Identical twins are not, of course, literally identical, but now it seems that they aren't genetically identical either thanks to "tiny differences" that are "relatively common" and "could have a major impact on our understanding of genetically determined disorders." I suppose, then, that we ought to just call them "very similar twins" which somehow or other reminds me that I haven't had a look at Reasons and Persons in a while.
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Identity Crisis
19 Feb 2008 02:15 pm
Comments (21)
Um, hasn't it been known for decades that identical twins aren't *literally* identical in their dna. Copy errors exist every time a cpoy is made.
Darn it, so they aren't physical duplicate twins? I was hoping we could study the heritability of being a zombie.
Yeah, Parfit, Mr. "cut my brain in half and there will be two of me!" Need to re-read that!
"Gore/Edwards"? Sounds like Parfit's already been at your brain with a hatchet.
OK, that's pretty fascinating.
Is it a developmental stage thing or what--ie, when twin splitting happens determines the degree of similarity, with earlier splits having less similarity than later splits?
Because I'm thinking that if it's not and twins aren't identical, then are all my own internal cells identical? IE, do some of my cells carry the parkinson's marker and some don't? Is that percentage, or perhaps the percentage variation within a single individual, itself genetic? Have they looked at groups of twins to try and figure out why?
This is a really neat finding. Makes me think of those odd cases where people have different sets of genes in different organs--chimeras. Maybe we're all more mosaics than we know...
Back in October 2004 when I was reading Parfit, I thought it'd be really cool to undergo several iterations of fission shortly before the voter registration deadline. All of me would register and vote for Kerry. If he ended up winning the election, I would stay divided at least for a while so that more people would enjoy victory. But if Bush won, I'd fuse again so that only one person would be depressed.
This isn't that new, nor surprising. The twins begin life with identical DNA, and then mutations, recombination and other modifications happen over time. Some are internal accidents, others are environmental assaults and influence.
This is the case for everyone.
Here's another way of putting it... the twins are on average as different from each other genetically as you are from yourself at birth.
Because I'm thinking that if it's not and twins aren't identical, then are all my own internal cells identical?No. Google "somatic mutations".
Re: anonymous
Your cells are not all DNA identical. Many will be due to mutations, as in the mole caused by UV exposure will only affect that skin cell's DNA, not the bone cells below which may be aided by the Vitamin D produced by the same UV.
Its also a reason why cancers can be in a cell's best interest. Why should that lymph do all that work to subsidize some genetically different muscle cell?
Something to keep in mind, though, is that cells used for reproduction (sperm, eggs) are usually isolated to prevent somatic mutations from piling up, as well forcing all the cells to behave if they any chance to exist the next organism. So those twins may be genetically different, but their eggs may be much more identical.
Matt Ripley has written some great stuff about how evolution works between cells of an organism, and how the genetics must be finely tuned so that everyone cooperates.
Sorry for length.
How tiny are the tiny differences? Identical twins seems pretty identical to me.
Maybe we should call them "for most useful intents and purposes identical twins"??? Or, just stick with the old name for convenience's sake. . .
There's a new post on my blog today:
http://swanpoliticsblog.blogspot.com
I resent this as a twin.. Darn you!!!!
Science takes all the fun out of everything. :(
No magic.
No fairies.
No identical twins.
About genetic differences inside the same person:
Women famously have 2 X chromosomes, but only 1 is needed. To prevent over-expression of proteins coded from on the X chromosomes, in each cell, one of the 2 X chromosomes is deactivated. This process appears to be random, so that 1/2 of the cells in a woman's body express the father's X chromosome and 1/2 express the mother's X.
This is thought to be one reason women tend to have better color vision than men, since the proteins that are color receptors are coded for on X. Women will have two slightly different sets of receptor proteins, and this can give better discrimination between colors, since they'll have a more complexly textured set of signals coming from the retina back to the brain.
RtR,
What you're talking about is less genetics than epigentics. The difference in form/function between a skin cell and a nerve cell has less to do with what genes each has (they both should have the same) than with how the genes are expressed and packaged
"epigenetics"
Anyone old enough, like me, to remember the Patty Duke Show ever wonder how "identical cousins" were possible? For a while, I thought someone had an affair with his wife's twin sister, but even that wouldn't have worked.
Oh thank you Anglo-American idiots for conquering the world and spreading such wisdom as the idea that different things might be identical. Sorry, too "meta", I guess.
You should read Ricoeur's Oneself as Another instead, as he gently points out that Parfit's argument is ridiculous.
"very similar twins"
Disgustingly sloppy wording.
The appropriate term, Matt, is monozygotic twins.
Matt's just obsessed with the Olson twins.
Otherwise, why did he find time to post this completely useless piece of information?
Get back to the Democratic horse race!
Comments closed March 04, 2008.
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Heads in the Sand
Very similar twins?
How about *AHEM* relatively identical twins?
Heh.
Posted by UncommonSense | February 19, 2008 2:45 PM