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May 19, 2008

BehindTheMedspeak: Identical Twins Aren't


Anahad O'Connor's March 11, 2008 "Really?" column in the New York Times Science section explains why in his typically elegant, concise fashion.

    The Claim: Identical twins have identical DNA.

    The Facts: It is a basic tenet of human biology, taught in grade schools everywhere: Identical twins come from the same fertilized egg and, thus, share identical genetic profiles.

    But according to new research, though identical twins share very similar genes, identical they are not. The discovery opens a new understanding of why two people who hail from the same embryo can differ in phenotype, as biologists refer to a person's physical manifestation.

    The new findings appear in the March issue of The American Journal of Human Genetics, in a study conducted by scientists at the University of Alabama at Birmingham and universities in Sweden and the Netherlands. The scientists examined the genes of 10 pairs of monozygotic, or identical, twins, including 9 pairs in which one twin showed signs of dementia or Parkinson's disease and the other did not.

    It has long been known that identical twins develop differences that result from environment. And in recent years, it has also been shown that some of their differences can spring from unique changes in what are known as epigenetic factors, the chemical markers that attach to genes and affect how they are expressed — in some cases by slowing or shutting the genes off, and in others by increasing their output.

    These epigenetic changes — which accumulate over a lifetime and can arise from things like diet and tobacco smoke — have been implicated in the development of cancer and behavioral traits like fearfulness and confidence, among other things. Epigenetic markers vary widely from one person to another, but identical twins were still considered genetically identical because epigenetics influence only the expression of a gene and not the underlying sequence of the gene itself.

    ''When we started this study, people were expecting that only epigenetics would differ greatly between twins,'' said Jan Dumanski, a professor of genetics at the University of Alabama at Birmingham and an author of the study. ''But what we found are changes on the genetic level, the DNA sequence itself.''

    The specific changes that Dr. Dumanski and his colleagues identified are known as copy number variations, in which a gene exists in multiple copies, or a set of coding letters in DNA is missing. Not known, however, is whether these changes in identical twins occur at the embryonic level, as the twins age or both.

    ''Copy number variations were discovered only a few years ago, but they are immensely important,'' said Dr. Carl Bruder, another author of the study at the university. Certain copy variations have been shown in humans to confer protection against diseases like AIDS, while others are believed to contribute to autism, lupus and other conditions. By studying pairs of identical twins in which one sibling has a disease and the other does not, scientists should be able to identify more easily the genes involved in disease.

    John Witte, a professor of genetic epidemiology at the University of California, San Francisco, said the findings were part of a growing focus on genetic changes after the parents' template had been laid. This and other research, Dr. Witte said, shows ''you've got a little bit more genetic variation than previously thought.''

    In the meantime, a lot of biology textbooks may need updating.

    Dr. Dumanski pointed out, for example, that as his study was going to press, the following statement could be found on the Web site of the National Human Genome Research Institute, the group that financed the government project to decode the human genome: ''Most of any one person's DNA, some 99.9 percent, is exactly the same as any other person's DNA. (Identical twins are the exception, with 100 percent similarity).''

    That, we now know, no longer appears to be the case.

    The Bottom Line: Identical twins apparently do not have identical DNA.


Anne Casselman, in an April 3, 2008 story on the Scientific American website about these surprising new findings, explored the subject in more detail; her piece follows.

    Identical Twins' Genes Are Not Identical

    Twins may appear to be cut from the same cloth, but their genes reveal a different pattern

    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.'"


Now that your brain's whirring, it's time for the abstract of the American Journal of Human Genetics paper cited above; it follows.

    Phenotypically Concordant and Discordant Monozygotic Twins Display Different DNA Copy-Number-Variation Profiles

    The exploration of copy-number variation (CNV), notably of somatic cells, is an understudied aspect of genome biology. Any differences in the genetic makeup between twins derived from the same zygote represent an irrefutable example of somatic mosaicism. We studied 19 pairs of monozygotic twins with either concordant or discordant phenotype by using two platforms for genome-wide CNV analyses and showed that CNVs exist within pairs in both groups. These findings have an impact on our views of genotypic and phenotypic diversity in monozygotic twins and suggest that CNV analysis in phenotypically discordant monozygotic twins may provide a powerful tool for identifying disease-predisposition loci. Our results also imply that caution should be exercised when interpreting disease causality of de novo CNVs found in patients based on analysis of a single tissue in routine disease-related DNA diagnostics.


Future twin studies will explore how environmental factors can alter one's genome, according to Charles Q. Choi's article on the subject that appeared in the May, 2008 issue of Scientific American.

He noted that Bruder's group is now investigating "... whether all of an individual's cells are genetically identical or whether, like twins, they diverge, making each of us mosaics of slightly different genomes."

As I consider these radically new views of the genome, I can't help but thing that Lysenko is up there looking down, smiling.

O'Connor's column heading this post represents the second time he's looked into a twin-related topic: the first was back on November 2, 2004, when he tackled the question of whether identical twins have identical fingerprints; that piece follows.

    The Claim: Identical Twins Have Identical Fingerprints

    The Facts: Identical twins often share personality traits, interests and habits. They come from the same fertilized egg and share the same genetic blueprint.

    To a standard DNA test, they are indistinguishable. But any forensics expert will tell you that there is at least one surefire way to tell them apart: identical twins do not have matching fingerprints.

    Like physical appearance and personality, fingerprints are largely shaped by a person's DNA and by a variety of environmental forces. Genetics helps determine the general patterns on a fingertip, which appear as arches, loops and whorls. An individual finger can have just one of these patterns or a mixture of them.

    While a fetus is developing, the ridges along these patterns are influenced by a number of factors, including bone growth, pressures within the womb and contact with amniotic fluid.

    These lead to unique ridge characteristics in each person and finger, said Gary W. Jones, a former fingerprint specialist with the Federal Bureau of Investigation.

    Often, identical twins will have a similar arrangement of patterns, but never the same minute details. "It's impossible for people to have identical fingerprints," said Mr. Jones, who now works as a private consultant in Summerfield, Fla.

    "The study of fingerprints has been around for about 100 years, and in all that time, two people have never been found to have the same prints." The patterns on a person’s fingers, palms and feet are fully formed by roughly the fifth month of pregnancy.

    They stay the same throughout life, barring any changes brought on by severe mutilation or a skin disease. But even then, they change very little.

    John Dillinger, the notorious Depression- era gangster, famously tried to alter his facial features and obliterate his fingerprints with acid to elude authorities. After Dillinger died, experts discerned a few of his remaining ridge patterns and identified him easily.

    The Bottom Line: Identical twins never have matching fingerprints.

May 19, 2008 at 04:01 PM | Permalink


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So, another butter vs. margarine instance of scientific debunking. I just don't know what to believe anymore except... mabybe... possibly... oh never mind.

Posted by: Milena | May 20, 2008 5:13:16 PM

A man biological clock is vitally important to the health of some offspring.

The male biological clock is the most potent cause of new genetic disorders!

http://ageofthefatherandhealthoffuture.blogspot.com/ Please look into this fact that is contrary to what we have been taught.



Posted by: Les Feldman | May 19, 2008 5:11:03 PM

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