Why are "identical" twins not identical?

[plindboe]

There's usually some differences in height and behaviour between identical twins, finger prints always differ as well. So what explains these differences?

I recently saw a documentary that mentioned mutations in mitocondria DNA as somehow being responsible, but it didn't go into details. I also recently read a scientist saying that the emerging field of epigenetics explained these differences.

So what is the current scientific view?

 

[kellyb]

Environmental factors can affect the expression of genes. Nutrition can affect height. Illness can affect eye color. Stuff like that.

 

[strathmeyer]

I hear they even grow up to work at different jobs, too.

 

[Taffer]

Genes + environment = phenotype.

 

[Gord_in_Toronto]

Chaos theory?

 

[Math Maniac]

Correct me if I'm wrong (anyone that knows for sure), but I believe that since women have two X chromosomes it's possible for any one area in the body to use the genes from the father's X chromosome while another area can use the genes from the mother's X chromosome. This could account for some variation seen in identical twins, among other things already mentioned. It would be small, however.

I don't know about mitochondrial DNA mutations but do know that mitochondrial DNA is passed only from the mother to child.

 

[Dogdoctor]

Environment can cause many effects. Genes get switched on and off and other changes may happen without genetic mutation however that would be another possible cause of changes. I am not familiar with the latest thoughts on this issue though

 

[scratchy]

We still have the fingerprints, which doesnt change from environment. The prints youre born with will stick for the rest of your life whatever the environment. The explanation might be that genes define the type of pattern you have but not the exact layout. If one loses a bit of skin which then grows back, the pattern has to line up with the undamaged areas according to the type of prints your genes define, and thus become the same as before.

Or not?

 

[Dogdoctor]

Fingerprints of twins are not identical because they form in the womb and are not totally a genetic trait. eta: The environment in the womb is not identical and there would be another reason why they aren't identical.

 

[Madalch]

Apparently, they are born at different times, so they have different astrological charts.

 

[blutoski]

There's usually some differences in height and behaviour between identical twins, finger prints always differ as well. So what explains these differences?

I recently saw a documentary that mentioned mutations in mitocondria DNA as somehow being responsible, but it didn't go into details. I also recently read a scientist saying that the emerging field of epigenetics explained these differences.

So what is the current scientific view?

I think you're misunderstanding the role of genetics in how a person develops. I try to teach my students that DNA is only a recipe.

In the case of twins, once the cells have divided into seperate zygotes or embryos or whatever, they're developing differently. Gestational development may even be more influential on outcome than the first ten years of life. Small differences during this period will have significant outcomes.

 

[Eos of the Eons]

At that early stage of gene expression you can get differences that result in differences that you can see. There are also differences that you don't see.

The authors have compared expression of 10,000 genes between 5 pairs of proven homozygotic twins and unrelated individuals. As no more than 2% of the genes show mostly low magnitude variations within twin pairs, compared to up to 14% of the genes in unrelated individuals, the authors suggest to use twin data as a reference to distinguish variations in expression due to genetic factors from those due to the environment.

http://genomebiology.com/reports/F1000/gb-2005-6-5-322.asp#Sharma

Even though the twins are identical (homozygotic), there is a 2% difference due to how the genes end up getting expressed in the end.


http://www.sciam.com/article.cfm?chanID=sa003&articleID=0001616A-93A1-12C5-93A183414B7F0000

http://www.news-medical.net/?id=19071

Scientists compared gene expression patterns of 11 pairs of monozygotic twins, who shared the same egg and were genetically identical, but only one of them had RA. They found three new genes that were significantly over-expressed in the twin with RA compared to the one without the disease. This is the first report for RA that examines gene expression patterns in monozygotic twins.

From egg to sperm to blastocyst, to embryo, there is a hugely complicated process that results in gene expression. There is shuffling, a recombination of genes, linked genes, dominant genes, etc.

Besides confirming previous findings about the evolution of sex chromosomes and duplications, we show that chromosomal rearrangements are associated with increased gene-expression differences in the brain and that rearrangements can have both direct and indirect effects on the expression of linked genes. In addition, our results are consistent with a role for some rearrangements in the original speciation events that separated the human and chimpanzee lineages.

http://www.sciencedirect.com/scienc...serid=10&md5=f82e48bffc7b72844d3e7b47775d8140

Genetics is mindblowingly cool.

 

[Eos of the Eons]

DNA shuffling: an in vitro process that generates genetic diversity and then selects genes optimized for a specific biological function. DNA shuffling can result in greatly enhanced performance by a gene and is being used in the development of various gene therapies.

DNA shuffling is performed by fragmenting a family of related DNA sequences into a pool of random DNA fragments that are then reassembled to create a collection or "library" of recombinant DNA molecules. High throughput screening is then used to select an improved sequence i.e. a DNA strand created by this process that performs a specific task in a more efficient or more specific fashion than was performed by the original form of the sequence.

This process is repeated again and again until an optimized product is obtained. This process mimics the gene exchange or "recombination" that occurs during sexual reproduction. However, instead of causing the recombination of an entire genome, this process is focused upon one specific gene or DNA sequence.

http://biojudiciary.org/glossary/index.asp?flt=d

There is another cause of differences in identicals which occurs in female identicals. When a female foetus is created; it has 2 copies of the X chromosome (one provided by Mum’s egg; one by Dad’s sperm; a boy has one X from Mum and a Y from Dad). I said above that generally the version of each gene that is ‘dominant’ is the one that is expressed – this is different for the X chromosome – this chromosome is unique in that one of the chromosomes is permanently ‘silenced’ very early in embryonic development so rather than gene ‘expression’ working from whichever version is ‘dominant’ as I mentioned above; the only genes that work are the ones on the X chromosome that is not ‘silenced’. This silencing can happen AFTER the identical twins have been created and are developing (when the embryo has grown to about 10-20 cells); and if by chance one identical twin ‘silences’ the X chromosome that came from Dad’s sperm; and the other twin silences the X chromosome that came from Mum’s egg; then effectively; they will have different genes working in their systems; which can result in noticeable differences. The X chromosome has about 120 different genes; so the effects of each twin silencing a different X chromosome can be quite obvious.

http://www.twins.org.au/index.php?page=93&PHPSESSID=a1aa3d773cf2e694a5f5d389127100cb

This last web page also explains epigenetics, and how that can effect what genes get switched on in either twin.

I hope that answers the question. There are many variables involved.

 

[plindboe]

Genetics is mindblowingly cool.

Agreed! I actually just returned from the lab where we knocked out a gene in a strain of yeast and replaced it with another. The cells lost the ability to differentiate sexually, but gained the ability to produce their own uracil. Fascinating stuff!

Thanks alot for your links, I'll read them all with great interest.

 

[Taffer]

Genetics is mindblowingly cool.

Absolutely!