Nature – Nurture Revisited and Resolved
Professor Michael Meaney recently gave a series of lectures at Harvard University as part of the institution's Mind Brain Behavior Interfaculty Initiative (MBB). In the first of the two-part series, he discusses the importance of epigenetics in redefining our understanding of the mechanism of heredity and the processes by which genetic and environmental forces weave the fabric of individual differences, including the potential transmission of "acquired" traits from parent to offspring.
The astounding success of agriculture over the past millennia relied on the practice of selective breeding, the mating of organisms of desired traits. Selective breeding is based on the fundamental observation that the offspring tend to resemble the parent(s), which defines heredity. The science of heredity refers to the statistical likelihood of similarity between parent and offspring for any given trait.
The identification of DNA as the source of genetic material and the subsequent characterisation of DNA structure was one of the great scientific achievements of the 20th century. Genes were now the embodiment of heredity. If a medical condition or trait was heritable, then ipso facto it will have a basis in the heritable DNA sequence variations transmitted from biological parent to offspring. This premise has borne out for rare disorders. But for common medical conditions such as diabetes, depression or asthma, genetic transmission cannot fully explain inheritance! It is not even close.
Here is the paradox: Studies of family pedigrees as well as those of twins reveal that diabetes, depression, asthma and scores of other common disorders, as well innumerable traits are heritable – some apparently very much so. The estimates of heritability for common disorders is anywhere from 20 to 80 percent. But large studies, some with over 1 million subjects, that associate common heritable genetic variants to these outcomes commonly explain only about 1 to 2 percent of the variation between people! In scientific terms, this gap is referred to as “missing inheritance” and is reflected in the difference between the known biological parent – offspring similarities and the degree of similarity (or heritability) explained by genetic variation.
There is a second problem for genetic explanation. Many populations have experienced massive increases in "heritable" conditions such as obesity and diabetes over a time frame that cannot be explained by rates of genetic mutation. Could it be that genetic variation is not the only process by which we might explain similarity between parent and offspring? Social interactions are an obvious process that might underly the transmission of traits from parent to offspring. And the contrasting view led to the nature (genetic) vs nurture (environmental) debate of the origins of individual differences.
The past decade has brought resolution into view in the form of epigenetics. Epigenetics is essentially the study of the biochemical processes that control the activity of the genome – the transcription (or expression) of DNA sequences into the RNA or protein signals that regulate cellular development and function. It is not a new science. What is new is the idea that these epigenetic mechanisms can be regulated by environmental signals. Hence, the environment can speak directly to the genome to affect cell development and function. Environmental conditions can stably alter the epigenetic signals that control the transcription of the genome. There is a nurture to nature. And the reverse is also true. The impact of environmental conditions on the ‘epigenome’ varies across individuals as a function of – yes – heritable genetic sequence variation. There is a nature to nurture.
(Adapted from Harvard University's MBB Distinguished Lecture)
ABOUT THE AUTHOR
Besides serving as Programme Director for the Translational Neuroscience programme at the Singapore Institute for Clinical Sciences (SICS), Professor Michael Meaney is also a James McGill Professor in the departments of Psychiatry and Neurology, and Neurosurgery at McGill University. Renowned for his research on stress, maternal care and gene expression, his scientific work focuses on epigenetics and the mechanisms by which adversity in early life might alter neural development and render certain individuals at risk of mental illnesses and/or learning disabilities later in life. One of the world’s most cited scientists, he has been invited to present his findings at research institutes, government health agencies, and scientific meetings globally. Through his work, he hopes to holistically depict what vulnerability to mental illnesses looks like, what constitutes resilience or risk, and what interventions can help stop these debilitating conditions before they take root in adolescence and early adulthood.
Harvard University's Mind Brain Behavior Interfaculty Initiative (MBB) was introduced in 1993 to encourage an interdisciplinary community of faculty from across the University to engage in research and other academic activities aimed at elucidating the structure, function, evolution, development, ageing, and pathology of the brain in the context of human behaviour, genetics, and society.