Twin Studies

[CBL4]

I keep reading about studies that compare the behavior of fraternal and identical twins in order to give a genetic portion of certain behaviors such as female orgasm and pychopathy. The contention is that twins have extremely similar environments and that differences in behavior would thereby be genetic.

This ignores the most important environmental time of a human's life - the womb. At first glance, the womb environment would be the same because all twins are in the same womb at the same time. However, this ignores the fact that there is another embryo in the womb.

For identical twins, the environment is basically identical between the other twin is identical. For fraternal twins, the other twin causes a different enviroment.

For example, let's assume that some embryos consume more kryptonite while in the womb. If Anne is a high kryptonite consumer, then her twin Betsy has a kryptonite shortage. If the lack of orgasms is due to lack of kryptonite in the womb, this would mean that Anne and Betsy are born destined to be orgasmically different even though it is not a genetic trait but an environmental one. If their mother ate more kryptonite then both of Anne and Betsy could have orgasms.

It seems to me that this is an almost insolvable problem with twin studies.

CBL

 

[drkitten]

This ignores the most important environmental time of a human's life - the womb. At first glance, the womb environment would be the same because all twins are in the same womb at the same time. However, this ignores the fact that there is another embryo in the womb.

For identical twins, the environment is basically identical between the other twin is identical. For fraternal twins, the other twin causes a different enviroment.

Different than what?

If you're saying that the womb-environment is different for fraternal twin than it is for single-birth offspring, then you are of course correct; fraternal twins tend, for example, to be of lower birth weight, with the corresponding long-term effects.

If you're saying that the womb-environment is different from fraternal twins than it is for identical twins, then I think that you are making a highly unsupported assertion. Just as a for-instance, your example of

For example, let's assume that some embryos consume more kryptonite while in the womb. If Anne is a high kryptonite consumer, then her twin Betsy has a kryptonite shortage.

... presumes that Anne is in fact capable of stealing kryptonite from her twin. An equally reasonable hypothesis is that the placenta supplies kryptonite equally to the two, and that Anne will be the one who ends up with a kryptonite shortage, because half of her supply is diverteed to Betsy. The environment for the two is the same, but Anne is the only one for whom it really matters.

 

[CBL4]

Originally posted by new drkitten
your example ... presumes that Anne is in fact capable of stealing kryptonite from her twin.

Yes, it does. The fact is that the environment is different because they are sharing the womb with a different embryo. I chose stealing kryptonite to indicate that I do not what, if anything, is significant. I would guess genetics are much more important but I do not have any evidence for that.

I do know that other studies have shown major differences in health due to the in utero environment. Aside from obvious ones like smoking, smaller things such as fat intake matter. I would bet that the genetic make up of the other twin matters as well.

All I am saying is that until twins embryos are decanted, it is impossible to remove the environmental factors from this type of study. These studies should continue but they should also be taken with a grain of salt.

CBL

 

[CBL4]

An equally reasonable hypothesis is that the placenta supplies kryptonite equally to the two, and that Anne will be the one who ends up with a kryptonite shortage, because half of her supply is diverteed to Betsy. The environment for the two is the same, but Anne is the only one for whom it really matters.

Yes but this shows another weakness with twin studies.

In your case, Anne's clone who is implanted alone in a womb would have different orgasmic characteristics than Anne. This would mean that the apparent difference is not genetic alone but genetic and environmental in nature. Again, the twin studies would overemphasize the genetic component.

CBL

 

[Mosquito]

Sorry to butt in here, but isn't this solved by researchers using samplesizes >> 1?

The idea being that it is much more difficult to eliminate other factors if you have only a few samples. (In effect "other effects" can be eliminated as explanations since they even out or something.)

With a large sample-size, you can make sub-samples and see if they show any signs of other effects playing in. Say a part of the mothers smoke, you could then check if their daughters are different from the daughters of non-smokers. Some mothers eat noticably more kryptonite than some other group of mothers, are there any differences?

Then playing with the numbers, eliminating factors that does not seem to play any real part (those who smoke a lot also eat less kryptonite, so the real effects may come from only one of these), you can draw some conclusions.


Mosquito - possibly showing off too much ignorance, in which case I'll learn something soon

 

[CBL4]

Originally posted by Mosquito
With a large sample-size, you can make sub-samples and see if they show any signs of other effects playing in.

