314 CENOTHERA AND THE MUTATION THEORY 



now appeared in about half the Fi flies, never in aU 

 of them. Dexter had shown that the character was 

 influenced by environmental and genetic modifying 

 factors, and that the principal gene concerned was 

 probably in the third chromosome. The fact that 

 the stock had at first not bred true and then later 

 had come to do so remained unexplained. Mufler 

 showed that the beaded factor itself is dominant, 

 and that it is lethal when homozygous — i.e., that 

 the homozygous beaded individual dies. The stock 

 had at first failed to breed true because all the 

 beaded individuals were heterozygous for the normal 

 allelomorph of beaded. But Muller also found that 

 in the final stock there was a new lethal factor in the 

 chromosome containing this normal allelomorph. 

 The event that caused the stock to begin to breed 

 true was evidently the mutation that produced this 

 lethal. Crossing-over between the beaded factor 

 and this lethal would ordinarily occur in a certain 

 number of cases, but it so happened that in the non- 

 beaded chromosome there was already present a 

 factor that practically prevents crossing over in this 

 region. The constitution of the individuals in the 



beaded stock then is as follows: ^ ^^^^^ , where 



Bd represents the beaded factor. Cm the factor limit- 

 ing crossing-over, and liiia the lethal factor. The 

 normal allelomorphs of these genes, present in the 

 opposite chromosomes, have not been represented 

 here. Such a ''balanced lethal" stock not only 

 breeds true though heterozygous, but parallels the 



