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The idea that DNA alone is responsible for all the hereditary differences between individuals is now so firmly fixed in people’s minds that it is difficult to get rid of it. When it is suggested that information transmitted through nongenetic inheritance systems is of real importance for understanding heredity and evolution, two problems arise. The first is that for most people the genetic system seems quite sufficient to explain everything. They invoke Occam’s razor: if one system can explain everything, why do we need to look for others? The second problem is that even when people agree that there is no escape from the mass of experimental data showing clearly that there are other, nongenetic, inheritance systems, they find it difficult to know how to think about them and their significance in evolution . We are all deeply conditioned by what we know about the genetic system, and tend to attribute its properties to other types of inheritance and to evaluate them in terms defined by the genetic system. We should not do this, of course, but it is difficult to change our habits of thought. Some years ago, after several very frustrating and largely unsuccessful attempts to get our point of view across to colleagues and students, we found an analogy for how different heredity systems could work alongside the genetic system. Since it seemed to satisfy many people, we will repeat it here. Think about a piece of music that is represented by a system of notes written on paper, a score. The score is copied repeatedly as it is passed on from one generation to the next. Very rarely, uncorrected mistakes occur during copying, and perhaps an impertinent copier sometimes makes a tiny deliberate alteration, but with the exception of such small and rare changes, the piece of music is transmitted faithfully from generation to generation in the form of the written score. The relationship between the score and the music is analogous to the genotype/phenotype distinction. Only the genotype (the score) is transmitted from one generation to the next; the phenotype (the particular performance, the actual interpretation II Three More Dimensions 108 Part II of the piece) is not. Changes in the genotype (mutations) are passed on; changes in the phenotype (acquired characters) are not. This was the situation until new ways of transmitting music were invented. The technologies of recording and broadcasting made it possible to transmit performances by recording them, editing them, copying them onto tapes or disks, and broadcasting them. Now, through these new technologies , the actual interpretations of the music can be transmitted as well as the written musical score. In terms of the genotype/phenotype analogy, the recording and broadcasting systems transmit the “phenotypes” of the pieces, rather than the “genotypic” instructions in the score. A phenotype, one particular performance, is affected by the notes in the written score, the skill of the musicians, the nature of the musical instruments, the general musical culture, and so on. Importantly, it is also affected by the interpretations of the score that the conductor and musicians have heard in the past—by earlier phenotypes. The relationship between the two systems of transmission is usually unidirectional—a change in the score alters performances , whereas the performance of the music usually does not change the score. However, occasionally a performance may alter the score: a particularly popular interpretation of the music may lead to a version of the score that includes notational changes that make it easier for the interpretation to be reconstructed. In this case, a phenotype affects a genotype. In all cases, by opening up a new channel of information transmission, the new technologies can affect the way the music is played. The recording-broadcasting transmission system is based on a completely different technology from that of copying of the score, and likewise the heredity systems that we are going to discuss in the next three chapters are completely different from the DNA system. They do not come instead of the DNA transmission system (the written score); they are additional to it. The genetic system is the basis of all biological organization, including the organization of the supragenetic heredity systems we are going to consider, but these additional systems allow variations in a different type of information to be transmitted. The variations occur at higher levels of organization —at the cell, organism, or group level. They may be quite independent of variations at the genetic level, in just the same way that...


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MARC Record
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