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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

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.

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.