2O6 



C. B. BRIDGES AND A. H. STURTEVANT. 



aB. In case of linkage, however, the two new combinations 

 (here Ab and aB} are not represented in as large numbers in the 

 gametes as are the original combinations (AB and ab). The 

 actual ratio in which these gametes are formed can be calculated 

 only very indirectly from an F 2 zygotic ratio, and if the two FI 

 individuals are forming the gametes in different ratios, the 

 calculation may be impossible. 



As suggested by Baur (Verh. naturf. Ver. Briinn, 'n) what is 

 needed is to test these double heterozygotes by an individual 

 all of whose gametes are of one kind, namely, recessive for both 

 factors in question (aa, bb) since in this case they will not mask 

 any combination in the gametes tested. Baur's method, then, 

 is to test double heterozygotes by double recessives. The pro- 

 portion in which the zygotes appear is a direct measure of the 

 gametic proportions. In such zygotic proportions there are two 

 equal classes representing original combinations (in the example 

 AB and ab) and two other equal classes representing recombina- 

 tions or crossovers. In calculating the percentage of crossing 

 over therefore, we add these cross over classes together and 

 divide by the total number of zygotes. Since in the case of sex- 

 linked characters the male producing sperm is analogous to a 

 double recessive, the F 2 males will in effect always be such a 

 back cross test. 



Flies with curved wings from pure stocks were mated to stock 

 black flies. The FI flies were gray not-curved, that is, like the 

 wild fly in appearance. F 2 consisted of: 



Wild Type. 

 391 

 458 



Total 849 



Black. 

 IQ4 

 226 



420 



Curved. 

 168 

 165 



333 



Black Curved. 

 O 



o 



The absence of black curved in F 2 is the result to be expected 

 if there is no crossing over in the male, no matter what the 

 percentage of crossing over may be in the female. However, 

 if there is crossing over in the female a few of the F 2 blacks should 

 be heterozygous for curved and a few of the curved heterozygous 

 for black. The most advantageous procedure to get the double 

 recessive from these F 2 flies is to mate in mass cultures the blacks 



