Is it possible to have one NT twin & one AS twin?

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Sorry if this has already been asked. And I mean both fraternal & identical twins.

Yes it's possible. There is a strong genetic component (about 37% chance a sibling or dizygotic twin also has it), but unknown environmental factors also play a role, so it's possible (though relatively improbable) that even monozygotic twins may not share the condition (concordance has been estimated at around 92% for MZ twins).

Yes! There is a set of twins in Ontario where one is NT and the other is LFA. Not even identical twins always turn out COMPLETELY identical because genetics play a role, but so do environmental factors, Once those twins are separated, environmental factors will impact them differently because they are two separate bodies. (I've read before that autism is often caused by a combination of genetics and environmental factors). If the twins are not identical, then they have a much greater chance at being different because they are really simply full siblings who happen to have the same birthday (two different eggs+ two different sperm...just like other full siblings who didn't share a womb).

If I had one twin that was AS and one that was NT wouldn't that make us triplets?

I've personally known two sets of twins where at least one was autistic. One set was an NT girl and an autistic boy, the other set were two autistic boys who I suspect were identical (I couldn't tell them apart, both in appearance and behavior).

Very possible. And not uncommon. And I am talking identical twins as well.

In fact, someone that goes to therapy with my son has a non-verbal, and almost non-communicative son with a very average NT son. I never would have thought. Crazy. Both kids are super smart in mathematics though. Both have an uncanny knowledge of all things numbers.

Aww, I got so excited that I could answer this, but it looks like everyone else beat me.

I have two little sisters who are twins. They're fraternal, though, so I don't know if you meant any twins or a specific type. One is an aspie and the other is NT.

Only if they are faternal twins. If they were idenical twins and one was autistic and the other was NT, they would not be idenical. That's like asking if idenical twins can still be idenical twins if one is a boy and the other is a girl.

Yes, even if they are identical twins they can still have different neurologies from each other. Be it any combo of NT, AS, HFA or LFA.

Identical twins can differ for two reasons:

* Random genetic mutation - there's a rare possibility that, in early cell division, a mutation will occur that makes one twin different. This tends to result in single gene conditions like achondroplasia. It could result in autism in rare cases, but very few autistic kids have single gene conditions causing their autism.

* Environmental effects - this is probably what happened when identical twins are discordant for autism. For example, if the firstborn twin came out easily and then the labour stalled out for a bit, the secondborn twin could have brain damage. Or if the twins had different placentas (true of most identical twins and all fraternal twins) one could've gotten hit harder by some prenatal infection or such. Or twin-to-twin transfusion syndrome could make them differ because an error in blood flow in a fused placenta caused anemia in one twin and polycytopenia in the other.

Nobody's explained why identical and fraternal twins are so interesting yet, have they?

Well, Aspie Lecture Mode go!

Identical twins split off from the same fertilized egg very early in the pregnancy--during the first few splits, it separates completely instead of just multiplying. The twins' DNA from the nucleus of that cell is 100% identical. There can be a few differences depending on which mitochondria each one got, though. Mitochondrial DNA is, obviously, in the little organelles called mitochondria, and they aren't in the nucleus, so identical twins can get different ones and so different mitochondrial DNA. They can also have different prenatal environments. I know this because my own identical twin and I had environments so different that she died at five months gestation and I did not. (I suspect twin-to-twin-transfusion syndrome, which happens when twins share a "leaky" placenta and one gets too much blood and the other not enough. It can cause heart problems in the twin who gets too much blood, which jives with my own heart murmurs and arrhythmias that I've been dealing with since I was a kid.)

Fraternal twins, on the other hand, are just like siblings sharing the same pregnancy. For some reason, their mother had two or more eggs finish developing during ovulation, and they were both fertilized with separate sperm. Fraternal twins are 50% identical, just like any siblings; but they share a similar environment. Because the tendency to have more than one mature ovum during one cycle is genetic, it can be passed on to one's offspring. That means that if you are a female fraternal twin, you are more likely to have twins yourself; and if you're a male fraternal twin, you can pass that trait on to your female offspring. Identical twins aren't genetic, so if twins run in the family, they're likely to be fraternal. Not that fraternal twins can't look an awful lot alike--they are siblings, and the same age on top of that; so figuring out whether they're fraternal or identical can sometimes be tricky.

So why are twins so interesting when it comes to genetics? Simply put, twin studies are a great way to figure out heritability and genetics-versus-environment.

Types of twin studies:
Identical twins raised together vs. identical twins raised apart: Compare these groups to figure out the effect of environment after birth.
Identical twins vs. fraternal twins or siblings: Compare these groups to figure out the effect of environment (in general) vs. genetics.
Fraternal twins vs. similar-age siblings: Compare to figure out the effect of twin pregnancy itself. Obviously complicated by the problem of age effects in the siblings. Ideally, one would compare fraternal twins from donated zygotes to same-age adopted siblings, but these are just not large enough groups to get any statistical significance.

If you're interested in the math of it, this Wikipedia page has the basics:
http://en.wikipedia.org/wiki/Twin_study

So, with autism:
Fraternal twins have a very slightly higher chance of both having autism than siblings do--about 5-6%. This shows that there's a slight prenatal-environment effect (though it could just be due to being twins and having to share resources and being more likely to be premature).

Identical twins are almost always both autistic--in the 95% range. When one twin is autistic and the other is not, the differences tend to be between a mild case of autism and NT with autistic traits, rather than an obvious case versus a social-butterfly NT.

Identical twins raised apart are also about 95% likely to both be autistic; so we're looking at genetic or prenatal-environment effects. From the evidence of fraternal twins vs. siblings, we can assume the prenatal-environment effect is small but present.

My identical twins have autism. They were DX at Yale. I learned there that if 1 identical twin has autism the other has a 70% chance of also having it.

My identical twins have autism. They were DX at Yale. I learned there that if 1 identical twin has autism the other has a 70% chance of also having it.