THE ACTIVATION OF GENES BY THE CYTOPLASM 363 



greater or lesser extent, in all tissues, though with different intensities in 

 different parts. 



A similar conclusion is suggested by the fact that careful metrical study- 

 often reveals an activity of a gene in a tissue v^hich it had previously been 

 thought to leave uninfluenced. Thus Dobzhansky and Holz (1943) in- 

 duced a number of mutations, affecting the eye colour, bristles or other 

 external characters, in long inbred strains of Drosophila melanogaster. 

 Each mutant strain thus differed from the race from which it was derived 

 only by the actual mutated gene. By comparing two corresponding races 

 it could be shown that nearly every mutant produced alterations in an 

 apparently quite unrelated character (the shape of the spermatheca) as well 

 as in the eye colour, etc. by which it had originally been detected. It is 

 difficult to suppose that this was merely fortuitous and it seems much 

 more probable that the evidence can be accepted as indicating that all, or 

 nearly all, genes are active in every tissue (Fig. 16.8). 



Another piece of evidence tending in the same direction is given by 

 some work of Demerec (1934, 1936) and depends on the phenomenon of 

 somatic crossing over. If a Drosophila is heterozygous for two linked 

 genes, such as singed (bristles) and yellow (body colour), during develop- 

 ment a process of crossing over will, in a few cells, take place at a mitotic 

 division, so that one of the daughter-cells becomes homozygous for 

 singed and the other for yellow. These cells will each give rise to a small 

 patch of tissue, and if this forms part of the body surface, one will see 

 small twin spots, a yellow-coloured one and one with singed hairs. 

 Demerec bred flies which were heterozygous not only for yellow and 

 singed but also for one of a number of small deficiencies located in the 

 same chromosome. He found that in most, but not quite all, cases the 

 somatic crossing over now gave rise only to a single spot, either a yellow 

 one or a singed one, the other partner spot being missing. This was inter- 

 preted to mean that the daughter-cell which had become homozygous 

 for the deficiency (and therefore lacked entirely the genes involved in it) 

 were not able to survive: the deficiency in fact was operating as a 'cell- 

 lethal'. If this is true, it means that all the genes involved in the cell-lethal 

 deficiencies are not only normally operating in the hypodermis cells, but 

 are active in such an important way that in their absence the cell dies. 

 Since the deficiencies were selected at random, and were not known 

 previously to contain genes active in the hypodermis, this is rather good 

 evidence that all genes are active in these cells, most in fact being essential 

 to life. And presumably the same conclusion must apply to all the tissues 

 in the body, although the relative importance of the various genes could 

 not be expected to be always the same. 



