NOT-SO-IDENTICAL TWINS: They look the same but their genes may reveal some fundamental differences.
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Identical twins are identical, right? After all, they derive from just one fertilized egg, which contains one set of genetic instructions, or genome, formed from combining the chromosomes of mother and father.
But experience shows that identical twins are rarely completely the same. Until recently, any differences between twins had largely been attributed to environmental influences (otherwise known as "nurture"), but a recent study contradicts that belief.
Geneticist Carl Bruder of the University of Alabama at Birmingham, and his colleagues closely compared the genomes of 19 sets of adult identical twins. In some cases, one twin's DNA differed from the other's at various points on their genomes. At these sites of genetic divergence, one bore a different number of copies of the same gene, a genetic state called copy number variants.
Normally people carry two copies of every gene, one inherited from each parent. "There are, however, regions in the genome that deviate from that two-copy rule, and that's where you have copy number variants," Bruder explains. These regions can carry anywhere from zero to over 14 copies of a gene.
Scientists have long used twins to study the roles of nature and nurture in human genetics and how each affects disease, behavior, and conditions, such as obesity. But Bruder's findings suggest a new way to study the genetic and environmental roots of disease.
For example, one twin in Bruder's study was missing some genes on particular chromosomes that indicated a risk of leukemia, which he indeed suffered. The other twin did not.
Bruder therefore believes that the differences in identical twins can be used to identify specific genetic regions that coincide with specific diseases. Next, he plans to examine blood samples from twin pairs in which only one suffers from asthma or psoriasis to see whether he can find gene copy number changes that relate to either of these illnesses.
The result might also call into question the many findings of previous twin studies that assumed identical twins were indeed identical, Bruder notes. "It's pretty unlikely they're going to significantly change any of the results found so far," counters Kerry Jang, a psychologist at the University of British Columbia in Vancouver, who runs Canada's largest twin study. "We can adjust our models to take [genetic differences] into account in the same way we've adjusted for different environments."
The discovery of this genetic variation gives hope for an obscure but pressing issue in the case of a criminal suspect who is an identical twin. "If one twin is a suspect and the whereabouts of the other twin cannot be determined, then the jury is often left without the ability to find guilt beyond a reasonable doubt" in cases that rely on DNA evidence, says Frederick Bieber, a pathologist at Harvard Medical School.
"If the twin issue comes up in a criminal investigation it's possible that if there are [copy number variants] that differ between the two twins that might help sort that out," Bieber says.
Given that there are 80 pairs of identical twins in Virginia's convicted offender database alone, this might not be as small an issue as it may sound. And such genetic variation also matters to the population at large.
Bruder speculates that such variation is a natural occurrence that accumulates with age in everyone. "I believe that the genome that you're born with is not the genome that you die with—at least not for all the cells in your body," he says.
Charles Lee, a geneticist at Brigham and Women’s Hospital in Boston, agrees. Genetic variations can arise after a double strand of DNA breaks when exposed to ionizing radiation or carcinogens. "It reminds us to be careful about our environment because our environment can help to change our genome," he says.
Plus, these variations may predict age-related diseases. Lee adds: "As you age … your chances for having a genomic rearrangement that causes a certain disease increases all the time."
The differences between identical twins increase as they age, because environmentally triggered changes accumulate. But twins can also begin their lives with differences, according to Bruder's study, and that calls into question their very name.
"Maybe we shouldn't call them identical twins," Harvard's Bieber says. "We should call them 'one-egg twins.'"
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22 Comments
Add CommentHas anyone really done a survey of multiple cell samples within a single person? Perhaps each person has some genetic variation throughout his or her body.
Reply | Report Abuse | Link to thisMy momma says twins are when god loves someone so much, he makes em twice. So, that means theys the same, no matter what some wacky doctor says. I think they should spend more time prayin' and less time snooping.
Reply | Report Abuse | Link to thisSo, when someone gets sick and dies...that means God doesn't like them any more? Should those "pesky doctors" leave them be and let them die? How about if it YOU who gets sick?
Reply | Report Abuse | Link to thisPeople seem to be rather ignorant about the topic of genetics. With comments posted like the ones here its obvious that they know nothing about genetics. This research is remarkable and will have some pretty big benefits. One Im currently exploring is its use in criminal investigations in court settings. Finally we can tell the difference between twins other than their fingerprints.
