INERT CHROMOSOMES 331 



The application of the principle is, however, less simple than Heitz 

 has supposed. The distinction between " heterochromatin " and 

 " euchromatin " is not an absolute one. Lorbeer (1934) and Tinney 

 (1935) disagree as to whether the Y chromosome of Sphcerocarpus 

 is inert, and as we have seen, chromosomes in different tissues of the 

 same individual differ in their differential behaviour which is 

 physiologically controlled. Probably in such an intermediate 

 position is the X chromosome of Sphcerocarpus Donnellii, which 

 according to all observers is differential in its behaviour. Yet it 

 is not by any means inert. Under X-ray treatment it produces 

 mutations as frequently as all the autosomes together, and when 

 differential parts are broken off and lost the plant is changed in 

 different ways affecting both the sexual and vegetative characters 

 (Knapp, 1935 a and h). These observations do not destroy Heitz's 

 contention, but they show (what is equally clear from the cytological 

 study of differential behaviour) that the differential properties of 

 chromosomes, or even perhaps of separate chromomeres, cannot be 

 described in terms of a simple all-or-nothing reaction. 



Whether the distinction between active and inert genes is an 

 absolute one we do not know. Inertness may itself be physiologically 

 controlled, although we know of no example yet. But we know 

 well enough that every transition occurs between an active chromo- 

 some and an inactive one. The change presumably takes place 

 gradually by mutation to inertness of separate genes. If there is, 

 therefore, a sharp distinction in behaviour of active and inert genes 

 such a distinction will not always be found between chromosomes 

 or parts of chromosomes at different stages of change, and we shall 

 have such gradual variations of behaviour as make for the observed 

 complexity. 



Inert chromosomes are thus presumably made up of inert genes. 

 Since the existence of a gene in the first instance is inferred from 

 its action, " inert gene " may seem to be a contradiction in terms ; 

 nevertheless it represents a step forward ; it enables us to see what 

 we mean by a gene more clearly than before. Thus the active gene 

 generally found in higher organisms must have two properties, 

 (i) of reaction with its substrate, by which it creates a milieu per- 

 mitting reproduction ; and (ii) reproduction. The inert gene has 



