no Outline of Genetics 



as in the homozygous.) They are mostly "loss" mutations 

 and recessive to the previous condition. Only a very few 

 dominant or "gain" mutations have ever been reported. 



Baur (i), working with Antirrhinum, concludes that 

 changes of this sort take place more frequently in the 

 vegetative tissues than in connection with gametogenesis . 

 The earlier work on the fruit fly indicated that the 

 locus changes took place late in gametogenesis, since only 

 one individual of the new type would appear in a pro- 

 geny. Later investigation, however, has indicated that 

 the change may take place at almost any point in onto- 

 geny (Bridges 4, Muller 8). (There are also indica- 

 tions that changes of this sort may take place in purely 

 somatic tissue, although in such cases, of course, the modi- 

 fication cannot be perpetuated. See also chapter viii 

 on this matter.) 



Zeleny (12) states that there is no periodicity to 

 these mutations, thus refuting one of the early ideas of 

 De Vries. The same investigator demonstrates that 

 reverse mutations are more frequent than original muta- 

 tions. (This, however, is simply because they are in 

 the reverse direction, and not because of their recent 

 origin.) In the case of these reverse mutations, the 

 changes are always full jumps back to the original 

 starting-point, and never result in an intermediate con- 

 dition; nor will the selection of extreme types at all 

 modify the rate at which these reverse mutations occur. 



MuLLER and Altenburg (10), who have conducted 

 a critical examination of the fruit fly for mutations 

 occurring on the first and second chromosomes, state 

 that the vast majority of locus changes have a lethal 

 or semilethal effect when present in the homozygous 



