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Twins may be identical physically, but they present a challenge to the assumption that being is merely physical because they are often completely different when

Twins may be identical physically, but they present a challenge to the assumption that being is merely physical because they are often completely different when it comes to who they are (and what is that, really?) I have lived with twins for 3 years (married to one of them) and you couldn't find two people more different. Physically they look the same, but as far as personality, you couldn't find two people more different. They were both "nurtured" from the same woman, same environment, yet they are opposites. I think there is an ontological question raised. Is there a non-physical component to our existence? Something to think about.

latortuga 17y ago
Why does a non-physical component need to be invoked? Why could there not simply be genetic mutation that affects personality but not physical stature? It seems to me that there's no reason to believe some non-physical component is at work until the physical components have been ruled out.
- philwelch 17y ago
  (Identical) twins are genetically identical. (edited--thanks patio11)
  - patio11 17y ago
    Monozygotic (identical) twins are. Dizygotic (fraternal) twins are not. Actually, clarification to the clarification: fraternal twins, like any two siblings from the same parents, are merely highly unlikely to be (close to) genetically identical. For any given chromosome, you either get one of Mom's or one of Dad's. That gives you a finite number of possible chromosomal choices. Since they're essentially chosen with replacement between fertilizations (in the discrete sense of "chosen with replacement", its not like the egg says "OK, I'm done with this X chromosome mom, you can have it back now"), you could randomly end up with two siblings (and, by extension, two fraternal twins) with the same chromosome choices (i.e. pretty close to genetically identical, allowing for some minor and likely consequence-less mutation). My naive calculation of the probability of any two siblings being genetically identical without being identical twins is 1 / (2 ^ 48), though. :)
    - gjm11 17y ago
      Much, much smaller than 1/(2^48). You don't get whole chromosomes from each parent. Suppose one of your father's chromosome pairs is (ABCDEF,abcdef) and your mother's corresponding pair is (PQRSTU,pqrstu). Then you might be, e.g., (ABcdef,pQrSTu). Except that actually there are hugely many points at which crossover (i.e., a switch between ABCDEF and abcdef) can occur, and it can happen a variable (though usually not very large) number of times on each chromosome. Oh, and after that you'll have maybe 100 mutations (which could be anywhere) relative to your parents. Crude back of envelope calculation: you have on the order of 10^8 base pairs on each chromosome; there might typically be two crossovers per chromosome pair from each parent, giving you on the order of 10^16 possibilities for the chromosome you get from your father, the same for your mother, and hence ~ 10^32 possibilities for each of your chromosome pairs, so ~ 10^700 possibilities in all. Then you have ~100 extra mutations in your ~10^9 base pairs, giving another 10^900 possibilities; so there are ~ 10^1600 ways to make a typical human child's genome from a given pair of parents. That's a super-crude calculation, of course, but it's much nearer the right order of magnitude 2^48.
gjm11 17y ago
Oh, twaddle. What differencse between "identical" twins challenge is the idea that being is purely genetic. Fortunately, no one actually believes in that idea. All apples of a given variety are (I think) genetically identical, because of the way they're grown. (You start with a rootstock of some kind of apple tree that doesn't produce nice fruit, and then graft on the thing you actually want. This has to be done because if you grow apples from seed you tend to get trees that are very unlike their parents.) But -- gasp! -- not all Granny Smith (or whatever) applies look identical. Is this evidence for a non-physical component to applehood?
tome 17y ago
In the back of my mind I recall a study that said identical twins raised separately will often act differently to try to differentiate themselves, whereas identical twins raised separately have no such motivation and thus often end up more similar. If anyone could verify my hazy recollection I'd be most interested.
Sforlips 17y ago
1) even if their environment was very similar, it wasn't identical, if only because they interacted with each other. As you probably know, living system are roughly chaotic (except that, formally, chaotic systems are strictly deterministic, whereas determinism in the real world is debatable), and a tiny difference at one time point can have drastic consequences later on. 2) identical genes can lead to different phenotypes. Take the example of handedness. There is a dominant allele for right handedness and a recessive allele for random handedness. The people with two random genes have 0.5 chances of being right handed, and 0.5 chances to be left handed or ambidextrous. The same gene also determines if the hairwhorl turns clockwise or counter-clockwise, with the same behaviour. The dominant allele mean clockwise, and the recessive one is random. AFAIK, in the random case, handedness and whorl direction are independant. I don't think that your example is a good case for dualism. Edit: Were you trolling?