If is something that can be overcome by an environmental impact this would be helpful. Of course, you need to ask the twins the right question which means you may need to know a lot more about the thing you are study than about female orgasm. I mean, who would have guess that kryptonite is related to orgasm.

The other problem is that there are only so many identical twins around. It gets hard to do all the studies. It is also hard to ask mothers to go back and remember whether their prenatal vitamins contained kryptonite or not.

CBL

 

[IndigoRose]

Identical twins are not born with identical weights despite the "identical" environment. ... interesting...

I keep reading about studies that compare the behavior of fraternal and identical twins in order to give a genetic portion of certain behaviors such as female orgasm and pychopathy. The contention is that twins have extremely similar environments and that differences in behavior would thereby be genetic.

This ignores the most important environmental time of a human's life - the womb. At first glance, the womb environment would be the same because all twins are in the same womb at the same time. However, this ignores the fact that there is another embryo in the womb.

For identical twins, the environment is basically identical between the other twin is identical. For fraternal twins, the other twin causes a different enviroment.

For example, let's assume that some embryos consume more kryptonite while in the womb. If Anne is a high kryptonite consumer, then her twin Betsy has a kryptonite shortage. If the lack of orgasms is due to lack of kryptonite in the womb, this would mean that Anne and Betsy are born destined to be orgasmically different even though it is not a genetic trait but an environmental one. If their mother ate more kryptonite then both of Anne and Betsy could have orgasms.

It seems to me that this is an almost insolvable problem with twin studies.

CBL

 

[IndigoRose]

Re: Re: Twin Studies

Different than what?
If you're saying that the womb-environment is different from fraternal twins than it is for identical twins, then I think that you are making a highly unsupported assertion.

You can have two placentas, and also two completely different amniotic sacs.

 

[CBL4]

Originally posted by IndigoRose
You can have two placentas, and also two completely different amniotic sacs.

I had forgotten that identical twins could share the same placenta and amniotic sacs. This is another environmental reason for differences between fraternal and identical twins.

Identical twins are not born with identical weights despite the "identical" environment. ... interesting

I was unaware of this. I found confirming evidence but nothing about how common or the difference compared to fraternal twins. Do you know more? I would assume that it would less different than for fraternal twins.

If that is the case, here is some evidence for womb environmental differences affecting adult twins:

When twin pairs differed markedly in their birth weights, the researchers found that the heavier twin was taller and slightly heavier in adulthood as well. And the greater the percentage in weight difference between a twin pair, the greater the adult height difference.

http://preventdisease.com/news/articles/birth_weight_body_comp.shtml

CBL

 

[Mosquito]

If is something that can be overcome by an environmental impact this would be helpful. Of course, you need to ask the twins the right question which means you may need to know a lot more about the thing you are study than about female orgasm. I mean, who would have guess that kryptonite is related to orgasm.

The other problem is that there are only so many identical twins around. It gets hard to do all the studies. It is also hard to ask mothers to go back and remember whether their prenatal vitamins contained kryptonite or not.

CBL

True, if you don't check for the right parametres, you'll end up with either a wrong answer or your study will be inconclusive. This is part of what makes it so interesting/confusing.

There are plenty of identical twins, I think about 1 in 80 pregnancies end up in twins, a fair bit if these should be identical. Don't ask for evidence, this is from my Swiss cheese memory.

You are also correct about the difficulties involved in making mothers remember the contents of their prenatal vitamins. This is a very serious problem, but can, at least sometimes, be solved by the mothers remembering which mark/model of vitimins they used, and then check with the producer what kind of stuff they put inside. If not, you can always ask them if they had/have a lot of orgasms (they'll remember THAT) and thus conclude that there's plenty of kryptonite in their diet

Mosquito - going to the shop looking for this kryptonite stuff everybody are talking about, it's supposed to do wonders, you know, for your you-know-what...

 

[MrMonty]

I am an identical twin and have been involved in twin studies.

Identical twins can most definitely be different because of womb factors. I "stole" food from my brother in the womb. I've been told it's not uncommon for one fetus to dominate the womb, sometimes to the fatal detriment of the other. When we were born, 8 weeks premature, I weighed around 4 pounds and he weighed around 3. I have always been taller, and at 29, am still at about 2 inches taller.

