today is your
biology, including your chromosomes, your
hormones, your gonads, and your genitals.
The catch is that these biological features
don’t always agree with each other.
And they certainly don’t always conform
to those high school health class diagrams
that tell us there is a single, universally
correct pathway to being male and female.
In fact, it’s estimated that nearly 2% of
live births are born with congenital conditions
of atypical sex development. That basically
means that something in their chromosomes,
hormones, gonads, or genitals is different
from what many people expect of a “boy”
or a “girl.”
This used to be known as being intersex, but
these days, it’s better described as having
differences of sexual development, or DSDs.
And while nearly 2% might not sound like a
lot, it means there could be 130 million people
or more with DSDs. If all those people were
in one country, it'd be among the top ten
most populous countries in the world!
Plus, DSDs are not always something you can
see. People can spend their whole lives thinking
they’re one sex based on anatomy only to
find at least part of them tells a different story.
You see, your sex is the result of both sexual
determination and sexual differentiation.
Sexual determination has to do with what chromosomes
you get. Those largely determine what happens
to your body during sexual differentiation—the
process by which you develop the physiological
characteristics associated with your sex.
And contrary to what you might think, that
differentiation doesn’t stop when you’re
born—it continues throughout your life.
That means there are a lot of moments where
differences between people can happen—so
of course there are a ton of different outcomes!
We tend to put those outcomes into two boxes
based on visible anatomy, or what scientists
call phenotypes. Phenotypical males have testicles
and a penis, while phenotypical females have
ovaries, a uterus, a vagina, and vulva.
But in reality, none of the traits we use
to discriminate between the sexes are truly
binary.
There’s a lot of variation within what we
call male or female, and there's a lot of
overlap that's normal, too.
Anatomically, someone might look phenotypically
female on the outside but not have ovaries
or a uterus, or have tissue from both ovaries
and testes.
And genetics aren’t any clearer, because
when it comes to chromosomes, people don’t
always get two Xs or an X and a Y.
Xs and Ys contain genes that help determine
sex, with the Y chromosome conferring the
genes that enable you to develop male reproductive
parts.
But the processes for producing sperm and
eggs are really complicated, and they can
lead to lots of different results.
In this process, (abbreviated version) specialized
cells basically duplicate themselves, then
undergo two rounds of division to produce
reproductive cells, or gametes, that have
half of the parent’s genetic material. So,
it makes one set of 23 chromosomes.
But sometimes, the chromosomes don’t split
into exact sets of 23—and that means there
are a whole bunch of possible combinations
of Xs and Ys that a person can end up with.
For instance, people can inherit three Xs
or an X and two Ys. These folks are normally
taller than average.
Those with three Xs have slender builds, and
sometimes have minor learning disorders. The
people who have an X and two YYs, on the other
hand, tend to have more acne because of the
extra testosterone in their systems. In both
cases, people retain full fertility.
Then, there’s Turner syndrome, which happens
when you get just one X. That results in female
characteristics, but the people who have it
tend to be shorter, don’t really go through
puberty, may have mental disabilities, and
are sterile.
And Klinefelter syndrome, which results from
two Xs and a Y, is the most common chromosomal
sex anomaly.
It happens in one in 600 male births and can
cause lower testosterone production and cause
incomplete testicular development, though
the symptoms can be minor enough that a person
isn’t diagnosed until later in life.
Now there’s also the fact that all your
cells in your body don’t necessarily have
the same chromosomal makeup. Which like, what?
Did I learn nothing but lies in high school?
But it’s true—someone with mosaicism can
develop from a single fertilized egg, but
have a patchwork of genetically different
cells.
And someone who’s a genetic chimera has
different cells because they develop from
two different fertilized eggs that merge in
the womb.
In both cases, it’s possible to end up with
a mix of cells with different sex chromosomes.
And depending on the distribution of those
cells, mosaicism and chimaerism can result
in ambiguous sexual characteristics or both
male and female reproductive body parts.
It’s even been shown that pregnant people
and their fetuses frequently swap stem cells
through the placenta in a phenomenon known
as microchimerism. That means a chromosomal
“female” can be carrying around XY cells,
and her son can have XX ones.
In some studies, these cells have been shown
to stick around in the mother for several decades.
But all that said… there are also plenty
of people with double-X or XY chromosomes
that also have differences of sexual development.
That’s in part because at least 25 genes
play a role in sex differentiation. So both
mutations and relocations of these genes can
result in a range of differences.
Genes necessary for male development can be
swapped onto the X chromosome, for example,
or someone can end up with multiple or mutated
versions of other sex-determining genes.
And some of these are on other chromosomes,
and are inherited as run-of-the-mill recessive traits.
All of these genes really start to be influential
around six weeks of development.
You see, at six weeks, the fetus has a pair
of bulges called the gonadal ridges next to
its kidneys—and they have the potential
to develop into ovaries or testes.
The fetus at this point also has two sets
of ducts. One set can develop into the uterus
and fallopian tubes, while the other set has
the potential to become the epididymis, vas
deferens, and seminal vesicles.
And what happens from there is somewhat of
a balancing act of different genes working
in concert.
