The idea that genes encode all the heritable features of living things has been a fundamental tenet of genetics and evolutionary biology for many years, but this assumption has always coexisted uncomfortably with the messy findings of empirical research. The complications have multiplied exponentially in recent years under the weight of new discoveries.
Classical genetics draws a fundamental distinction between the “genotype” (that is, the set of genes that an individual carries and can pass on to its descendants) and the “phenotype” (that is, the transient body that bears the stamp of the environments and experiences that it has encountered but whose features cannot be transmitted to offspring). Only those traits that are genetically determined are assumed to be heritable—that is, capable of being transmitted to offspring—because inheritance occurs exclusively through the transmission of genes. Yet, in violation of the genotype/phenotype dichotomy, lines of genetically identical animals and plants have been shown to harbor heritable variation and respond to natural selection.
Conversely, genes currently fail to account for resemblance among relatives in some complex traits and diseases—a problem dubbed the “missing heritability.”1 But, while an individual’s own genotype doesn’t seem to account for some of its features, parental genes have been found to affect traits in offspring that don’t inherit those genes. Moreover, studies on plants, insects, rodents, and other organisms show that an individual’s environment and experiences during its lifetime—diet, temperature, parasites, social interactions—can influence the features of its descendants, and research on our own species suggests that we are no different in this respect. Some of these findings clearly fit the definition of “inheritance of acquired traits”—a phenomenon that, according to a famous analogy from before the Google era, is as implausible as a telegram sent from Beijing in Chinese arriving in London already translated into English. But today such phenomena are regularly reported in scientific journals. And just as the Internet and instant translation have revolutionized communication, discoveries in molecular biology are upending notions about what can and cannot be transmitted across generations.
Biologists are now faced with the monumental challenge of making sense of a rapidly growing menagerie of discoveries that violate deeply ingrained ideas. One can get a sense of the growing dissonance between theory and evidence by perusing a recent review of such studies and then reading the introductory chapter from any undergraduate biology textbook. Something is clearly missing from the conventional concept of heredity, which asserts that inheritance is mediated exclusively by genes and denies the possibility that some effects of environment and experience can be transmitted to descendants.
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