IN THE SECOND CHROMOSOME. 319 



fr Pr C nr , ft _- . C H i c m r 



7 - , from 677 was mated to a cross-over male, T - 

 o p r c ' o p r c 



from 678. The resulting wild-type offspring were 7 - ^ - A 



Pr WJr 



female of this constitution, in culture 713, gave 4.3 per cent crossing-over 



between pr and c, none between 6 and pr or between c and mr. The 



Q 



same general method was followed in making up seven other - 7; 



W/r 



females. The same C//j c chromosome was present in all these 

 females, but Cn r from different sources was used. The results from 

 these females are given in tables 15 and 16 and the fifth line of figure 1. 

 These data agree with those obtained from Cm C IIr , except 

 that p r c shows a slight rise (from 1.1 to 2.9). Owing to the statistical 

 difficulty of handling such small ratios it is not possible to say whether 

 this difference is significant or not until more data can be collected. 

 The point is of interest in its bearing on the mechanism of the action 



of GUI and C IIr , but must be left unanswered for the present. 



Q 

 The second brood data here presented for ni are entirely 



Cllr 



inadequate for the purpose of detailed comparison with first broods. 

 They do, however, show an increase for p r c over the C/// C// r 

 second broods (from 1.2 to 2.2). 



HOMOZYGOUS C// r . 



We have seen that heterozygous C IIr greatly decreases crossing- 

 over in the region from purple to speck, but does not appreciably 

 affect the region from star to purple. The data now to be presented 

 show that homozygous C IIr gives a value for purple speck that is 

 very close to that found in "normal' ' flies, but again does not influence 

 the region from star to purple. 



It has so far not been found possible to obtain a chromosome con- 

 taining C// r with vestigial or curved, since heterozygous C// r practi- 

 cally prevents all crossing-over in the region in which these three genes 

 are located. For this reason none of the data on homozygous C// r 

 deal with loci between purple and speck. 



HOMOZYGOUS C// r WITH C ni . 



In the course of the experiments with C IIr a chromosome of the 

 constitution S f b p r C IIr s p was obtained. 1 When males with this 



Pr (-s 1 1 r &-n 

 1 This chromosome was derived from culture 570 (see p. 3 14) , in which was a female r - 



A cross-over in this female gave & b p r Cn T s v chromosome. This chromosome, or a derivative 



C// T 



of it, since it had perhaps been passed through a r - ^ - female, was placed opposite star 



Pr I/// T 8 



8 v 

 in the females of cultures 696 and 699a. The chromosome referred to above 



(S f b p r Cu T s p ) was produced by crossing-over in these females and was kept intact thereafter 

 by breeding from males heterozygous for it, in which no crossing-over occurred. 



/& _ \ 



I ~~T - 7; - I i 

 \ o p T Cu r s p / 



