CALVIN B. BKIDGES 



The most efficient experiment by which to determine the 

 amount of crossing-over between gens is the back-cross. Here 

 a multiple heterozygote is tested by mating to the corresponding 

 multiple recessive. When this is done, in the next generation 

 produced, there are two contrary classes representing the original 

 or Pi combinations, and two other contrary classes of cross- 

 overs. The percentage of crossing-over is given directly by 

 division of the sum of the crossover classes by the sum of all 

 the classes. It was in this way that the percentage of cross- 

 overs was calculated for the following tables. 



TABLE 1 



Pi purple vestigial X wild. B. C. Fi wild type 9 X purple vestigial c? cf 



