CONTINUOUS VARIATION 



extreme cases suppress, recombination, so that the chromosomes 

 tend to se2;rec;.ite as wholes at mciosis. 



Two wild-type strains o{ Drosophila were used in the experiment. 

 These showed no differences from one another which could be 

 followed by the mcndclian method, but they did differ from one 

 another in their average numbers of abdominal hairs. The one, 

 which we may denote as S, showed an average of 59*2 hairs on 

 females and the other O, only 43 • 5 hairs per female. Tiiese strains 

 were each crossed to a third carrying the marker genes, B, Pm and Sh. 

 The Fl females, which showed the effects of jB, Pm and Sh in their 

 phenotypes, were crossed back to males from their respective wild- 

 type parental strains. Eight classes could be distinguished by the 

 simultaneous segregation of 5, Pm and Sh in the backcross progenies, 

 and the hairs were counted on a number of female flics in each of 

 these classes. The averase hair numbers are shown in Table 6. 



TABLE 6 



A. AVERAGE NUMBERS OF HAIRS ON FLIES OF EIGHT 

 CLASSES DISTINGUISHED BY THE ANCESTRY OF 

 THEIR CHROMOSOMES 



B. THE DIFFERENCES IN AVERAGE HAIR NUMBER 

 ATTRIBUTABLE TO THE THREE LARGE CHROMO- 

 SOMES 



Now the B Pm Sh class has one representative of each of the three 



74 



