Jane E. Brody, New York Times, August 20, 2018

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Decades-long studies of identical and fraternal twins — and in some cases, triplets — who had been separated at an early age and reared in what were often strikingly different environments have documented the important interaction of nature and nurture and help to explain the relative contributions of each to how a child develops.

“A strict dichotomy between genes and environment is no longer relevant; they work in concert,” said Nancy Segal, a psychologist at California State University, Fullerton, and herself a fraternal twin who has made a career of twin studies, starting with the famous Minnesota Twin Family Study. She is the author of “Born Together — Reared Apart: The Landmark Minnesota Twins Study,” published in 2012 by Harvard University Press.

The many studies of thousands of pairs of identical and fraternal twins, both those reared together and those reared apart, have made it possible to assess the relative contributions of genes and the environment to a large number of characteristics.

“It’s trait-specific,” Dr. Segal said, with different ratios depending on the characteristic in question. “In an individual person, the contributions of genes and the environment are inestimable,” she explained, “but on a population basis we can estimate how much person-to-person variation is explained by genetic and environmental differences.”

The studies of reared-apart twins have shown that in general, half the differences in personality and religiosity are genetically determined, but for a trait like I.Q., about 75 percent of the variation, on average, is genetic, with only 25 percent influenced by the environment.

Furthermore, there can be gender differences in the influence of genetics. A study of 4,000 pairs of twins in Sweden found that genetics has a stronger influence on sexual orientation in male twins than in female twins.

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Genetics researchers now know that while an individual’s DNA is essentially immutable, a wide range of environmental factors can confer what are called epigenetic differences. Epigenetics influences which genes in an individual’s genome may be turned on or turned off. Such factors as exercise, sleep, trauma, aging, stress, illness and diet have been shown to have epigenetic effects, some of which can be passed on to future generations.

Researchers are seeking ways to deliberately alter gene expression in hopes of finding preventives or treatments for diseases like diabetes with a strong genetic component.

There can also be changes in the genome of an identical twin when the egg divides, resulting in a defect in a particular gene, Dr. Segal said. In a pair of identical twin girls, one can experience a phenomenon called X-linked inactivation. Two of the identical Dionne quintuplets were colorblind as a result of such a genetic effect.

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