Identical twins have the same DNA and the same genes. They have many similarities such as resemblances in facial and body expression, their laugh, and much more. Despite these similarities, as these twins get older the differences that develop between their appearances can be remarkable. It is also very possible for one twin to get a deadly disease, such as cancer, while the other does not. The first question that may run through your mind is how? The answer is the epigenome. In contrast to the genome, which is the set of your entire DNA, the epigenome is what tells your genome what to do and when. In other words, if the genome is thought of as the hardware of the computer, with its physical parts being genes, the epigenome can be thought as the software of the computer, influencing how and when these physicals parts should be used. The epigenome also dictates the cell's behavior and function.
The epigenome marks the genome, determining whether or not a gene is expressed and if so, to what level. It does this in two ways, DNA methylation, and histone modification. DNA methylation is where a methyl group, a tag of carbon and hydrogen, connects to a part of DNA (to the gene) and decides for it to be expressed or not. Histone modification is where a chemical tag secures a histone, or a protein, and tightens or loosens the gene's coil around it to determine how greatly the gene is expressed.
But what does the epigenome have to do with identical twins? A study done by Dr. Randy Jirtle (Duke University) gives a partial explanation. His study was based off of genetically identical mice. Although these mice were identical, some were fat and yellow while others were brown and skinny. This difference of weight and color was caused by changes in their epigenomes. A gene, called agouti, was in all of these mice. The agouti gene determines the coat color of the mouse and the weight. In the yellow mice, the agouti gene was "turned on" all the time, causing the gene to be expressed. This consequently caused the yellowness and obesity in the mice. On the contrary, the brown mice, because of DNA methylation, had a methyl group attached to its agouti gene, causing the gene not to be expressed. This, in turn, caused the mouse to be thin and brown. Because of this epigenetic variation in the identical mice, it caused a very noticeable change in their appearances.
It has also been shown through another study that as twins age, their epigenomes change significantly. Though the two people will still possess the same genome, their lifestyles may have been completely different. These changes in their epigenome can be based off of what they eat, smoking habits, and their managements of stress. Based off the environmental factors of this person's life, important genes can be turned off, as a result of tags being in the wrong place. This can give rise to diseases like cancer. This is one of the reasons why one twin can get diagnosed with cancer while the other does...