THE GENr. AS A UNIT OT CHANGE 



All these residual or minimal units of genetic structure are 

 therefore conditional and there is no reason why they should be 

 proved to correspond in regard to action and change in different 

 experiments. Unless, that is, the unit of action is so built up that 

 any change of its structure whatever has the same effect; then, of 

 course, all its mutations would be alike and no recombination 

 would be detectable within it. 



The genetical unit into which we divide the chromosome, there- 

 fore, depends on our experimental technique or our theoretical 

 purpose. The cytological unit must also depend on the wave-length 

 of light used in the resolution of chromomeres. In ultra-violet photo- 

 graphs of polytene bands in Drosophila we must be near to an 

 ultimate analysis, for Muller and Prokofieva have found them to 

 agree with the limits of X-ray breakage. This correspondence, 

 however, is not so important as the general principle that the 

 chromosome consists of a linear arrangement of particles which are 

 frequently, or even usually, different from their neighbours in 

 structure, in action, and in capacity for change. But though different, 

 they are not wholly independent, for they have to move and to 

 work together. The question of how they move and work together 

 is on the border-line of chemistry and to answer it we shall have to 

 get down to the chemical level of approach. 



Meanwhile we can admit that the visible chromomeres of the 

 cytologist, and the genes as units of crossing-over and mutation of 

 the geneticist, agree to a first approximation. They are the ultimate 

 units of structure, change, and action in the chromosome and in 

 heredity. 



REFERENCES 



BATTAGLiA, E. 1947- La "Seniigamia," singolare comportamcnto del nucleo 

 spermatico nelle uova diploidi delle specie apomittiche del genere Rudheckia 

 (Asteraceae) e consequente embriogenesi di tipo chimerico. Rendiconti Acad. 

 Naz. Lined, Ser. 8, 2: 63-67. 



BRIDGES, c. B. 1936. The bar "gene" a duplication. Science, 83: 210-211. 



DARLINGTON, c. D. 1940. The origin of iso-chromosomes. J. Genet., 39: 351-361. 



DARLINGTON, c. D., and ROLLER, P. C. 1947. The chemical breakage of chromosomes. 

 Heredity, i: 187-222. 



DARLINGTON, c. D., and LA COUR, L. F. 1945. Chromosome breakage and the 

 nucleic acid cycle./. Genet., 46: 180-267. 



TT9 