Essentially, different networks of genes shout
MALE and FEMALE, and when that balance gets
knocked slightly askew, it can move a person
along the sex spectrum.
Take SRY. Discovered in the 1990s, this is
the male programming gene, and it has a big
effect on development.
If it ends up on the chromosome of someone
who is XX, it can cause them to develop testes
instead of ovaries.
This can happen because there’s a step in
sperm and egg production when chromosomes
swap some DNA with their partner chromosomes.
And even though the X and Y chromosomes generally
don’t join in on this DNA swapping process,
they sometimes do.
Plus, other mutations that occur during the
production of gametes can result in multiple
or mutated versions of SRY or other sex-determining
genes—because it’s not
the only gene that matters.
There are also genes that actively encourage
the fetus to develop female characteristics.
For instance, the gene WNT4 suppresses testicular
development and promotes ovarian development,
and multiple copies of it can cause incomplete
female gonads to develop in people who are XY.
Gonad development also triggers the production
of sex-specific hormones, which results in
further sex-specific development.
But some people have differences of sex differentiation
that limit their ability to respond to those hormones.
Complete androgen insensitivity syndrome is
one of these. People who have it are unaffected
by male sex hormones, because they have some
kind of mutation to the protein that these
hormones bind to, called the Androgen Receptor.
And that means that while they have testes
and a Y chromosome, their exterior genitals
appear female or in between.
There’s also congenital adrenal hyperplasia,
the most common DSD out there.
That’s when the adrenal glands underproduce
cortisol and overproduce androgens, the male
hormone group that includes testosterone.
The underproduction of cortisol can lead to
health problems, while the overproduction
of androgens can lead to external male genitalia
paired with internal female gonads in people
with XX chromosomes.
Some of these conditions don’t fully present
themselves until puberty or later.
In fact, some aren’t realized at all until
a person seeks some kind of medical care that
reveals them. Like, in 2014, doctors reported
one case of a 70-year-old father of 4 whose
quote “hernia” turned out to be a uterus
with fallopian tubes.
But, in many cases, differences in sexual
development are notable from birth; for those
newborns, it may be possible to assign a gender
based on what they are more likely to identify
as, as they grow up.
The thing is, with all of the things that
can happen during sexual development, when
a child is born with an obvious difference
of sex development, it's not always clear why.
Looking at chromosomes often isn’t enough,
and sometimes a hormonal test isn’t either.
And even if the child’s doctors have a sense
of what’s going on, determining what, if
any, treatment is necessary can be challenging.
Back in the 1960s, it was thought that growing
up without clearly defined sexual organs would
cause emotional trauma. So, there was a push
towards performing surgery on infants to clearly
assign them a sex.
And because of social stigmas surrounding
DSDs, parents were often encouraged to keep
all this a secret, even from the child. So
people grew up without knowing kind of important
details about their own bodies.
It’s hard to get numbers on how many of
these surgeries were—or even are being—performed.
It’s also hard to know exactly how these
surgeries affect patients, but as adults,
many report pain, scarring, and a loss of
sensation.
Also, people with DSDs do report high rates
of gender dysphoria, where their chosen gender
does not align with their assigned sex.
And there is an association between gender
dysphoria and mental health issues, like self-harm
behaviors, so these surgeries may contribute
to mental health problems later in life.
Though, it’s important to note that such
issues are less likely if people have supportive
and affirming parents who accept them as they
are.
And, sometimes, surgery is medically necessary,
like to unblock the urethra.
Also, surgery can help to preserve fertility
or, in the case of complete androgen insensitivity
syndrome, to reduce the risk of testicular
cancer.
But from a medical perspective, those surgeries
don’t need to be performed on infants.
In fact, most of the time, differences in
genital anatomy at birth aren’t something
that needs to be fixed. At least, not until
the person is old enough to make their own
choices about what they want their bodies
to look like.
So nowadays, healthcare is moving away from
a surgical approach. If a DSD is identified
at birth, treatment is more likely to include
therapy and hormonal replacement than surgery.
Often, a DSD team is involved in care, which
can include geneticists, endocrinologists,
and psychologists or psychiatrists.
They help the family decide if any interventions
are immediately and medically necessary, and
help provide care and support to the child
with DSD and their family throughout childhood.
Unfortunately, this kind of care still isn’t
available everywhere.
For now, researchers are working to better
understand the development of both sex and
gender over time, and to gain a clearer sense
of when kids begin to understand
their own gender identity.
The problem, of course, is the fact that from
clothes to restrooms to organized sports,
they are raised in a society that is set up
around a binary that just… isn’t binary.
But researchers are thinking about how we
can make our overall discussions and understanding
of sex even more inclusive—and more accurate.
Because even though biological sex may seem
like one of those things that is relatively
straightforward in a very, very complicated
world… it’s not!
And while there’s probably still a long
way to go to understand it,
we are making progress.
Before we go, we’d like to give a special
thank you to our patrons on Patreon. It’s
because of their support that we’re able
to tackle complex, difficult topics like this
one. So thank you, patrons! And if you want
to support us, too, you can learn more
by joining our patron community at Patreon.com/SciShow
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