Academic journal article Genetics

The Evolving Definition of the Term "Gene"

Academic journal article Genetics

The Evolving Definition of the Term "Gene"

Article excerpt

IN 1866, Gregor Mendel, a Moravian scientist and Augustinian friar, working in what is today the Czech Republic, laid the foundations of modern genetics with his landmark studies of heredity in the garden pea (Pisum sativum) (Mendel 1866). Though he did not speak of "genes"-a term that first appeared decades later-but rather of elements, and even "cell elements" (original German Zellelemente p. 42), it is clear that Mendel was hypothesizing the hereditary behavior of miniscule hidden factors or determinants underlying the stably inherited visible characteristics of an organism, which today we would call genes. This is apparent throughout his publication in his use of abstract letter symbols for hereditary determinants to denote the physical factors underlying the inheritance ofcharacteristics. There is no doubt that he considered the mediators of heredity to be material entities, though he made no conjectures about their nature.

The word "gene" was not coined until early in the 20th century, by the Danish botanist Johannsen (1909), but it rapidly became fundamental to the then new science of genetics, and eventually to all of biology. Its meaning, however, has been evolving since its birth. In the beginning, the concept was used as a mere abstraction. Indeed, Johannsen thought of the gene as some form of calculating element (a point to which we will return), but deliberately refrained from speculating about its physical attributes (Johannsen 1909). By the second decade of the 20th century, however, a number of genes had been localized to specific positions on specific chromosomes, and could, at least, be treated, if not thought of precisely, as dimensionless points on chromosomes. Furthermore, groups of genes that showed some degree of coinheritance could be placed in "linkage groups," which were the epistemic equivalent of the cytological chromosome. We term this phase the "classical period" of genetics. By the early 1940s, certain genes had been shown to have internal structure, and to be dissectable by genetic recombination; thus, the gene, at this point, had conceptually acquired a single dimension, length. Twenty years later, by the early 1960s, the gene had achieved what seemed like a definitive physical identity as a discrete sequence on a DNA molecule that encodes a polypeptide chain. At this point, the gene had a visualizable three-dimensional structure as a particular kind of molecule. We will call this period-from roughly the end of the 1930s to the early 1960s-the "neoclassical period."

The 1960s definition of the gene is the one most geneticists employ today, but it is clearly out-of-date for deoxyribonucleic acid (DNA)-based organisms. (We will deal only with the latter; RNA viruses and their genes will not be discussed.) Here, we review the older history of the terminology, and then the findings from the 1970s onwards that have undermined the generality of the 1960s definition. We will then propose a contemporary definition of the "gene" that accounts for the complexities revealed in recent decades. This publication is a follow-on paper to an earlier paper by one of us (Portin 2015).

The Classical Period of Genetics

The development of modern genetics began in 1900, when three botanists-the German Carl Correns, the Dutchman Hugo de Vries, and the Austrian Erich von Tschermak- independently cited and discussed the experiments of Mendel as basic to understanding the nature of heredity. They presented results similar to Mendel's though using different plants as experimental material (Correns 1900; de Vries 1900; Tschermak 1900). Their conceptual contributions as "rediscoverers" of Mendel, however, were probably not equivalent. De Vries and Correns claimed that they had discovered the essential facts and developed their interpretation before they found Mendel's article, and they demonstrated that they fully understood the essential aspects of Mendel's theory (Stern 1970). In contrast, Tschermak's analysis of his own data was inadequate, and his paper lacked an interpretation. …

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