CROSSINGOVER IN DROSOPHILA 149 



kept in the same place while laying. When this is done it is 

 found that the values are uniformly so close as to be within the 

 probable limits of error. When one compares different stocks 

 having the two original characters in different combinations, 

 however, the values often differ by a significant amount. In 

 certain cases this difference can be accounted for, and allowance 

 made. When certain crossover classes have a markedly high 

 viability, it is possible to balance such data by adding more 

 data in which the class in question is a non-crossover class (cf. 

 Balanced Inviability, Morgan and Bridges, '16). In cases where 

 the same cross is made with similar stock, the reason for a statisti- 

 cally significant difference is rather harder to understand. That 

 cases of this sort occur in other forms than Drosophila is prob- 

 able from the variations in linkage suggested by the results of 

 Baur in Antirhinum, Punnett in the sweet pea, and Tanaka in 

 the silk- worm moth (as pointed out by Sturtevant, '14). Two 

 such cases have been analyzed in Drosophila. Sturtevant (' 17) has 

 recently shown that three definite Mendelian factors occur which 

 influence the amount of crossingover between other factors, 

 while the work of Bridges with second and third broods has 

 made it plain that the amount of crossingover in the second 

 chromosome bears some relation to the age of the female parent. 



A significant difference in the amount of crossingover is often 

 shown, however, between cultures which are genetically alike, 

 and which have been laid by a female of the same age. A speci- 

 fic example may make this clear. The summaries given in 

 table 1 show the results of two control series. They were made 

 up in the same way, from the same stock cultures, kept in the 

 same place at room temperature, and counted after the same 

 interval, but one set was recorded about a month later than the 

 other, so that there was probably a slight variation in tempera- 

 ture — between 20°C. and 25°C. — and possibly some difference 

 in the food. The characters used were black body color, purple 

 eyes and curved wings. 



The X 2 value shows that the probability that the two results 

 are the same and due to random sampling is about fifteen in 

 ten thousand. Yet when No. 2, table 1, is compared with another 



