288 THE SECOND-CHROMOSOME GROUP 



Seven pairs of F 2 (or more likely F 3 ) wild-type flies gave offspring 

 (table 132). Of these pairs, two (2840, 2863) proved to have both 



parents of the original constitution ( - - J. Two others (2861, 2864) 



produced only small wild-type offspring, from which it was evident 

 that at least one parent of each had carried neither black nor curved, 

 and had come from the non-crossover chromosome which was alter- 

 native to the b 1 c chromosome. The remaining three pairs contained 

 various cross-over chromosomes. Thus, 2859 came from 



b + + b I c 



nocn e + I C b I C b + C b I C 



2860from -- #+ -- $ ,2862from 9+ cf. 



The mother of 2859 contained two cross-over chromosomes ( - - ) 



\+ I c / 



and must therefore have been an F 3 individual, as more were expected 

 to be. 



LOCUS OF LETHAL Ha. 



The composition of these flies is important, since from their off- 

 spring calculations of the amounts of crossing-over between the lethal 

 and both black and curved were made. The first culture in which the 

 lethal appeared (2675, table 131) was of the type most advantageous 

 for the study of the crossing-over relations because of being in the form 

 of a back-cross for black and curved. Unfortunately, no counts had 

 been taken on black, so that only the lethal curved value could be 

 calculated. Both the curved and the not-curved classes produced by 

 this cross are composite, the curved class being composed of two cross- 

 overs and a non-crossover class (2x+ri), while conversely the not- 

 curved class is 2n+x. The solution of the equations 



gives x =3.4 and n = 60.7. The fact that x has a minus sign is 

 easily accounted for by probable error, and only means that the loci 

 of the lethal and curved are close enough together so that a small 

 deviation of the classes gives an apparently impossible cross-over value. 

 In cultures 2840 (and 2863) the composition of the not-curved class 

 is 3n-j-2x and of the curved class simply x. The not-curved flies 

 totaled 327 and the curved 14, from which x = 14 and n = 99.7. The 

 lethal curved cross-over value is thus 12.3. Likewise the black lethal 

 cross-over value is 9.8. Culture 2859 furnishes data on the lethal 

 curved value according to the equations 



from which, the cross-over value is 11.4, comparable with the 12.3 just 

 found from 2840+2863. The condition in 2859 with respect to black 



