No. 594] THE MECHANISM OF CROSSING-OVER 



ready had been made up, as had j c v ba. These two stocks 

 were crossed together, and the F x was back-crossed to 

 da bi. In the F 2 generation all the dachs that are not black 

 are cross-overs in the region between these two factors, 

 and so must contain, in the same chromosome with dachs, 

 instead of black, the factors j c v and b a , for black is in such 

 a position in the chromosome that a cross-over between 

 da and b x must nearly always be a cross-over between d a 

 and j Cv ba. 



In the case of the second chromosome the individuals 

 tested for linkage contained, in one chromosome, the fac- 

 tors streaked thorax, black body color, purple eyes, ves- 

 tigial wings, arc wings, and specked thorax, and in the 

 other chromosome, the factors dachs, jaunty, curved and 

 balloon. The order of these factors is as follows: 

 Str d a bi p u v g Cv a r Sp b a . The way in which they were com- 

 bined is as follows : 



As da j Cv ba is very inviable it is kept in heterozygous 

 condition by back-crossing, in each generation, normal 



appearing males of the composition t )i p ttVg a r8 * ( called % 

 for short) to b,puV g a r s P females (called 5). Since there 

 is no crossing-over in the male, all the offspring are either 

 apparently normal, % or the homozygous quintuple reces- 

 sive, "5." The same process can then be repeated in 

 every generation, by crossing the normal-appearing sons 

 to their recessive sisters. It is evident that the "5" fe- 

 males which are used need not be virgin, as they could 

 have been fertilized only by % or by 5 males. When % 

 males are crossed by 5 females which have been made up 

 so as to contain in addition the dominant factor streak 

 (these 5 females need only be heterozygous for streak; 

 homozygous streaks are hard to handle), the daughters 



'From Bridges. 



