114 



GENETICS IN RELATION TO AGRICULTURE 



mated to black curved males, according to the same investigators, 

 gave the results tabulated in the last two columns in Table XXIII. Here 

 the percentage of crossing-over amounted to 22.74 per cent., a value 

 substantially in agreement with the results of the reverse factor tests. 



TABLE XXIII. CROSSING-OVER BETWEEN B AND C, IN DROSOPHILA 



Black curved d" (be,) (be,) mated to 



No Crossing-over in the Male. The above results show clearly that 

 crossing-over in the female between the loci B and C v of the second chro- 

 mosome results in the production of approximately 23 per cent, of cross- 

 over gametes irrespective of the particular combination of the factors 

 concerned. Sturtevant and Bridges, however, have shown that there is 

 no crossing-over in the male so that males of the genetic constitution 

 (#C S ) (6c r ) produce only two types of sperm in the ratio l(BC v ) :l(6c c ) 

 and males of the genetic constitution (Bc v }(bC v } produce sperms in the 

 ratio, l(fic,):l(6C,). 



It is not known just what the absence of crossing-over in the male 

 depends upon. In the case of factors in the X-chromosome or first 

 group, crossing-over would involve exchange of chromatin material be- 

 tween the X- and ^-chromosomes. Since these differ strikingly it is 

 not surprising that interchange of chromatin does not take place between 

 these chromosomes for it is difficult to see how the difference could be 

 preserved if crossing-over should occur. But the other chromosomes are 

 alike in both sexes, nevertheless no matter how high the percentage of 

 crossing-over in the female, none whatever has been observed in the male. 

 This has been found true for factors lying in the third group as well as 

 for those lying in the second group, and it is without doubt a general 

 phenomenon. 



The knowledge that no crossing-over occurs in the male has often 

 been turned to advantage in experimental work. When gray curved 

 flies, (Bc,)(BcJ are mated to black normal, (6C.)(6C,), the FI flies are 



