314. GQNOTHERA AND THE MUTATION THEORY 
now appeared in about half the F;, flies, never in all 
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. Muller 
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 
‘Ci Ina 
Bd represents the beaded factor, Cru the factor limit- 
ing crossing-over, and lia 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 
