328 GENETICS 



by their broken ends, but often in a different order from be- 

 fore. In the case of a single chromosome that has broken 

 in two places, thus yielding three pieces, the three pieces 

 often reunite in such a way that the middle piece is reversed 

 in position. That is, the order of the genes is the opposite 

 of that which prevailed before the break and reunion, the 

 opposite of the order shown on such gene maps as figure 38. 

 Thus, if the original order of the genes was a— b— c— d— e- 

 f— g— h— i— j— k— 1— m— n-o— p— q— r— s-t, after the break and re- 

 union the order may be a— b— c— d— e— n— m-1— k-j— i— h— g— f— 

 o-p-q-r— s— t, the genes between e and o being reversed 

 In their order. This reversal in the order of the genes can 

 be discovered by tests of breeding and linkage in later gen- 

 erations. A great number of cases of such inversions of the 

 middle part of the chromosome have been studied — the 

 order of the genes reversed in the middle part but not at 

 the two ends. This reversal is an extraordinary phenome- 

 non, requiring explanation. 



(3) In other cases the middle piece drops out completely 

 and is lost. The two end pieces then reunite at their broken 

 ends. Thus a short chromosome is produced, lacking all the 

 genes of the middle region, but still carrying those near the 

 two ends. These are known as deletions, the middle part 

 being deleted. A large number of such deletions are known. 



A third strange result is often produced when the chro- 

 mosomes are broken by radiation. If the usual two breaks 

 are In two different chromosomes of the same cell, then fre- 

 quently 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 other 

 two 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 chromo- 

 somes. Such exchanges, or translocations as they are called, 

 are known in many cases. 



