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  • The secrets of the X chromosome.

  • 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.

The secrets of the X chromosome.

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TED-ED】X染色体の秘密 - ロビン・ボール (【TED-Ed】Secrets of the X chromosome - Robin Ball)

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    吳D に公開 2021 年 01 月 14 日
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