154 Vniiersity of California Fuhliratioiis in Ac/ricuUural Sciences [\'ol. 4 



In the Drosophila case just mentioned, the "principal" factor for 

 the character in question is "dominant for its visible effect and 

 recessive for a lethal effect," so that no pure beaded individuals 

 appear among the progeny of beaded. The original race regularly 

 gave progen}^ partly heterozygous beaded and partly homozygous 

 normal, while after a long period of selection a true-breeding beaded 

 race appeared. This latter form, it proved, fails to give normals not 

 because of being duplex for beaded — it is still simplex — but because 

 of its possession of another factor, known only l)y its lethal effect 

 when homozygous, which is carried by the chromosome bearing the 

 normal allelomorph of the factor for beaded. The locus of this reces- 

 sive lethal factor gives in general about 10 per cent of crossovers with 

 the locus of beaded, but in tlris case, because of the presence of a factor 

 "which almost entirely prevents crossing over" between the loci of 

 the two lethal factors, viable non-beaded zygotes are very rarely 

 produced. Thus every zygote receiving either two beaded-carrying 

 chromosomes or two non-beaded-carrying chromosomes of the pair 

 concerned fails to develop, and all the insects produced are necessarily 

 heterozygous for both lethal factors. 



A point of special interest in this case is the fact that by certain 

 crosses individuals can be produced which give certain types among 

 their progeny in yery small percentages. Muller sviggests that part 

 at least of the supposed mutants of Oenothera may be due to crossing 

 over between chromosomes carrying lethal factors, by which certain 

 recessive factors are permitted to come to expression in viable zygotes. 



For the inheritance of doubleness of flowers in Matthiola he gives 

 a "balanced-factor" explanation essentially identical with mine (Frost, 

 1915). 



There seems to be little reason to doubt that the differential factors 

 for these aberrant Matthiola tj'pes have originated by mutation. On 

 the analogy of Drosophila we might expect that the true mutations 

 would be relatively rare, and that most of the apparent mutants, in 

 cases where they appear frequently, would be due to segregation, 

 appearing as the result of crossing over in chromo.somes carrying 

 balanced lethal factors. The evidence seems to indicate, however, that 

 the differential factors for the mutant types at all extensively studied 

 are dominant for their visible effects and usually (probably imper- 

 fectly) recessive for a lethal effect, the mutant factors thus being 

 genetically similar to the factor for beaded wings in Drosophila. 

 This would seem to imply the occurrence of certain mutations in pro- 