Another interesting tidbit. We started doing twin studies around the age of 12 I think. We did the studies as fraternal twins. When we were born, they did something wrong and incorrectly determined we had different blood types so the doctors, my parents, and my brother and I had always assumed we were fraternal. Wasn't until we were 18 that one of stuides involved genetic testing. That's when they came back to us and said our genetic makeup was so exact that we had to be identical. So, there's five years or so of skewed data for twin studies coming out of Case Westerne Reserve University.

Monty

 

[CBL4]

Originally posted by MrMonty
Identical twins can most definitely be different because of womb factors. I "stole" food from my brother in the womb. I've been told it's not uncommon for one fetus to dominate the womb, sometimes to the fatal detriment of the other.

Interesting. Do you know if this is more or less common with fraternal or identical twins?

The identical twins I knew were clearly identical. Sometimes there was a size difference but it was in the order of an inch or two like you say. I find it amazing it was not obvious that you were identical. Fraternal twins (at least the ones I have seen) are clearly not identical when seen together.

CBL

 

[MrMonty]

Interesting. Do you know if this is more or less common with fraternal or identical twins?

The identical twins I knew were clearly identical. Sometimes there was a size difference but it was in the order of an inch or two like you say. I find it amazing it was not obvious that you were identical. Fraternal twins (at least the ones I have seen) are clearly not identical when seen together.

CBL

I'm not sure how common it is, heck, it could have been a total woo who was telling me, it was years ago so I don't remember. I'm terrible when it comes to recognizing people so, ironically I guess, I'm not good at telling twins apart. I've known eight or nine twins, identical and fraternal, and unless they're boy/girl, I could never tell who was who.

Just googled it and found this from the StraightDope. Learn something new everyday, guess I'm a "MoMo" Here's an even better link. Hm... they say only 1-2%, so maybe I'm not. Who knows, guess I'll be doing some reading on the subject. I feel terrible that I'm a twin and have never done any serious research on the subject.

It wasn't obvious that we were identical because of the blood type thing and because of our physical differences. However, growing up, maybe 50% of people would say they thought we were identical and were surprised when we told them we were fraternal. There was always someone who'd say we look exactly alike and always someone who'd say we look so different.

Being a twin is fun, especially growing up. Probably part of my journey to skepticism was my brother and I playing on people's beliefs that twins have some kind of psychic bond. We'd convince people we could read each other's minds or do stuff like hit ourself and have the other brother pretend to feel the pain. Not to mention stuff like switching shirts to mess with people or playing card games, on the same team, and messing around by "knowing" what the teammate was going to play. Lot's of slight physical clues, counting cards, guessing, and remembering the hits and forgetting the misses. Then there's also stuff like people coming up to you that you've never met and talking to you. "Wait a minute, I don't know you. You think I'm my brother."

Monty

 

[Eos of the Eons]

Sometimes an identical twin will get a mole where another won't. Sometimes an identical twin will be taller than another.

Is one more likely affected by a womb environment than another?

I don't think so.

This is so hard to study. It's so hard to draw conclusions. Genetics is highly complex. Identical twins may have the same genes, but there are things happening to affect expressions of those genes in an individual. This is genetic, but also a variance that is almost impossible to predict. This is also hard to explain.

http://eprints.kcl.ac.uk/archive/00000024/01/4.pdf

osteoporosis, we identified genes showing differential expression.

There is a game of "spin the wheel" going on when genes are actually selected in the embryonic development.

http://www.the-scientist.com/news/20040528/01
.

Many cellular processes are known to show variability, said Michael Elowitz, assistant professor of biology and applied physics in the Biology Division at the California Institute of Technology. "It's not genetic variation, it's what's usually known as epigenetic variation," he said.

Unless we include this information in gene studies, then the study will be somewhat flawed. You must allow for the variance in gene expression, or more specifically-epigenetic variation.

 

[CBL4]

Originally posted by Eos of the Eons
Unless we include this information in gene studies, then the study will be somewhat flawed. You must allow for the variance in gene expression, or more specifically-epigenetic variation

I agree with pretty much everything you said.

