SEWALL The Unie~ersity
In textbooks of physiology, it is customary to treat the cell as the ultimate living unit. The properties of the cell are interpreted as resulting from the interactions of individually non-living substances when organized in the physical system characteristic of protoplasm. A different treatment isfound in genetics. The ultimate unit of life here is not the cell but the gene. The purpose of this paper is to review briefly certain aspects of the role of genes in cellular physiology. Its scope forbids any attempt at an exhaustive review of the pertinent literature (cf. 101, 256). The gene concept is based primarily The Gene as a Unit of Heredity. It is found that most of the on results frombreeding experiments. differences among individuals of a species can be interpreted successfully as due to combinations of differences in numerous separable components of a postulated hereditary material; and in the environments. A gene, from this standpoint, is one of a number of alternative conditions of a hereditary component, occurring in an individual only, as a rule, if in continuousunchanging existence along one of the ancestral lines; incapable of resolution by any known experiment into separately transmitted subentities, transmissible asexually to all offspring, but sexually only to a certain proportion (usually 50 per cent). The data directly imply the existence of numerous intracellular entities, each capable of synthesizing exact duplicates irrespective of the nature of the restof the cell or of the organism or of the external environment, and producible only by such synthesis. The differential effect of any pair of alternative genes (alleles) on the characteristics of individuals is constant among all individuals in which the other conditions (genetic and environmental) are the same, however variable, apparently, in intervening generations in which these otherconditions have not been controlled. The most important qualification of these principles is that a gene may occasionally undergo an abrupt change (mutation) to an alternative condition, which thereafter persists and multiplies as of the new sort. The likelihood of undergoing such a change may be increased by certain environ487
mental conditions (treatment with ionizingradiations, high temperature, etc.) but not apparently in a directed fashion. Mutation rate is also sometimes increased by particular associated genes (56, 21 1). Finally it may be noted that genes differ greatly in stability, the rate of mutation in most cases being apparently less than 10S5 per generation but with much higher rates in the relatively rare class known as unstable genes. The unitarycharacter of genes may be illustrated by an experiment (196) in which more than 300 successive back crosses of long winged flies of the species Drosophila melanogaster to a pure vestigial winged stock have not modified in any way the character long wing or its frequency (50 per cent) in each generation, although in the 300th generation the flies were only (+)300 long winged by ancestry. The autonomyof genes is especially strikingly illustrated in experiments in which genes from one species have been introduced into another by repeated back crossing to the latter (59, 266). The Gene as a Physical Entity. A great deal has been deduced about the nature of the system of genes without recourse to any other technique than that of breeding, but, while adequate, these deductions take on a greaterappearance of reality when it is found that they are exactly paralleled in all respects by the visible behavior of the thousands of individualized granules (chromomeres) distributed along the unbranched, threadlike chromosomes. A single set of the latter is typically present in germ cells, a double set in the somatic cells, but other numbers are found in just those cases in which the breeding...