These women are identical twins.

They have the same nose,

the same hair color,

the same eye color.

But this one is color blind for green light,

and this one isn't.

How is that possible?

The answer lies in their genes.

For humans, the genetic information that determines our physical traits

is stored in 23 pairs of chromosomes in the nucleus of every cell.

These chromosomes are made up of proteins and long, coiled strands of DNA.

Segments of DNA, called genes, tell the cell to build specific proteins,

which control its identity and function.

For every chromosome pair, one comes from each biological parent.

In 22 of these pairs, the chromosomes contain the same set of genes,

but may have different versions of those genes.

The differences arrive from mutations,

which are changes to the genetic sequence

that may have occurred many generations ago.

Some of those changes have no effect,

some cause diseases,

and some lead to advantageous adaptations.

The result of having two versions of each gene

is that you display a combination of your biological parents' traits.

But the 23rd pair is unique,

and that's the secret behind the one color blind twin.

This pair, called the X and Y chromosomes, influences your biological sex.

Most women have two X chromosomes

while most men have one X and one Y.

The Y chromosome contains genes for male development and fertility.

The X chromosome, on the other hand,

contains important genes for things other than sex determination or reproduction,

like nervous system development,

skeletal muscle function,

and the receptors in the eyes that detect green light.

Biological males with an XY chromosome pair

only get one copy of all these X chromosome genes,

so the human body has evolved to function without duplicates.

But that creates a problem for people with two X chromosomes.

If both X chromosomes produced proteins, as is normal in other chromosomes,

development of the embryo would be completely impaired.

The solution is X inactivation.

This happens early in development when an embryo with two X chromosomes

is just a ball of cells.

Each cell inactivates one X chromosome.

There's a certain degree of randomness to this process.

One cell may inactivate the X chromosome from one parent,

and another the chromosome from the other parent.

The inactive X shrivels into a clump called a Barr body and goes silent.

Almost none of its genes order proteins to be made.

When these early cells divide, each passes on its X inactivation.

So some clusters of cells express the maternal X chromosome,

while others express the paternal X.

If these chromosomes carry different traits,

those differences will show up in the cells.

This is why calico cats have patches.

One X had a gene for orange fur and the other had a gene for black fur.

The pattern of the coat reveals which one stayed active where.

Now we can explain our color blind twin.

Both sisters inherited one mutant copy of the green receptor gene

and one normally functioning copy.

The embryo split into twins before X inactivation,

so each twin ended up with a different inactivation pattern.

In one, the X chromosome with the normal gene was turned off

in the cells that eventually became eyes.

Without those genetic instructions,

she now can't sense green light and is color blind.

Disorders that are associated with mutations of X chromosome genes,

like color blindness,

or hemophilia,

are often less severe in individuals with two X chromosomes.

That's because in someone with one normal and one mutant copy of the gene,

only some of their cells would be affected by the mutation.

This severity of the disorder depends on which X got turned off

and where those cells were.

On the other hand, all the cells in someone with only one X chromosome

can only express the mutant copy of the gene if that's what they inherited.

There are still unresolved questions about X inactivation,

like how some genes on the X chromosome escape inactivation

and why inactivation isn't always random.

What we do know is that this mechanism

is one of the many ways that genes alone don't tell our whole story.