NATURE OF THE GENETIC EFFECTS 457 



specific matings. They allow deductions regarding the time and manner 

 of onset, in the embryo, of effects traceable to the genes introduced by 

 spermatozoa, and regarding the interaction of these with the stored 

 gene products derived from the chromosomes which had been present in 

 the egg and in the surrounding soma before fertilization. 



This account far from exhausts the varied ways in which the production 

 of genetic changes by radiation has already been used for throwing light 

 on questions of genetics and evolution. Thus, in the Drosophila work, 

 both the gene mutations and structural changes so obtained have vastly 

 increased the number and diversity of genetic tools available for the 

 attack on problems of the most diverse types. In many cases these tools 

 have been constructed to order. For, even though no control can be 

 exercised over the type of mutation that will occur in any given germ cell, 

 nevertheless mutations can be produced in such abundance that, with a 

 suitable genetic setup for the detection of given, desired types of mutants, 

 it is often practicable to carry through operations expressly designed for 

 finding changes of these particular types. The latter can then be recom- 

 bined, by crossing, into a variety of arrangements, useful for diverse 

 types of investigation. Similarly, in maize, Anderson and Randolph 

 (1945) have produced translocations by irradiation and thereby "tagged" 

 given chromosomes in order to follow and control the distribution of 

 genes useful in practical breeding and in investigation. In silkworms, 

 Tazima (in press) has by inducing deletions elucidated sex determination. 



Many important stocks of Drosophila contain recessive lethal, near- 

 lethal or sterilizing genes or gene combinations which it is desired to 

 preserve. Since these cannot be bred as homozygotes, continual selec- 

 tion of the appropriate individuals would be recjuired, unless there was a 

 genetic "balancing" arrangement present which resulted in the death or 

 sterilization of individuals not carrying the desired gene or genes as well 

 as of those homozygous for them. This balancing is accomplished by 

 the introduction of one or more lethals or steriles into the chromosome 

 homologous to that carrying the desired gene or genes, and it is usually 

 necessary also to have one or more inversions present, heterozygously, 

 which will effectively prevent crossover individuals, free from the lethals 

 or steriles, from being produced. Varied "balancing chromosomes," 

 equipped with the required lethals or steriles and also with suitable 

 inversions, have been provided by irradiation, some of them in work 

 designed for obtaining them. Thus the maintenance of Drosophila 

 stocks of most of the desired types has been rendered automatic, in the 

 sense that no artificial selection is required, and the number of stocks 

 which it is feasible to keep has thereby been greatly increased. 



In addition to their use in the mere maintenance of stocks, such "bal- 

 ancing chromosomes" of Drosophila, most of which owe their origin in 

 part at least to radiation, are increasingly being put together in combina- 



