VARIATION DUE TO CHANGE IN THE 

 INDIVIDUAL GENE^ 



DR. II. J. MULLER 

 Dfpartmext of Zoology, Uxiversitv of Texas 

 I. The Relation between the Genes and the Char- 



ACTEES OF THE OeGANISM 



The present paper will be concerned rather with i)rol)- 

 lems, and the possible means of attacking thenu tliaii with 

 the details of cases and data. The opening up of tlicsc 

 new problems is dne to the fundamental coiit lihiit ion 

 wliich ire.ietics has made to cell ].liysiolo-y witliln tlu^ la>1 

 d,ra.l... Tills .•ontril.iitioii. wl,i.-li l,a^ far M-an-rlv 

 hrcii a.>iniihitt"(l by the ovniM-al i.liy>iolo-i>ts tluMiiM.Uvs, 

 consists ill the demonstration that, besides tli<- oi'dinary 

 proteins, carbohydrates, Upoids, and extractive-, of their 

 several types, there are present within the cell tlnnismids 

 of distinct substances — the "genes"; these mMie- exist 

 as ultramicroscopic particles; their inthience- neverthe- 

 less permeate the entire cell, and they play a fiindatnental 

 role in determining the nature of all cell suh>taii<-es, cell 

 structures, and cell activities. Tlirouuh tlie-e cell effects, 

 in turn, the genes affect the entire oi'uani>iii. 



It is not mere guesswoi'k to -ay that the unie- are 

 ultra-microscopic bodies. For the woik (.n Dinsnj,h 'il,i 

 has not only proved that the genes are in the ehroino 

 somes, in definite positions, but it lias shown that there 

 must be hundreds of such genes within each of tlie larger 

 chromosomes, although the length of these elironiosomes 

 is not over a few microns. If. then, we divide the size 

 of the chromosome by the niininunii nnuihcr of it> uenes 

 we find that the latter aiv partieh- too >mall to -dve a 

 visihl,. iniaue. 



The chemical <-o,npo.iti„n of the uvM,.-. an.l the for- 

 nnila" of their react!.. ns, remain a. vet (piite unknown. 

 We do know, for exan.].le. that in certain cases a given 



