548 



MARY B. STARK 



frequency of their appearance. For the present, therefore, the 

 fact of the occurrence of the lethals in the inbred stocks is the 

 one important result of this examination. 



THE PRESENCE OF 'TWO LETHALS 



Number 4 of 13, 2, of table 6 yielded a ratio of 27.33 : 1. It 

 seemed probable in this case that two different lethal factors 

 were present, one in each chromosome. This might seem to 

 prevent all the sons from developing since each son must get 

 one or the other maternal sex chromosome ; but the survival of 

 the three males would be possible through the crossing over 



TABLE 12 fa) 

 Heterozygous Bar Female X Bar Male 



Total number of c?s, 1734; Crossovers = 144. 

 Crossover percentage = 144 H- 1734 or 8.3. 



