THE FACTOR HYPOTHESIS 165 



Morgan and his associates have demonstrated the 

 existence of over twenty different lethal factors in 

 Drosophila which when inherited from both parents 

 not only prevent the development of any unit charac- 

 ters but also doom the individual to death. Only 

 heterozygotes for such lethals, who receive the death 

 warrant from one parent alone, may escape and hand 

 on this fatal determiner. 



In plants lethal genes have been demonstrated by 

 Baur in snapdragons and by Lindstrom in maize. In 

 these instances the lethal factor is a lack of chlorophyll 

 which is not fatal if inherited from a single parent 

 because the deficiency is supplied by a gene for 

 chlorophyll from the other parent, but when the lack 

 comes from both parents it produces a seedling unable 

 to survive. 



Recently'G. H. Shull has demonstrated the existence 

 of two balanced recessive lethal factors in one pair of 

 the fourteen chromosomes in (Enothera, one pair pro- 

 ducing a lethal effect in the zygote, the other pair 

 destroying the gametes. This fact explains many of 

 the hitherto confusing ratios obtained in breeding this 

 classical plant. 



"Such lethal factors modify the expected Mendelian 

 ratios and greatly complicate the study of genetics, 

 but they do not destroy its fundamental principles, 

 indeed when properly understood they furnish one of 

 the strongest proofs of the truth of the factorial 

 theory of heredity" (Conklin). 



