Identical Twins are not so identical as one would think
Most parents of Identical Twins can easily tell their twins apart. There are subtle, and not so subtle, differences between the twins that clue parents in to the unique identity of each child. In fact, often times parents (supported by misinformation from their healthcare professionals), inaccurately classify their identical twins as fraternal twins, in part because of the differences they can easily see between their twins.
Scientists have long studied twins, especially identical twins, in their search for answers to the age old question of nature versus nurture. Yet, it has only been in the last decade or so that we started to understand the influences of epigenetics.
Dr. Courtney Griffin from the Cardiovascular Biology Research Program at the Oklahoma Medical Research Foundation is the parent of identical twins, and an expert on Epigenetics.
“Identical Twins have had a profound impact on scientists’ understanding of nature and nurture. Studies on identical twins who were separated at birth and raised in separate households, have helped us understand different traits that are more effected by nature, or DNA, versus nurture, or the home environment. For example, some traits like IQ or criminal tendencies are more affected by your DNA than the house that you grew up in. On the other hand, other traits, like depression in men, or your preference for a particular political party are more influenced by your environment, than by your genes.
So what about identical twins who are raised in the same home environment? Their nature and their nurture are almost the same, and yet any parent of identical twins, myself included, can quickly point out differences in their children. One twin may have more of a preference for certain types of foods, or may have more of an aptitude for a certain sport or musical instrument. And sometimes, health differences can arise in these children. For example, there are reports of autism, or asthma, or bi-polar disorder arising in one twin at a young age, while the other one remains unaffected. How do we explain these differences given that the DNA is the same in these children and for the large part their home environment has been the same, too. Well, it turns out that some of these differences can be explained by a third, very powerful influence on our lives besides nature and nurture. And this is epigenetics.”
~ TED lecture by Dr. Courtney Griffin from the Cardiovascular Biology Research Program at the Oklahoma Medical Research Foundation
Twin studies have greatly contributed to our understanding of which factors are clearly inherited characteristics (nature) versus those that are shaped by our environment (nurture). But, while scientists were able to clearly classify some characteristics, others seem ambiguous – mostly pointing to genetics – yet sometimes the rules were broken – causing confusion.
Why would one identical twin develop autism at a young age, while the other does not? As our understanding of genetics have evolved, the field of epigenetics has emerged as a possible answer.
Epigenetics was defined back in the 1950’s by Conrad Waddington as, “the study of heritable changes in genome function that occur without a change in DNA sequence”, and re-defined in December, 2009, by Berger, Kouzarides, Shiekhattar, and Shilatifard as, “An epigenetic trait is a stably heritable phenotype resulting from changes in a chromosome without alterations in the DNA sequence.” Huh?!
Most of us have learned that our DNA, specifically our genes, control things like our eye color, hair color and that funny shape to our nose that runs in our family. “Epi” means “on top of” or “above”, so Epigenetics literally means “on top of genetics”. Epigenetics explains how DNA is exposed to external factors resulting in modifications to the DNA to turn genes “on” or “off”. This turning on and off of genes impacts the way that cells respond to the genes. Genes that are turned off, are prevented from being expressed.
One reason for this is the packaging of the DNA material. There is so much, there is a need to save space in order to carry all of the genetic sequencing necessary for a complex organism. Think of this like a zip file on your computer. When we zip up a file or set of files, we save space, but we cannot easily read the contents. When we unzip files – those files can now be read through our computer. A similar thing happens with DNA and the expansion of the epigenetic marks, allowing the cells to read the DNA. But, think of it not so much as a switch, but more like the volume control knob on your stereo.
Have you ever wondered why a bone cell is different from a brain cell? Or a skin cell is different than a blood cell? All of our cells contain the same DNA, but the expression of genes in each type of cell are turned on and off in different combinations. Each combination creates the basis for the different types of cells in our bodies.
Why is all of this important for pregnant women? What you eat, and your behaviors during your pregnancy will impact the DNA of your offspring, through epigenetics. Not only that, but there is mounting evidence, that these genetic implications can be passed through multiple generations.
Why is this important for anyone that might someday wish to conceive children? A study in Sweden and England demonstrated that the diet and smoking habits of ten year old boys impacted the lifespan of their offspring. In fact, things that you do long before having children might impact the lives of multiple generations after you’re long gone.
What if you’ve already spent a good deal of your life eating unhealthy foods and practicing bad habits? It’s not too late. You can still positively influence your epigenome. Ms. Griffin says,
“It’s not too late to start eating healthier foods, foods that we already know are good for us, like leafy vegetables, whole grains, avoiding cigarettes, cocaine, and stress. All of which have been shown, experimentally, to impact our epigenomes negatively. These are things that you can do to impact your genes and your long term health. And if that’s not incentive enough, they can also impact the health of your future children and grandchildren. I think this concept that we can positively impact our genes, is really profound and empowering. Because we’d always worked under the assumptions that our genes are set in stone, that they’re beyond our influence.”
All of us can start living a healthier lifestyle to make positive impacts on our epigenome tomorrow. I’m going to get started right away. As soon as I finish my bowl of ice cream. It’s been a long day.
Interested in learning more? Visit our YouTube playlist, The Epigenetics of Identical Twins.