latortuga 17y ago

Why does a non-physical component need to be invoked? Why could there not simply be genetic mutation that affects personality but not physical stature? It seems to me that there's no reason to believe some non-physical component is at work until the physical components have been ruled out.

philwelch 17y ago
(Identical) twins are genetically identical. (edited--thanks patio11)
- patio11 17y ago
  Monozygotic (identical) twins are. Dizygotic (fraternal) twins are not. Actually, clarification to the clarification: fraternal twins, like any two siblings from the same parents, are merely highly unlikely to be (close to) genetically identical. For any given chromosome, you either get one of Mom's or one of Dad's. That gives you a finite number of possible chromosomal choices. Since they're essentially chosen with replacement between fertilizations (in the discrete sense of "chosen with replacement", its not like the egg says "OK, I'm done with this X chromosome mom, you can have it back now"), you could randomly end up with two siblings (and, by extension, two fraternal twins) with the same chromosome choices (i.e. pretty close to genetically identical, allowing for some minor and likely consequence-less mutation). My naive calculation of the probability of any two siblings being genetically identical without being identical twins is 1 / (2 ^ 48), though. :)
  - gjm11 17y ago
    Much, much smaller than 1/(2^48). You don't get whole chromosomes from each parent. Suppose one of your father's chromosome pairs is (ABCDEF,abcdef) and your mother's corresponding pair is (PQRSTU,pqrstu). Then you might be, e.g., (ABcdef,pQrSTu). Except that actually there are hugely many points at which crossover (i.e., a switch between ABCDEF and abcdef) can occur, and it can happen a variable (though usually not very large) number of times on each chromosome. Oh, and after that you'll have maybe 100 mutations (which could be anywhere) relative to your parents. Crude back of envelope calculation: you have on the order of 10^8 base pairs on each chromosome; there might typically be two crossovers per chromosome pair from each parent, giving you on the order of 10^16 possibilities for the chromosome you get from your father, the same for your mother, and hence ~ 10^32 possibilities for each of your chromosome pairs, so ~ 10^700 possibilities in all. Then you have ~100 extra mutations in your ~10^9 base pairs, giving another 10^900 possibilities; so there are ~ 10^1600 ways to make a typical human child's genome from a given pair of parents. That's a super-crude calculation, of course, but it's much nearer the right order of magnitude 2^48.

gjm11 17y ago

Oh, twaddle. What differencse between "identical" twins challenge is the idea that being is purely genetic. Fortunately, no one actually believes in that idea. All apples of a given variety are (I think) genetically identical, because of the way they're grown. (You start with a rootstock of some kind of apple tree that doesn't produce nice fruit, and then graft on the thing you actually want. This has to be done because if you grow apples from seed you tend to get trees that are very unlike their parents.) But -- gasp! -- not all Granny Smith (or whatever) applies look identical. Is this evidence for a non-physical component to applehood?

tome 17y ago

In the back of my mind I recall a study that said identical twins raised separately will often act differently to try to differentiate themselves, whereas identical twins raised separately have no such motivation and thus often end up more similar. If anyone could verify my hazy recollection I'd be most interested.

Sforlips 17y ago

1) even if their environment was very similar, it wasn't identical, if only because they interacted with each other. As you probably know, living system are roughly chaotic (except that, formally, chaotic systems are strictly deterministic, whereas determinism in the real world is debatable), and a tiny difference at one time point can have drastic consequences later on. 2) identical genes can lead to different phenotypes. Take the example of handedness. There is a dominant allele for right handedness and a recessive allele for random handedness. The people with two random genes have 0.5 chances of being right handed, and 0.5 chances to be left handed or ambidextrous. The same gene also determines if the hairwhorl turns clockwise or counter-clockwise, with the same behaviour. The dominant allele mean clockwise, and the recessive one is random. AFAIK, in the random case, handedness and whorl direction are independant. I don't think that your example is a good case for dualism. Edit: Were you trolling?