Moss - What genes cant do - 2003, страница 9

Like chemical molecules, these elements haveno transitional stages between them. . . . The lack of transitional forms betweenany two simple antagonistic characters in the hybrid is perhaps the best proofthat such characters are well delimited units. . . . In the hybrid the two antagonistic characters lie next to each other as anlagen. In vegetative life only the dominating one is usually visible. . . . I draw the conclusion that the law of segregationof hybrids as discovered by Mendel for peas finds very general application in theplant kingdom and that it has a basic significance for the study of the units ofwhich the species character is composed (de Vries 1900).A central outcome of de Vries’s encounter with Mendel was the modification of his notion of pangenes, which he had previously deemed to bevariable in number per cell and which he then revised to pairs that segregate in the production of gametes. Mendel’s studies in transmissionthus served to alter de Vries’s conception of the units of inheritance, butnot with respect to their status as Anlagen.

Where Mendel’s reductionof the units of transmission to Anlagen—i.e., to the preformed units ofdevelopment—was for specific practical purposes (Falk 1995), de Vries’sMendelism proffered a conception of the inheritance of particulateAnlagen as a universal theory. Transmission and development were fundamentally linked in de Vries’s preformationism. Traits were nothing butthe expansion of the inherited Anlagen. In order to account for why onekind of Anlagen would be dominant and another recessive in the casesof hybrids (heterozygotes) with antagonistic characters, De Vries lookedfor substantive differences between them. He suggested the following:“Ordinarily the character higher in the systematic order is the dominating one, or, in cases of known ancestry, it is the older one” (de Vries1900).Bateson also sought to ground a discontinuous model of heritablevariation in a theory of encapsalized, developmentally preformed unitsof hereditary transmission.

His term “allelomorph” for the units of transmission captures this sense of an encapsulated piece of organismic form.He too addressed the issue of dominance in hybrids and, in order to28Chapter 1simplify further the explanation for this phenomenon, he proposed thatdominance and recessivity were really about the presence or absence ofsome heritable Anlagen. In this way, the pure-breeding recessive linesimply lacked that heritable unit-character–allelomorph which shows upas dominant in hybrids (heterozygotes) whose other parent comes froma pure breeding line that possesses it. What this does not explain is whya trait would fail to appear or appear in a highly attenuated form whenthe allelomorph is present.

To explain (away) such deviations fromMendelian expectations without recourse to complicating the story byreference to developmental interactions, the environment, and so forth,the terms penetrance and expressivity, were introduced, which simplyturned phenotypic variability into intrinsic propensities of theallelomorphs.13Johannsen’s Critique of the New Preformatonism—Origins of thePhenotype-Genotype DistinctionThe terms “gene,” “genotype” and “phenotype” were introduced by theDanish botanist Wilhelm Johannsen, but his contribution to the advancement of the gene concept ran far deeper than terminology.

Keime andAnlagen in nineteenth-century biology were concepts that pertainedfirst and foremost to development. Holistic and teleological when takentogether, they were not readily amenable to a reductive analysis. Theteleomechanist program refined its understanding of inheritance to theextent that it established the continuity of the cell. The Kantian heuristic required that the wherewithal for producing new forms be alwaysalready contained within the potential of the germ. An explanation forthe appearance of new species (past or future) would thus be a furtherextension of its theory of development, i.e., a new and more adaptivelyspecialized expression of the potential of the germ.The question of transmission across generations only became animportant topic unto itself when the view arose that evolutionarychanges were based at least in part upon novel and possibly fortuitousvariations in the germ.

Darwinian natural selection requires that progenyof a species vary in what they contain in their egg, that is, in what istransmitted from their parents. In attempting to formulate a way to con-Genesis of the Gene29ceive of the contents of the egg such as to be consistent with a gradualist theory of evolution based on natural selection, Weismannians and biometricians alike required a conception of the determinants of organismicform which would allow for all kinds of minor variations of characterthat can be packaged within the egg. Johannsen (1911) considered thislegacy to be one steeped in the popular experience of heredity as theinheritance of personal properties, as, for example, in the inheritanceof an estate. In later writings (1923) he extended his critique to what hecalled the whole “morphological tradition” in which he included, notjust Darwin, Weismann, and Galton, but also Mendel, Bateson, and deVries.

