THE EFFECT OF GENES ON THE DEVELOPMENT OF SIZE AND FORM

Summary. 1. Genetics has concerned itself almost entirely with an analysis of the hereditary constitution of organisms in terms of genes. For a complete understanding of hereditary differences, however, it is important to determine how genes control the processes of development so that specific characters are produced. Traits of size and form lend themselves most readily to studies of this sort, and in a few instances direct evidence of changes in development brought about by specific genes has been obtained from both animals and plants. This evidence is briefly reviewed. 2. The influence of genes in determining size has been clearly demonstrated. Instances are described in which these genes produce their effects by a control of cell size and of cell number, and also, as is perhaps more generally the case, by determining in early stages a developmental schedule in which these two elements of growth are co‐ordinated in a specific fashion. 3. The size of parts seems often to be genically determined independently of body size, but in the few cases which have been critically analysed local size differences are found to be due to general effects upon growth occurring at such a stage of development that a particular part is more affected than the rest of the organism. 4. Characters of shape and form, either of the body as a whole or of its parts, have been shown in a number of cases to be determined by genes, which evidently operate through a control of the relative rates of growth in specific directions or dimensions. 5. The underlying mechanisms by which genic control of the development of size and form is brought about are as yet unknown. In a search for them, however, it is important to recognise two general concepts which developmental genetics has established: (1) Genic balance, or the dependence of a given character upon the interaction of all the genes rather than upon a single one, each gene producing its effect not alone but by virtue of its activity as a part of the whole genic complement. (2) Correlation of growth rates in development, by which the rates of specific processes, known in many cases to be genically controlled, are so interrelated in a definite schedule or pattern that development becomes a unified and orderly process. 6. The necessity is pointed out of obtaining a much broader basis of the facts of development in genetically analysed material before generalisations of importance can be made.