Culinary Metaphor Used To Understand Gene-Environment Interactions
A recent article in the Archives of General Psychiatry by Hallmayer et al. discussed the role of genetic and environmental factors in autism and autism spectrum disorders (ASD). The study was a heritability analysis of 192 pairs of twins, which attributed 37 percent of the variation in risk of autism to genetic factors and 55 percent to shared environmental factors. The authors contrasted their findings with those of previous studies, which had given genetics a much higher share (up to 90%).
Rather than contradicting previous research, the new results provide more evidence that autism, like many other common diseases, results from both genetic and environmental factors. The way that these elements – often called “nature and nurture” – influence health outcomes has been discussed for decades but is often misunderstood, even among scientists.
Disease Causation is Not as Easy as… Pie
Heritability analysis focuses on sources of variation in specific populations. A common misinterpretation of these types of analyses is that the causes of a particular disease are cleanly, though errantly, summed up as slices of a pie – or in pie chart fashion. For example, it is not uncommon to have the explanation of disease presented as “25 percent genetic and 75 percent environmental,” adding up to 100 percent of cases. Furthermore, from the perspective of “either/or,” a person with gene variant “X” is thought to be destined to develop colon cancer no matter what his diet, whereas a person with gene variant “Y” can smoke all she wants yet will never develop lung cancer, and so on. However, what we have learned from gene-disease association studies is that, in reality, human disease is rarely a product of such simple and clearly defined relationships. Causation of human disease is not about nature OR nurture but more about nature AND nurture.
Disease Outcomes “Stew”
Most common diseases–such as coronary heart disease, cancer, and diabetes, which affect millions of Americans–are caused by modifiable environmental risk factors such as cigarette smoking, diet, and sedentary lifestyle. Because gene-environment interactions underlie almost all human diseases, a large role for the environment does not preclude an equally large role for genetic factors. Interaction among genetic and environmental factors allows the total contribution of individual risk factors to exceed 100%.
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