What he meant by the morphological tradition was the penchantfor believing that units of inheritance were chunks of morphology, aspatently expressed in Bateson’s term “allelomorph.” However, themorphology of an organism is the contingent result of many factorsinteracting over time. Johannsen coined the terms genotype andphenotype and distinguished between them in order to depart from thismorphological (and preformationist) legacy and establish, he felt,the grounds for a proper science of the inheritance of the genotype. It isprecisely the conflation of the phenotype—a product of environmentaldevelopmental interactions—with the inheritance of Mendelian unitswhich constituted a new brand of preformationism.What Johannsen called for in distinguishing between the genotype andthe phenotype was a separation of the inheritance of Mendelian unitsfrom development, thereby constituting the study of genetics as an independent discipline.

All of the theories of inheritance from Darwinthrough Bateson and de Vries had confused the two. Genetics emergeswith Johannsen as a science of the acquisition of the genotype and notof the phenotype. Heredity was not about the passing on of properties,which were always historically acquired and developmentally contingent,but rather about the presence of identical genes in ancestor and descendent. Although Johannsen had no specific suggestion for the (physical)nature of genes, he recommended treating them as chemical-like in theirahistorical nature:The genotype conception is thus an “ahistoric” view of the reactions of livingbeings—of course only as far as true heredity is concerned.

This view is an analogto the chemical view, as already pointed out; chemical compounds have no30Chapter 1compromising ante-act, H2O is always H2O, and reacts always in the samemanner, whatsoever may be the “history” of its formation or the earlier stateof its elements. I suggest that it is useful to emphasize this “radical” ahistoricgenotype-conception of heredity in its strict antagonism to the transmission-orphenotype-view (Johannsen 1911).Having distinguished between development and inheritance Johanssenhad no need to interpret the gene in a reductivist-preformationist fashion.He recommended that the genotype be understood along the lines ofthe German term Reaktionsnormen used by Woltereck to refer to the fullrange of an organism’s potential (1911).

The phenotype of an organismfor Johannsen is the product of the whole genotype reacting to the environmental conditions of its development. Phenotypes can be seen to varyalong a continuum because the Reakionnormen of the genotype arecapable of plastically adapting to variant conditions. Genotypes vary discretely, but the consequences on phenotype are realized at the level ofthe Reaktionnormen as a whole:Hence the talk of the “genes for any particular character” ought to be omitted,even in cases where no danger of confusion seems to exist. So, as to the classical cases of peas, it is not correct to speak of the gene—or genes—for “yellow”in the cotyledons or for their “wrinkles,”—yellow color and wrinkled shapebeing only reactions of factors that may have many other effects in the pea-plants(Johannsen 1911).From Cytoplasmic Anlagen to Morgan’s ConversionJohannsen clarified the conceptual basis for an independent science ofgenetics, but it was T.

H. Morgan who turned it into an actual researchprogram. Morgan, unlike the founders of modern Mendelism, was nota plant breeder or evolutionary naturalist but rather an embryologist.He was steeped in the controversies introduced in previous sections(“Chopping up the Anlagen” and “A Bifurcation in Embryology”) pertaining to how the potential of the organism is distributed betweennucleus and cytoplasm and between egg cell and progeny cells.

The factthat the course of this history does not lead in a logically compelled orconclusive way to the gene-centered paradigm is what makes it interesting and important. Embryology did not culminate in genetics—rather,Morgan converted to the practice of the new discipline, leaving the unre-Genesis of the Gene31solved problematics of embryology to fare for themselves (Allen 1985,Darden 1991). As embryologists fractured the cell into nucleus and cytoplasm, so the life sciences fractured into a center and periphery, withgenetics becoming the center and with the legacy of developmentally (andorganizationally) oriented biology relegated to the periphery. Philosophically, it will be important to see how many central problems werebanished to the margins and yet naively thought to be solved (or almostsolved) in the name of the gene.