Reply | Report Abuse | Link to thisPreach on bubblez5505! I am very interested in your findings on identical twins' differences and its use in criminal investigations in court settings. Can you share that information with me?
Reply | Report Abuse | Link to thisI am always looking for new information concerning twin studies. My two year old twins were thought to be fraternal (dizygotic) becasue they were dichorionic/diamniotic, but science has proven that at least 30% of all monozygotic (identical) twins are actually di/di (usually proven by dna or placental testing. They have the exact same weight/height, hair and eye color, but there are very slight differences in thier face shapes. I have always believed they were actually 'identical', but because of thier slight differences, I have been told this could not be (not by any 'reliable' source, mind you. Only by some family members). "Identical twins MUST be perfectly identical", I've been told. This article seems to place some doubt on that statement. Does anyone care to share their knowlege on the subject?
Reply | Report Abuse | Link to thisMy twins are 51 years old and one has recently been diagnosed with a very rare disease. She will be needing a bone marrow transplant. I always suspected they were identical as they are mirror twins who can ONLY be identical, although the doctor said there were 2 placentas. They just had a DNA test which proved that they are identical right down to the same blood type. Now the bone marrow transplant can be done with one twin donating to the other.
Reply | Report Abuse | Link to thisI Have a set of twin daughters. I am not sure if they are identical or fraternal, because I could not afford testing. They were in two different sacks inside one large sack, but on the same placenta. They have the same eye color, almost the same weight, and almost the same height. But one twin has asthma and psoriasis, but the other twin doesn't. The younger twin had psoriasis as an infant but it went away.
Reply | Report Abuse | Link to thisI had twins years ago. They had Twin to Twin Transfusion Syndrome whick , I have read, is related to identicle twins. One daughter was born with dark brown eyes and hair and the other was born with blue eyes and blonde hair. Although Samantha's eyes and hair have darkened, they are no where nearly as dark as Alyssa's. The articles I have read online discuss differences re: diseases and developement but not eye and hair color. Just curious if any one has explanation.
Reply | Report Abuse | Link to thisI have identical twin girls, when I had my second scan at twenty weeks pregnant the hospital told me my twis were not identical as they had their owm sac and placentas. When they was one years old I was contacted by TEDS (twins early developement study) they wanted them to take part in their study, which I agreed. At the age of six I was asked for their DNA which I gave them. Months later they wrote back telling me that my twins were identical. I was shocked I rang them up and spoke to the scientist who did the DNA tests , I told him what the hospital told me, and told him of the slight differences, I was upset I didnt want them to be identical. He told me there was scientific proof that twins whose egg splits into two in the first week of conception can have their own sacs and placentas. One of my girls has mild spina bifida and the other does not. and the other twin has asthma and the other does not . This web site might explain this hope this helps.
Reply | Report Abuse | Link to thisInteresting. Raises also fascinating philosphical questions about will and consciousness. Philosoper David Chalmers posits that that consciousness, the phenomenon of conscious experience itself, cannot be explained by biology or physics. The fact that twins have different experiences seems consistent with this notion because their biological sameness or extreme biological similarity would suggest identical thoughts and experiences (assuming the same stimulation/environment), yet we seem to know they are two persons, two consciousnesses and two wills.
Reply | Report Abuse | Link to thisIt is widely accepted that the genetic material determines the phenotype, s0 as to humanbeings. To the best of my knowledge, there are no pretty identically leaves in the existence, the same law can be applied to the twins, no matter they are one-egg twins or two-egg twins. There must be some differences between the twins, both in genome and phenotype.
Reply | Report Abuse | Link to thisI could have told the doctor that before all the research!! My identical twin sister and I never looked alike! Although, everyone tells us we do; with the exception of the last 5 years (we are now in our late 20s). When I look at baby pictures of us, I have to admit, that I only know which one I am is by the full head of hair on my head! However, when my sister and I were in grade school and our adult teeth were coming in , the shape of our faces did change. I had much worse teeth (had to wear braces 6months longer than my sister) , therefore my jaw line was much thinner than my sisters. Her face was more full and round. Nobody saw that massive physical difference between us....just our immediate family. My point is that DNA must have played a small role in the developing of our adult teeth, how else did I get a massive overbite and undeveloped lower jaw; while my twin sister got bad teeth, just not as bad as mine!