My one difference is that these things probably are not quite "spin the wheel." I guess there are subtle environmental difference in some of them. My second guess is that identical twins share a more similar environment which makes it more likely that the gene would express themselve more similarly.

I could be wrong. Mr Monty "stealing" food from his identical twin is counter evidence. If this is a common occur and the effect is great, it could make it so identical twins have a less similar environment. I doubt it but I do not have the evidence.

CBL

 

[Eos of the Eons]

I agree with pretty much everything you said.

My one difference is that these things probably are not quite "spin the wheel." I guess there are subtle environmental difference in some of them. My second guess is that identical twins share a more similar environment which makes it more likely that the gene would express themselve more similarly.

I could be wrong. Mr Monty "stealing" food from his identical twin is counter evidence. If this is a common occur and the effect is great, it could make it so identical twins have a less similar environment. I doubt it but I do not have the evidence.

CBL

Thank you. Yes, I'm grasping for ways to explain this, so any help is appreciated. The womb environment may affect the gene expression. Right now we know so little, and have so many questions.

 

[CBL4]

Originally posted by Eos of the Eons
Right now we know so little, and have so many questions.

Exactly. With our current knowledge all inheritance studies need to be taken with a grain of salt. Unless, the evidence is overwhelming, we need do further research and see what else we can discover.

I have read a few things about embryonic development and it makes me believe that this part of the environment has a huge effect. If I am correct, then, with twin studies, the grain of salt needs to be pretty large.

If you are looking for stuff about embryonic development, the name for this subscience is evolutionary development (or evo-devo.) I am reading "Endless Forms Most Beautiful: The New Science of Evo Devo and the Making of the Animal Kingdom" by Sean Carroll. . Depending on where and when the gene is turned on, it can do totally different things - wing spots and the end of limbs on butterflies. Also it is surprising that this same gene is also on chickens and most (all?) other animals.

CBL

 

[Soapy Sam]

I have a deep seated objection to the word "identical".

No two things are identical. To be identical they must share all the same properties. Which would include position. Hence there can only be one of them. Identical is a word whose own definition precludes the existence of type examples in the extreme case. In practice we use it to mean "very like".

We call "identical" twins "identical" because they are alike enough in appearance to make them hard to tell apart. But their life experiences differ from the moment the original cell splits into two embryos. They are not as different as the experience of fraternal twins, for many reasons, but they are still different.

Still, twin studies are the best control available for many testable conditions, especially if we have a lot of them.

But any control is imperfect and conclusions should always be reached with that in mind.

 

[Tanja]

I keep reading about studies that compare the behavior of fraternal and identical twins in order to give a genetic portion of certain behaviors such as female orgasm and pychopathy. The contention is that twins have extremely similar environments and that differences in behavior would thereby be genetic.

Sorry to derail the thread, but I think you are looking at it the wrong way round. In twin studies, they look at identical twins, who share 100% of their genes, so therefore any difference between them is due to environment. To compare twins growing in a "similar" (not same but similar) environment to those living in different environments you can compare results for twins who grew up together to those of twins who grew up apart.
I was always wondering how they manage to find a sufficient number of twins who grew up apart? They must be like golddust to twin studies.

 

[Eos of the Eons]

Re: Re: Twin Studies

Sorry to derail the thread, but I think you are looking at it the wrong way round. In twin studies, they look at identical twins, who share 100% of their genes, so therefore any difference between them is due to environment.

We've already gone over why this is not true because of the epigenetic variation that will occur. Just because they have all the same genes doesn't mean they will end up the same. You yourself contain genes for different types of hair color, different types of eye color, etc. You only express some of them.

I've been trying to explain how a developing blastocyst or embryo will have many genes to choose from when actually forming. Some are dominant, and will have a better chance of being expressed. Others will be combined in ways that will be expressed together no matter what.

When the eggs split into "identical" twins you suddenly have two individuals that will grow differently because of this "randomized" gene selection. They will contain the same genes, but will end up with different features sometimes depending on many factors.

To look at one trait:


Variations in genome-wide gene expression in identical twins – a study of primary osteoblast-like culture from female twins discordant for osteoporosis

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=436052

Using the example of osteoblast-like cells in our monozygotic discordant twins for osteoporosis, we identified genes showing differential expression

I really value the genetics course I took in college. It showed how complex these issues are, and I keep that in mind.