"R” Genes Us ?
We live in a genocentric world.7 The ‘‘genes ‘r’ us’’ habit is so deeply imbued in our thought processes that it seems impossible to think otherwise. We think of our genes as a blueprint for development, linear information that need only be read out of the book of life. We go to movies in which the major premise is that a DNA sequence isolated from a fossilized mosquito is all we need to create Tyrannosaurus rex. (The nicety, clearly found in Jurassic Park, that the
DNA needed an egg to become a T. rex is lost in the shuffle).8 And we hear almost daily on the news that the project to sequence human DNA molecules has led us from the genes for breast cancer and diabetes to Parkinson’s and more. Present-day students of human genetics can do the rest, ‘‘discovering’’ genes for alcoholism, shyness, and—yes—homosexuality.9
Even when scientists are themselves cautious about imbuing all power to the gene, popular renditions of new scientific findings dispense with linguistic subtlety. When Dean Hamer and his colleagues published evidence that some male homosexuals possessed the same region of DNA located on the X chromosome, for instance, they used fairly cautious language. Phrases such as ‘‘the role of genetics in male homosexual orientation,’’ ‘‘genetically influenced,’’ or ‘‘a locus related to sexual orientation’’ abound in the paper.10 Such caution did not, however, extend to other pages in the same issue of Science, the journal in which the Hamer group’s report appeared. In the Research News section of the same issue, the headline ran: ‘‘Evidence for Homosexuality Gene: A genetic analysis. . . has uncovered a region on the X chromosome that appears to contain a gene or genes for homosexuality.’’11 Two years later, coverage in a more popular venue, The Providence Journal, had, on the same page, headlines referring to ‘‘gay gene’’ research and ‘‘schizophrenia gene search.’’12
But what does it mean to speak of gay genes or genes for some other complex behavior? Do such phrases, or Hamer and colleagues’ more circumspect language advance our understanding of human sexuality? I think that the language not only fails to illuminate the issues at hand; it gives us intellectual cataracts.13
A brief review of basic genetic physiology demonstrates why: genetic function can be understood only in the context of that developmental system we call the cell. Most protein sequence information in a cell can be found in DNA located in the cell’s nucleus. The DNA itself is a large molecule composed of linked chemicals called bases.14 Genetic information is not continuous in the DNA molecule. A stretch that codes for part of a protein (called an exon) may be linked to a noncoding region (called an intron). Before a gene’s information can be used in protein construction, the cell must make an RNA cast for both the coding and noncoding regions of the DNA. Then enzymes snip out the introns and stitch the exons together into a linear sequence containing the template for a specific protein. Making the protein requires the coordinated activity of additional special types of RNA molecules and many different proteins.
In shorthand, we sometimes say that genes make proteins; but it is precisely such shorthand that gets us into trouble. Naked DNA cannot make a protein. It needs many other molecules—special RNAs to carry the amino acid to the ribosome and secure it, like a vise, so that other proteins can link it to its next neighbor. Proteins also help transport the DNA’s message out of the nucleus and into the cytoplasm, help the DNA unwind so that other molecules can interpret its message in the first place and cut and splice the RNA template. In short, DNA or genes don’t make gene products. Complex cells do. Put pure DNA in a test tube and it will sit there, inert, pretty much forever. Put DNA in a cell and it may do any number of things, depending in large part on the present and recent past histories of the cell in question.15 In other words, a gene’s actions, or lack thereof, depend on the microcosm in which it finds itself.16 New work, suggesting that as many as 8,000 genes can be expressed in a developmentally stimulated cell, shows just how complex that microcosm can be.17
Development, to paraphrase the philosopher Alfred North Whitehead, is a moving target. As an organism emerges from a single fertilized egg cell, it builds on what has gone before. By analogy, consider how a forest grows back in an empty, unmowed field. At first annuals, grasses, and woody shrubs appear, then a few years later scattered cedars, willows, hawthornes, and locusts. These trees need full sun to grow, so as they get larger, they create so much shade that their own seedlings cannot survive. But the white poplar does well under the conditions created by the cedar and its companions. Eventually, the poplar and other trees create a cool, leaf-covered forest floor on which the seedlings of hemlock, spruce, red maple, and oak thrive. Finally these create conditions for hemlock, beech, and sugar maple to grow. These new trees, in turn, create a microclimate under which their own seedlings thrive, and a stable constellation of trees, called a climax forest, finally develops. The regularity of such a succession of growth does not result from some ecological program found in the genes of cedar, hawthorne, and willow trees, ‘‘rather it arises via a historical cascade of complex stochastic [random processes that can be studied statistically] interactions between various’’ living organisms.18
The work of M. C. Escher offers a helpful analogy. In the early 1940s he produced a series of woodcuts designed to divide a plane into interlocking figures. Two features of these images help us see how developmental systems theory applies to cells and development (see figure 9.2). First, as one stares at the image, the birds jump into view, then the fish swim up. Both are always there, but how one focuses at a particular moment makes one animal more visible than the other. Second, each line simultaneously delineates the outline of both a fish and a bird. If Escher were to change the shape of the bird, the
figure 9.2 : Symmetry drawing E34B, by M. C. Escher. (© Cordon Art, reprinted with permission)
fish would change shape as well. Thus it is with a systems account of cellular physiology. Genes (or cells or organisms) and environment are like the fish and the bird. Change one change all. See one see all.