Reply | Report Abuse | Link to thisIt seems as though these geneticists were approaching this question presuming that processes of gene transcription and expression were infallible. We know this is not completely correct and that transcription errors contribute to genetic mutation, along with external damage. As I understand, even my own genes do not match the ones I had when born. A cell is not a precision machine but a complex, dynamic operational environment.
Reply | Report Abuse | Link to thisI have two friends that are identical (or mono-zygotic) twins. When I first knew them I could not tell them apart - in fact only by their hair styles. Over time though I began to notice their differences (other than one had a small mole on the left and the other on the right). One has a slightly thinner build for example. I would expect that they had minute differences in their genes and not just their phenotype.
Reply | Report Abuse | Link to thisI also have twins who were considered fraternal because they had two placentas and sacks. The same doctor that told me that when I was pregnant came to see me in the hospital after the twins birth and asked me if they were identical or fraternal! I told him he had told me fraternal and he asked why...I told him because he had told me that because they were in separate sacks and each had their own placenta they were fraternal. The then said that he was wrong. I asked how I could know if they were identical and he said I would just know. That answer didn't work for me so after much research I found a website that did DNA testing to find if twins were identical or fraternal. They sent me a test kit and I swabbed their cheeks and mailed it back. I found out my twins were indeed identical. They look the same, same height, weight, hair and eye color but like above their heads are a little different in shape. Hope this helps. Here is the link to the website I used to test my twins. http://www.proactivegenetics.com/?gclid=CITJ_PKCsKQCFYs65Qod111NyA
Reply | Report Abuse | Link to thisNow we could question, if we clone an animal do we get a real clone? It shoudn't if we extrapolate from this study...
Reply | Report Abuse | Link to thisActually, I just saw a documentary that shows evidence to contradict this. Identical twins actually have incredibly similar consciousnesses. I can confirm this as an identical twin myself. When twins share the same experiences by being together most of the time, they tend to think and act exactly the same. I don't have time or space to go into the experiences I've had to support this, but suffice for now to say that looking back on my life, back when my twin and I basically had the same lives (lived in the same room, went to the same school, had the same interests, friends, etc) our lives and consciousnesses were more and more identical. Now that we live on other sides of the country, we look different, act different, think different, have different tastes and interests, and are just generally 2 different people as opposing to before when we were almost clones.
Reply | Report Abuse | Link to thisThis is not surprising. Snowflakes are said to be different, so why wouldn't twins? Also, many mutations can occur in genes. This happens differently and individually when DNA is being copied and transcribed. Bases shift, are added, subtracted, and changed. Even with the correction/proofreading that occurs, errors are made. If one mutation occurred in one twin on one single strand of DNA, this mutation could cary over to thousands of other strands of replicated DNA. This alone could cause significant change amongst the DNA of twins. Even though they may not be apparent at first glance, identical twins have subtle differences in appearance or other defining qualities. For example, one twin may be slightly taller or have a deeper voice. Height is a trait that is multi-gene dependent. If one twin is taller, this could mean that this twin has different genes expressing the trait than the other. This is just one example of twin DNA being different. Just like anything else in the world, there is nothing exactly identical about every single human, including identical twins.
Reply | Report Abuse | Link to thisStunningly silly comment. By this reasoning God loved beetles about 300,000 times more than humans. There are an awful lot of different kinds of those crawly critters.
Reply | Report Abuse | Link to thisI am parsuing Ph.D.in forensic dna analysis and i want to work more on your findings on "identical twins differences" and its use in criminal investigations in court settings. Can you please share some information with me or please give me some guide line about your technique.
Reply | Report Abuse | Link to thisFirst off I would like to say I am a very religious person and I am also an identical twin. I would have to disagree with you Bluebuilder, for although that is what your mother told you, no where in the bible does it talk about identical twins and say that they cannot be different. God created life and science and he created me and my identical twin sister. Although we are identical our genes are not the same exactly. I have dimples, different eye color and many different genetic conditions like insomnia, dyslexia, and Celiac disease, and we have many differing allergies which are all genetics. There are different obstetrical that God has chosen to place in my life through my genetic health issues that my sister hasn't had to go through; my sister even has told me that she doesn't think she would be able to go through what I go through with all of it, and that I am stronger in this sense than her. God had a plan even when I was in the womb and he was making me, cause although he made me a twin the genetic differences have placed a giant part in my life and faith. I think it is ignorant for someone to just ignore science due to religion because I think God is a very scientific person, just look at every thing he has created and every detail in his creation.
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