Genetic determinism

Genetic determinism is the mechanism by which genes, along with environmental conditions, determine morphological and behavioral phenotypes.

Alternative use

The term genetic determinism has sometimes been mistakenly applied to the unscientific belief that genes determine, to the exclusion of environmental influence, how an organism turns out. Such views have sometimes been attributed to opponents, or forwarded in hypothetical arguments, without having been actually held by anyone: as CH Waddington wrote in 1957, "It is of course a truism which has long been recognised that the development of any individual is affected both by the hereditary determinants which come into the fertilised egg from the two parents and also by the nature of the environment in which the development takes place."[1]

The use of genetic determinism in this sense of an accusation of holding unscientific beliefs originates in the historical "nature versus nurture" dispute, especially during the 1970s and 1980s.

A related error is the supposed misconception holding that geneticists and molecular biologists have only recently come to the realization that environment is essential in the development of the organism from egg to adult. It was in factual history understood long ago that genetic effects cannot be studied in isolation of the environment and that all measurements of such effects are only relative to stable external conditions.[2] Also known since at least the 1950s is the means by which the environment influences embryonic and juvenile development, namely the epigenetic control of gene activation and deactivation.[3]

Origins

Genetic determinism, which identifies the gene as the biological source of morphology and instinct, can be traced back to Austrian theorist August Weismann, who proposed in the 1890s that the key actors in the struggle for survival are not organisms but their genes, which he called determinants. While Darwin's concept of natural selection was intended to apply to whole organisms, Weismann modified Darwin's idea according to a process he called "germinal selection." Since the fittest determinants would be whichever ones correlate to the most useful phenotypic traits, germinal selection would result in the fittest organisms surviving and reproducing.[4] Weismann referred to the chemical carrier of these determinants as the germ plasm, now known to be DNA.

Weismann's view was founded on the belief that biological inheritance is inconceivable except by way of germ plasm from parents to offspring. As Stephen Jay Gould pointed out, this belief was not based on empirical observation. "We accept it," wrote Weismann, "not because we are able to demonstrate the process in detail... but simply because we must, because it is the only possible explanation that we can conceive."[5] On the assumption that behavior cannot affect genes, Weismann argued that only genetic mutation, not adaptations on the part of a struggling organism, could significantly alter the developmental patterns inherited by progeny. Though contrary to Darwin's view,[6] Weismann's belief that determinants shape the body, and never vice versa, has long been known as the central dogma of modern biology.

The machine theory

Weismann believed that the initial division of the egg into two cells causes determinants to be divided into two groups, such that one cell will develop, say, the left half of the embryo, while the other cell will generate the right half. With each subsequent division, determinants continue to be meted out differently to different cells until the stage is reached where each cell type is in place. At this point the distinct set of determinants in each cell type produce the developmental machinery that will generate the tissue, organ or system associated with that type of cell.[7]

Weismann's developmental concept was based on a mistaken interpretation of an 1882 experiment carried about by Wilhelm Roux, in which Roux killed one of the cells of a frog embryo at the two-cell stage. The remaining cell then led to half an embryo, leading Weismann to believe that the determinants for the embryo were divided along with each cell division. As Hans Driesch and other researchers discovered, when the dead cell is removed, the other cell produces a whole organism, not half.[8] It's now well known that all the cells in a given organism contain the same set of genes.

Developmental genes

Weismann also assumed that different species contain different developmental determinants. The lizard's germ plasm naturally contains different developmental instructions than the cow's germ plasm. While genetic differences do help determine the variety of forms across the phylogenetic tree, genes regulating development, now known as homeobox genes, have been found to hardly vary at all across vast portions of it. Everything from flies to humans share roughly the same set of developmental genes, differing in the way they are expressed.[9]

The modern synthesis

Following the rediscovery of Mendel's principles of genetics, several theorists such as RA Fisher, JBS Haldane, Sewall Wright, Ernst Mayr, and Theodosius Dobzhansky contributed to the synthesis of Mendel and Darwin's concept of natural selection. In contrast to Weismann, the modern synthesis rejected the notion that the organism is ultimately reducible to physical principles. According to Mayr, the organism responds to a dual causation, one based on laws of physics, the other based on a genetic program. Thus the organism reflects not just the mechanics of its constituent parts but the phylogenetic history encoded in its genes. Genetic information both reflects a species' descent and directs an organism's development.[10] The key to genes is not their chemical makeup but the information they carry. According to esteemed researcher Christiane Nusslein-Volhard, what distinguishes genes from proteins is that genes are chemically identical and differ only in terms of their sequence of nucleic acids and the information encoded in that sequence.[11]

