Name: 一卵性・兄弟性を超えて。6つの珍しいタイプの双子 (Beyond Identical or Fraternal: 6 Rare Types of Twins)
Uploaded: 2021-01-14T10:44:35.000Z
Duration: 14 min 29 s
Description: VoiceTubeの動画で発音を聞きながら英語表現を覚えよう！学べる英語：

様態の副詞

There's a saying that there are two things in life that you're never truly prepared for: twins.

転換語

And that is in large part because twinning is somewhat rare in our species.

Only about three percent of live births involve multiple fetuses.

Most of these are dizygotic, or what's commonly called fraternal twins.

They occur when a person just so happens to release two eggs instead of one,

both of which get fertilized and successfully implant.

A smaller percentage of twins are monozygotic or “identical” twins.

Those are the twins that share essentially all of their DNA

because they come from a single fertilized egg which later split into two.

And both of these types of twins are useful for scientific studies

because they can help us sort out the influences

of genetics and environmental factors on different traits.

But it turns out that twinning is a lot more complicated than just identical or fraternal.

And the rarer types of twins suggest that

we have a lot more to learn about human development.

The name “mirror image twins” basically says it all.

where the two people are physical mirror images of each other.

Like, if one twin is right-handed, the other might be left-handed.

The mirror twin's internal organs might even be on the opposite side;

So while that degree of mirroring only occurs in about one in every 10,000 people,

some degree of mirroring in identical twins is pretty common.

Some scientists think this is a hint as to when the twins' embryo actually split.

You see, within the first couple of weeks of embryonic development,

the cells that will eventually give rise to organs become programmed

for the right or left side of the body, a process called left-right asymmetry.

During this window, left and right are determined,

but the embryonic cells haven't yet migrated

to the positions where they will ultimately grow into body parts.

So it's thought that full mirror image twins

only occur when a single embryo splits after this has happened;

sometime around two weeks after fertilization.

So, the concept of “identical” twins isn't really true;

there are always some slight genetic differences between twins.

So it's possible that those genetic variations

might actually explain mirroring better than when the embryo split.

And even if it is a timing thing, the research isn't clear on

exactly when splitting will result in mirrored traits like hair whorls or handedness,

as opposed to full blown asymmetry of organs.

And that's because we can't actually see a human embryo splitting in utero.

Ultrasound technology is good, but it is not that good.

Though, even if we could see twinning happen,

we'd probably miss it for most pregnancies,

since it happens during the earliest stages

when a person might not even know that they're pregnant.

Still, whether it's about when the split happens,

or genetic differences between twins, or something else,

piecing together the mechanism behind mirroring has the potential to teach us a lot

about how our developing bodies determine where our different pieces go.

You're probably already familiar with conjoined twins;

twins that remain at least somewhat attached at birth.

For better or worse, they've received a lot of attention both medically and culturally.

Still, we're not sure how conjoining happens.

And examining cases more closely has kind of

upended everything we thought we knew about twinning.

The earliest and still common explanation for conjoinment

is that it's a result of incomplete splitting;

This is actually how all monozygotic twins supposedly happen:

for whatever reason, one embryo splits into two.

And, so the theory goes, if this happens super early,

both twins develop their own amniotic sacs and their own placentas.

If it happens a little later, they share one or both of those.

two or more weeks into development then you get conjoined twins.

It's thought that, for some reason, splits that occur that late fail to fully separate.

And that makes a lot of sense intuitively,

and there are other reasons to think that conjoinment has to do with late embryo splitting;

like, that there are higher rates of mirroring in conjoined twins.

But, remember, mirroring might not be a timing thing after all.

And many researchers think conjoined twins don't arise from incomplete splitting.

posits that the embryo does completely split, and this split occurs a lot earlier on.

they end up physically colliding and they grow back together.

This might explain why most cases of conjoined twins

are joined at the chest and often share a heart.

See, early on in development, the primordial heart is one of the few parts of the body

that isn't covered in the type of cells that will become skin.

That may mean that that area is more vulnerable to rejoining.

Sorting out which of these two theories is right, or if they both are, at times,

would help scientists better understand how and when cells become

programmed into different tissues and why twins happen in the first place.

About ten percent of conjoined twins are considered external heteropagus twins.

Essentially, one twin doesn't develop all of their organs or body parts,

so they rely on the other twin for survival.

This undeveloped or asymmetrical twin may even look

more like a mutation than a separate, attached entity.

Like, a while back there was a viral video of

a pretty cute puppy with a tail that was sticking out of its head?

That was probably an external heteropagus twin.

Much like other conjoined twins, it's long been assumed that

such twins arise when an embryo splits late in the game and doesn't split perfectly.

And for some reason, the split is also fairly lopsided,

so one twin either doesn't develop well enough to survive,

or is essentially part of a person attached to their twin.

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一卵性・兄弟性を超えて。6つの珍しいタイプの双子 (Beyond Identical or Fraternal: 6 Rare Types of Twins)

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