Here’s a conundrum: identical twins originate from the same DNA, so how can they turn out so different even in traits that have a significant genetic component? For instance, why might one twin get heart disease at 55, while her sister runs marathons in perfect health? Nature versus nurture has a lot to do with it, but a deeper related answer can be found within something called epigenetics. That’s the study of how DNA interacts with the multitude of smaller molecules found within cells, which can activate and deactivate genes. If you think of DNA as a recipe book, those molecules are largely what determines what gets cooked when. They aren’t making any conscious choices themselves, rather their presence and concentration within cells make the difference. So how does that work? Genes in DNA are expressed when they’re read and transcribed into RNA, which is translated into proteins by structures called ribosomes. And proteins are much of what determines a cell’s characteristics and function. Epigenetic changes can boost or interfere with the transcription of specific genes.