252 



THE SECOND-CHROMOSOME GROUP 



INHERITANCE AND CHROMOSOME OF PLEXUS. 



One of the plexus spread males was out-crossed to a black female 

 and produced in Fi only wild-type flies, showdng that the character 

 was recessive. Several r2 cultures were raised from pairs of the Fi 

 flies, and in these plexus reappeared as about a quarter of the F2 indi- 

 viduals. The plexus venation was present equally among the F2 

 females and males, from which it was known not to be sex-linked. 

 Likewise plexus was known not to be third-chromosome from the free 

 recombination of plexus and spread among the F2 individuals. This 

 distribution of plexus and spread might have been thought due to 

 very free crossing-over between spread and plexus instead of the 

 random assortment, were it not that the black and plexus appeared 

 in the typical 2:1:1:0 ratio, which showed that the locus of plexus 

 is in the second chromosome. 



From the F2 cultures black and plexus flies were crossed to each 

 other, with the two-fold purpose of obtaining the double-recessive form 

 and of elimination the third-chromosome recessive spread. 



LOCUS OF PLEXUS. 



The double recessive black plexus was easily obtained, and a back- 

 cross test was made of the amount of crossing-over in the female 

 between these two loci (table 99). The back-cross cultures furnished 

 1,026 flies, of which 417 or 40.6 per cent were cross-overs. This very 

 free crossing-over located plexus in the region of arc, if its locus were 

 to the right of black, or at a point even farther to the left than streak 

 if it were to the left of black. 



Table 99. — Pi, black plexus 9 X wild cf ; B. C, Fx wild- 

 type 9 X black plexus cf . 



A means of easily testing these alternatives was soon afforded by the 

 discovery and location of "star," which proved to be a dominant 

 mutation whose locus is some distance to the left of streak. A star 

 black plexus back-cross was made by testing the Fi star daughter, from 

 the mating of a star female by a black plexus male, to black plexus 

 males (table 100). Four such pair cultures gave 1,352 offspring, of 

 which 233 were double cross-overs, 343 simple cross-overs between 

 black and plexus, 289 single cross-overs between star and black, and 

 487 were non-cross-overs. Plexus gave 42.6 per cent of total observed 



