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7 Interacting Dimensions—Genes and Epigenetic Systems In this chapter we deal with the interplay of the genetic and epigenetic systems, leaving the other interactions for chapter 8. This means that we shall be returning to some slightly tricky genetics and cell biology, which nonspecialists may find rather tough going. We will do our best to make it palatable, but in parts it may be necessary for nonspecialists to do what the distinguished English zoologist Sir Solly Zuckerman said he did when he encountered a complicated mathematical equation—hum through it.1 The details are not too important, although the general message is. In particular, we hope that readers will get the gist of genetic assimilation, because we believe this concept to be extremely important. It will crop up again in the next chapter, when we look at interactions between genes, behavior, and symbolic communication. Earlier in this book we used a music analogy to highlight the differences between genetic and nongenetic inheritance, and it may be helpful to use it again to illustrate what interactions between the genetic and epigenetic systems may mean. We suggested that the transmission of information through the genetic system is analogous to the transmission of music through a written score, whereas transmitting information through nongenetic systems, which transmit phenotypes, is analogous to recording and broadcasting, through which particular interpretations of the score are reproduced. A piece of music can evolve through changes being introduced into the score, but also independently through the various interpretations that are transmitted through the recording and broadcasting systems. What we are interested in now is how the two ways of transmitting music interact . Biologists take it for granted that changes made in genes will affect future generations, just as changes introduced into a score will affect future performances of the music. Rather less attention is given to the alternative possibility, which is that epigenetic variants may affect the generation and selection of genetic variation. 242 Chapter 7 A recorded and broadcast interpretation of a piece of music could affect the copying and future fate of the score in two different ways. First, a recorded interpretation could directly bias the copying errors that are made. For example, a copyist might be so influenced by hearing a particular record over and over again that she makes a mistake that reflects this version of the music. The popular interpretation has an extra trill, so she unthinkingly adds it to the score. A second, more indirect effect would occur if a new and popular interpretation affects which versions of a score are copied and used as the basis for a new generation of interpretations. Think of something like traditional folk music, where there is no “master score.” Similar, yet nonidentical versions of the music are played and recorded by various bands, each using its own score, its own instruments, and its own interpretation. If a new recorded interpretation becomes very popular, and is played over and over again, it is likely that versions of the score that resemble it will be used, recorded, and copied, and thus become more common. After a long period of such cultural evolution, it will eventually seem as if the beautiful fit between the score and what is heard could never have been otherwise— that the music flows seamlessly from the now dominant version of the score. In this case the recorded interpretation of the music has affected the selection of the version of the score, while in the first case that we described the recorded interpretation biased the generation of variations in the score. Epigenetic systems could have either or both types of effect on the genetic system: they could directly bias the generation of variations in DNA, or they could affect the selection of variants, or they could do both. We will start by looking at the first possibility—that the epigenetic systems directly bias the production of genetic variation. We will then use the rest of the chapter to explore the ways in which epigenetic variations construct the cellular and physiological niche in which genes are selected. The Effects of Epigenetic Systems on the Generation of Genetic Variation Before looking at the interplay of the genetic and epigenetic systems, we need to briefly recapitulate some of the points about genes and their activity that we made in earlier chapters. The most important is that DNA molecules do not sit naked in the cell. They are associated with many different proteins and RNA molecules, which together...


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