(ENOTHERA AND THE MUTATION THEORY 315 



CEnothera situation in otlier respects. If this stock 

 is crossed to wild type flies the I'l individuals are 

 beaded and wild-type in equal numbers, suggesting 

 at once the ''twin hybrids" (Iseta and velutina) of 

 certain (Enothera crosses. If one of the ''balanced" 

 chromosomes contains one or more recessive mutant 

 genes (and such have been purposely introduced into 

 it by appropriate crosses), crossing over between 

 any of the recessive genes and the lethal region may 

 cause the appearance of a small number of specimens 

 showing the corresponding recessive characters, and 

 these will be produced in small numbers generation 

 after generation, exactly as in the case of such 

 "mutants" of O. Lamarckiana as nanella or oblonga. 

 The frequency with which, these types will appear 

 will depend simply on the frequency with which 

 crossing over occurs — i.e., on the loci of the genes 

 in question. 



Similar cases of balanced lethals have since been 

 observed in other races of D, melanogaster, such as 

 truncate and certain races of dicha^te. In at least 

 one instance such a race has been artificially made 

 up in order to save labor in kee])ing stock of certain 

 lethal characters. There are strong indications that 

 a somewhat similar case of balanced lethals occurs 

 in stocks (Matthiola). 



This work of Muller's, then, furnished definite 

 information that ]oermanent heterozygotos due to 

 balanced lethals can and do exist, and fiuthermore 

 correlated this phenomenon with the repeated a])- 

 pearance of apparently new types in small numbers. 



