GENE MUTATIONS AND EVOLUTION 71 



die part being deleted. A large number of such deletions are 

 known. The process by which they are produced demands 

 explanation. 



A third strange result is often produced when the chromo- 

 somes are broken by radiation. If the usual two breaks are in 

 two different chromosomes of the same cell, then frequently 

 these two chromosomes exchange parts. The broken end of 

 one piece from one chromosome unites with the broken end 

 of one of the pieces from the other. The two other pieces of 

 the two chromosomes similarly unite. Thus we get two newly 

 combined chromosomes, each made up of two halves that 

 formerly belonged to two separate chromosomes (Figure 11, 

 D and E). Such exchanges, or translocations as they are 

 called, are known in many cases. Their formation calls 

 loudly for explanation. 



Thus several questions demand answer. First, how does it 

 happen that in a cell, two chromosome breaks occur instead 

 of one ? Second, how do the broken ends find each other and 

 unite, often resulting in a new combination ? And third, how 

 does it happen that in such reunited chromosomes the order 

 of the genes in the middle piece is commonly reversed } 



There is a single simple and convincing explanation for all 

 these strange relations; and it is one that throws a sharp light 

 on the way gene mutations are produced. This explanation, 

 originally due to Serebrovsky,*' is as follows: 



The numerous long thread-like chromosomes in the cells 

 undergoing radiation lie close together and are more or less 

 curved, coiled and tangled. They may cross each other and 

 come in contact, but under ordinary conditions when this 

 happens they do not stick together. 



But under the action of radiation or high temperatures, the 

 chromosomes change physically, in such a way that when 

 they come in contact by crossing at a certain point they stick 



