Naked Science Forum
Life Sciences => Physiology & Medicine => Topic started by: ROBERT on 23/01/2006 15:51:53
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Why are monozygotic twins not identical ? :
e.g. they have different fingerprints and different iris patterns.
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Some of your development is random, or succeptable to environmental influences. This is particularly true of women, who have 2 X-chromosomes, each of which must be powerful enough to work in a whole person (just look at blokes) So one of them is turned off at random in different bits of the body. It is fixed at random when the embryo has about 100 cells (google for Bar Body). Which bit has which active chromosome will cause variations in identical twins.
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Googling for "Bar body" probably won't help. You want "Barr body".
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Thank you Dave & Rosy.
Do these "random" processes mean that, contrary to popular belief,
there is some difference in the genomes of monzygotic ("identical") twins ?. i.e.
would the "mosaic" pattern of gene activation be apparent in a "DNA test" ?.
This could have forensic significance:-
http://news.bbc.co.uk/1/hi/england/tees/4446218.stm
http://news.bbc.co.uk/1/hi/england/london/4595348.stm
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As far as I can make out from what I've learned from studying genetics over the last few months (which isn't a lot), I don't think it necessarily follows that both of monozygotic twins would have mosaicism.
Mosaicism is where not all of the cells are affected. That would imply that it's not meiosis that causes it, but myosis. Therefore 1 of the monozygotic twins could be perfectly normal while the other has whatever symptoms are associated with the particular mosaicism concerned.
I would imagine, therefore, that DNA testing would be able to differentiate between a mosaic & a non-mosaic twin. I'm not sure whether any differences would show where neither of the twins has this type of defect although, as Dave has said, some differences occur when there are about 100 cells so I would think that it is indeed possible.
I'd be interested to know the answer to this so hopefully someone with more knowledge will reply.
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quote:
The decision on whether twins are fraternal or identical is usually based on the Doctor's examination of the placenta after delivery but this method is not always completely reliable. Now, we can perform a simple DNA test on twins by collecting loose DNA cells from the inside of the cheek using a buccal swab. This is a painless procedure that can be easily administered on infants as well as adults. The DNA sample is analyzed and produces the donor's genetic profile. If the twins are monozygotic (identical), their genetic profiles will be exactly the same and if the twins are dizygotic (fraternal), their genetic profiles will be different. It is therefore possible to determine with 99% certainty whether twins are identical or fraternal.
I found that at http://paleo-dna.lakeheadu.ca/ancientdna/twinzygosity.html
That seems to say that there is no difference in the DNA. I wonder if there is a difference between monochorionic & dichorionic twins? Hmmmmm [?]
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Doctor Beaver- we're not talking about mosaicism as I think you mean it (as discussed previously with ref. to chromosome 22). This is something that absolutely DOES apply to ALL women, as they have two x-chromosomes and if they used (all of) both in each cell it'd be very harmful (like having too many copies of a chromosome, as in Down's syndrome, only more so as the X is a very big and important chromosome).
There will be no difference in the DNA *present* in the cells, only in what is transcribed.
There's a neat experiment they've done on mothers of red-green colourblind boys which shows that carriers of the recessive gene for colourblindness are colourblind on the areas of the retina in which the chromosome carrying the defective gene is active. But they're not phenotypically affected because 50% (ish) of the retina works normally so they can perceive red-green.
I believe the differences between identical twins is actually more or less wholely environmental- starting in the womb. Usually one twin does better in terms of placental blood supply and ends up bigger, often brighter, etc. The extreme of this is of course the case of engulfed twins, where one embryo ends up inside (!!) the other.
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OK, Rosy. Thanks for correcting me. I'm still very much a beginner where genetics is concerned, so reading posts by someone such as yourself is extremely useful & interesting.
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does this mean your clone wouldnt have your finger prints
"Defender of the Sea"
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quote:
Originally posted by sharkeyandgeorge
does this mean your clone wouldnt have your finger prints
"Defender of the Sea"
Monozygotic twins are clones, they have different fingerprints,
so the answer to your question is no.
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Here is an article on Mosaicism and Chimerism, interesting if for no other reason than
it has a picture of a "Geep", (a goat-sheep chimera).
http://arbl.cvmbs.colostate.edu/hbooks/genetics/medgen/chromo/mosaics.html