330 GENETICS 



leaving the loop free. In that way are produced the "dele- 

 tions," short chromosomes lacking all the middle part. In 

 such cases the free loop degenerates or is lost. 



In other cases the two end pieces may reunite with the 

 ends of the central loop. Sometimes the union may be with 

 the same ends as before; then there might be no evidence 

 that any break had occurred. But in other cases, since the 

 four ends are all close together, each end piece may unite 

 with the other end of the loop from that with which it was 

 before united. This produces the inversions (figure 67, C). 

 The chromosome now has its two end parts as before, but 

 in its middle part the order of the genes is reversed. 



If the accidental crossing or contact is between two dif- 

 ferent chromosomes (figure 67, D), then they adhere at the 

 point of crossing, and both may break there. Then the ends, 

 in becoming reunited, may in some of the cases readily ex- 

 change partners. Thus are produced the translocations: two 

 chromosomes that have exchanged parts (figure 67, E). 



Thus all these remarkable phenomena find a simple ex- 

 planation through consideration of the conditions that exist 

 within the cells. No other explanation than one along the 

 lines suggested seems possible. It appears clear therefore 

 that the breaks and other chromosome irregularities result- 

 ing from radiation are not due to the striking of an electron 

 against the chromosome at the point of breakage, but to the 

 sticking together of the chromosomes. The radiation seem- 

 ingly makes some general physical or physiological change 

 in the cell contents, resulting in the ready adhesion and 

 breaking of the chromosomes.^ 



We now come to the genetic results of these changes, the 

 alterations in inheritance that they bring about. There are 

 a number of different types: 



When a chromosome is broken into two pieces, genes that 

 were formerly present in a single chromosome, and were 

 therefore linked in inheritance, have become separated into 