Crick and Watson

The chemical nature of the germ plasm was discovered in 1953, when Francis Crick and James Watson explained the structure of DNA. This event was heralded as the discovery of the book of life, a view still prevalent in the popular press and among many scientists.[12] However, nearly 50 years later, when a "first draft" of the human genome became available, the book of life turned out to be considerably less extensive than had been anticipated. Instead of the expected 100,000 genes, the genome contains fewer than 30,000. As Nusslein-Volhard points out, morphological complexity does not correlate with size of genome.[13]

The contemporary view

Despite surprises about the quantity and distribution of genes, genetic determinism remains the standard model. According to Nusslein-Volhard, who won a Nobel for her research into molecular mechanisms of development, the genome is a "building plan," DNA is a "language of four letters that can be read faultlessly... In the fertilized egg, the genetic program is complete."[14] She reiterates the reductionist approach even as she fleshes out a model of development based not on genetic information but the way genes are "expressed." She notes that it's rare that one gene determines a specific structure in a specific position.[15] Rather than provide anything like a blueprint for the finished organism, homeobox genes guide development by regulating other genes.[16]

A given organic form is thought to emerge at a particular place in the embryo on the basis of its distance from an "organizer," a set of cells that influence how other cells develop. By generating a chemical gradient that permeates the embryo, the organizer establishes zones of development. Depending on which zone a cell is located in, some of its genes are expressed while others are repressed.[17] "Through an interplay of mutual activation and repression, more and more complex molecular patterns emerge."[18]

In fiction

See also

References

  1. ↑ CH Waddington, The Strategy of the Gene, London: George Allen & Unwin, 1957, p. 88
  2. ↑ CH Waddington, The Strategy of the Gene, London: George Allen & Unwin, 1957, pp. 88-104
  3. ↑ CH Waddington, The Strategy of the Gene, London: George Allen & Unwin, 1957, pp. 30-39
  4. ↑ Stephen Jay Gould, The Structure of Evolutionary Theory, Cambridge: Belknap Press, 2002, p. 207
  5. ↑ Stephen Jay Gould, The Structure of Evolutionary Theory, Cambridge: Belknap Press, 2002, pp. 201-03
  6. ↑ Charles Darwin, The Origin of Species, New York: Modern Library, pp. 29, 169, 175, 290-93, 324, 331, 338, 593-96, 606-07, 636-37
  7. ↑ Ludwig von Bertalanffy, Modern Theories of Development, London: Oxford University Press, 1933, p. 32
  8. ↑ Ludwig von Bertalanffy, Modern Theories of Development, London: Oxford University Press, 1933, pp. 73-74
  9. ↑ Sean Carroll, Endless Forms Most Beautiful, New York: WW Norton & Company, 2005
  10. ↑ Ernst Mayr, What Makes Biology Unique? Cambridge: Cambridge University Press, 2004, p. 30
  11. ↑ Christiane Nusslein-Volhard, Coming to Life: How Genes Drive Development, Kales Press, 2006, p. 37
  12. ↑ Boyce Rensberger, Life Itself, New York: Oxford University Press, 1996
  13. ↑ Christiane Nusslein-Volhard, Coming to Life: How Genes Drive Development, Kales Press, 2006, p. 127
  14. ↑ Christiane Nusslein-Volhard, Coming to Life: How Genes Drive Development, Kales Press, 2006, pp. 21, 32, 145
  15. ↑ Christiane Nusslein-Volhard, Coming to Life: How Genes Drive Development, Kales Press, 2006, p. 52
  16. ↑ Christiane Nusslein-Volhard, Coming to Life: How Genes Drive Development, Kales Press, 2006, p. 55
  17. ↑ Christiane Nusslein-Volhard, Coming to Life: How Genes Drive Development, Kales Press, 2006, p. 43
  18. ↑ Christiane Nusslein-Volhard, Coming to Life: How Genes Drive Development, Kales Press, 2006, p. 57

External links

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