The problem of the nature of the materials which in the reproductive cells constitute the basis for the transmission of hereditary characters is a central question in Biology. In pre-mendelian times this was a theme for the general biologist rather inclined to speculate on fundamental points. Indeed, in those times such speculations were almost deprived of a factual basis, which was obtained only after the rise of Genetics and Cytology, in the present century. Nevertheless, the two fundamentalthe particulate or atomic and the architectural or structuralwhich may yet be recognised in actual discussions on the nature of the hereditary materials, were already present in those times.
Modernly the particulate hypotheses have been followed by a greater number of workers than the architectural ones. It must be said, however, that most workers seem to be unaware of the conceptual bases upon which they build their generalizations; it seems that many a student of experimental Genetics employs little of his time and thinking in trying to understand what are the "genes" which form the basis of his science. Some seem to take as granted that the lesser units which some day will be seen with the aid of the best "optics" of the most refined observational means, instance with the electron or proton microscope, must necessarily be the "genes". Following this line of empirical reasoning, the atoms themselves could finally be found to be the genes.
On the other hand, many workers of the more physiological chapters of Genetics are rather inclined, also, to admit implicitly that the physiological differences they analyse correspond to unitary material components of a corpuscular nature. In fact, for the great majority of the geneticists, and with them for the biologists in general, the gene is a particle localized at a certain point of the chromosome and which, it is believed, some day will be directly observable, when a suitable resolving power, or a suitable material, will become available.
Only a few workers in the field of Genetics have recently advocated the architectural theories of the gene, and among them an outstanding position is occupied by Goldschmidt (1938-1946) who, at least until recently, favoured an extreme view in which the chromosome, not the gene, is the unit. We have proposed an hypothesis (Serra, 1944, 1946) which was almost intermediary between the two opposed directions of development of the gene theory. The gene was postulated as having a composite structure whose modifications of architecture should correspond to gene mutations, while position effects were explained as cases of more or less marked interference of peripheric nucleoproteins with the action of shorter or longer segments of the chromosomes.
Meanwhile, other data have been accumulating which allow that a further step towards a comprehensive theory of the gene, gene mutation and position effect be attempted. This is especially true of the chemo-cytological data on the chromosomes and the chemo-genetical and physico-genetical data on the mutational effects of chemicals as compared to those of penetrating radiations and particles. The theory we present here, which is a natural development of the hypotheses put forward in our former work, may be designated as the cyto-physiological theory of the gene and gene mutation. In presenting it, we have no doubt that our theory is no more than a mere guess at the true nature of the fundamental units of the hereditary materials; it will be satisfactory enough if it is in the right direction. Speculations on the nature of the genes not only have a value in themselves, as basic ideas for the entire field of Genetics, as also may possess an heuristic value for what may perhaps be the most relevant field of Genetics in a not too distant future: the experimental control of the direction of provoked mutations, and therefore of the evolution of living species. For this reason, basic speculations must be encouraged in Biology, though their value is, in the actual little advanced state of biological knowledge, much less important than, for instance, speculations in the field of theoretical physics are for the understanding and guidance of experimental atomic science.
A confront of the apparently contradictory opinions of outstanding geneticists in what concerns the nature of the gene plainly demonstrates how little advanced Biology is, as yet, in this domain. However, the importance of the subject justifies all attempts to deal objectively with it. An attempt will be made in the following to separate clearly fact and interpretation, immediate data and underlying conceptual explanations, as well as the logical implications of the established relations between isolated interpretations and incorporated doctrines or theories. Special attention will be paid to the explanation of the phenomenon of position effect, as its interpretation is of paramount importance for the gene concept.