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Understanding Twins

Identical Twins Are Not Genetically Identical

Potential consequences for the Minnesota Twin Study

When I was a child one of my favorite books was about a pair of identical twins who decided to switch clothes. They looked so much alike that their parents had had to dress one in blue and the other in green. The twin boys fooled their parents for a long long time. An obedient 3-year old, I was thrilled by their ingenuity and boldness.

Though parents can usually tell the difference between their identical twins, grandparents, teachers, neighbors and peers sometimes cannot. And for good reasons. Identical twins very often look almost exactly alike. No surprise there, if identical twin share all of their DNA.

Recent research presented at the 2012 American Society of Human Genetics meeting, however, suggests that identical twins may not be as genetically similar as hitherto suggested. Identical, or monozygotic, twins come from the same fertilized egg. So, at some point during cell division (before 14 days post-conception), identical twin embryos share virtually all of their DNA. During early fetal development, however, identical twins undergo more than 300 genetic mutations, or copy errors, on average. As human cells divide trillions of times during their lifespan, a few hundred genetic mutations could lead to millions or trillions of genetic differences in the DNA of identical twins over the years. Chemical factors can furthermore activate or suppress gene expression, which means that the same subset of genetic material can lead to the formation of different proteins.

The results, which were presented by McGill University epidemiologist Rui Li, could have drastic consequences for what we know about the heritability of diseases, addictions, personality and intelligence—or what is more popularly known as the nature versus nurture debate.

A good chunk of the information we have about whether traits are passed down from parent to child through genes or whether they are a result of the environment comes from the Minnesota Study of Twins Reared Apart. The Minnesota Twin Study is a project originally led by Minnesota Professor of Psychology Thomas Joseph Bouchard, Jr. The initial project took place from 1979 to 1999 and consisted in periodical educational, psychological, medical and dental testing of individuals in an extensive population of identical (monozygotic) and fraternal (dizygotic) twins and their families.

Starting around 1990 Professor Bouchard and his team published numerous results from the twin study project. The majority of the conclusions of the twin studies are based on answers to the question of whether identical twins (who were thought to share all their genes) are more similar than those of fraternal twins (who share an average of 50 percent of their genes). It was concluded, among many other things, that identical twins are about 85 percent similar for IQ, whereas fraternal twins are about 60 percent similar. This would seem to indicate that half of the variation in intelligence is due to genes.

What, then, are the consequences of the recent discovery that identical twins are not completely genetically identical for the results of the Minnesota Twin Study? The answer to that question depends on how different identical twins are. Assuming that early fetal genetic mutations multiply significantly as time passes, there may be reason to question some of the previous twin study results. The main conclusions at risk are those that concern traits, diseases and conditions that we thought were a result of environmental influences.

Suppose a large number of pairs of identical twins separated at birth turn out to have very similar IQs. Setting aside skepticism about IQ tests as a measure of intelligence, we should be able to conclude that the environment does not significantly affect intelligence. The studies done by Li and her colleagues do not affect this conclusion in any interesting way. However, suppose that we find that a large population of pairs of twins separated at birth have very different IQs. Can we conclude that intelligence isn't inherited? The answer to this question is "no." The reason for this is that variation in intelligence may be grounded in genetic material that identical twins do not share. So the environment could affect intelligence a lot less than we once thought.

Li and her colleagues used blood cells to calculate mutation rates. As blood cells divide slower than most other cells in the body, the consequences for diseases (e.g., cancer) could be more austere than we think. That is, some diseases that we believe are not genetic could be genetic after all. Blood cells, however, divide faster than brain cells. So, genetic differences in brain-based traits, such as personality and intelligence, may not be as austere as differences elsewhere.

More from Berit Brogaard D.M.Sci., Ph.D
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