204 



THE SECOND-CHROMOSOME GROUP 



The presence of the double recessive arc pink in the F 2 of the 

 "repulsion" cross would now be regarded as conclusive proof that 

 arc was not third-chromosome; but at that time the fact that there is 

 no crossing-over in the male had not yet been discovered, and the 

 above result meant to us either free crossing-over or separate chromo- 

 somes. Only further tests could decide which of these alternatives 

 was correct. There were still other reasons for further tests. At this 

 early period it was important to prove to the satisfaction of everybody 

 that if a new mutation showed linkage to any one member of a group 



TABLE 49. PI, pink 9 X arc cf ; FI mild-type 9 9 

 + ^i wild-type d* cf. 



it must show linkage to every member, even though in some cases 

 this linkage be very slight because of the long distance between the 

 loci. Conversely, if a mutant failed to show linkage to one member 

 of a group it was still necessary to show that it would likewise fail to 

 show linkage to other members. For this reason arc had been crossed 

 to maroon at the same time as to pink. It is true that it was not then 



TABLE 50. PI, arc 9 X maroon tf;B.C.,Fi mid-type 9 9 X 

 arc maroon 



definitely known that maroon was third-chromosome, but it was seen 

 that these tests would determine to which chromosome maroon be- 

 longed. The arc-maroon "repulsion" likewise gave an approach to a 

 9 :3 :3 :1 ratio. (Culture B 50; + 158, a 54, m a 41, a m a 20.) 



To make more certain that no appreciable linkage was present, 

 three back-cross cultures were made (table 50). 



In the total of 2,728 flies of this arc maroon back-cross there were 

 1,348, or 49.05 per cent of recombinations, where 50.0 is expected 

 from free assortment. 



