Evidently some of the "higher fields," postulated above, may be systems like those discovered by McClintock in maize. Furthermore, there is a connexion worth considering. We noted the enormous increase between lower and higher organisms in DNA per unit map. Let us keep in mind that this increase might reflect that of "controlling elements." In other words, the number of genes, and therefore the number of different proteins, need not be several orders of magnitude greater in a mammal than in a bacterium, but the number of "controlling elements" may be. Evolution may have taken place by finer adjustments of reactions rather than by an increase in the number of different molecular species.If there is anything in this idea, the difference in the amount of DNA per nucleus between, e.g., Urodela and Anura, or between Dipnoi and other fishes, could represent an orthogenetic trend which has led almost to the extinction of the Urodela and the Dipnoi. Too much of "controlling elements" may be at least as embarrassing as too big a size or too long a tooth or too branched an antler.A discussion on the relations between arrangement and activity of the chromosomal material should, of course, include a consideration of heterochromatin. The trouble here is that the study of heterochromatin is at a prescientific level (e.g., Pontecorvo, 1944). We have no alternative but to ignore it.
CONCLUSIONS
For the time being, we must restrict a working model of the genetic material to the level for which there begins to be some solid ground, i.e., the intra-genie level. Summarising again:
1. The chromosome is subdivided in sections within which recessive mutants are noncomplementary to one another. These sections of allelism--the genes of classical genetics--are called cistrons ( Benzer, 1957).
2. A proportion of the mutants of a cistron represent changes at individual positions which can be recombined by crossing over and which are in a linear arrangement; the positions within a cistron separable by crossing over are called (mutational) sites ( Pontecorvo,

-67-

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Trends in Genetic Analysis
Table of contents

Table of contents

  • Title Page iii
  • Contents vii
  • Tables ix
  • Contents x
  • Introduction 1
  • Chapter I - Genetic Analysis and Its Resolving Power 8
  • Chapter II - Allelism 28
  • Chapter III - Structure and Function of the Genetic Material 54
  • Conclusions 67
  • Chapter IV - Recombination 72
  • Chapter V - Mapping Chromosomes Via Mitotic Recombination 101
  • Conclusions 112
  • Chapter VI - Novel Genetic Systems 114
  • Conclusions 128
  • Works Cited 135
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