LINKAGE VARIATION IN DROSOPHILA 



be very small. If an equal amount of data for 'coupling' and 

 'repulsion' be combined, each possible class will appear in the 

 required manner both as a non-crossover and as a crossover. 

 Table 3 combines in this manner the results for the first broods 

 of tables 1 and 2. 



Of the 5174 flies of the first broods 608 or 11.8 per cent were 

 crossovers. A similar balancing of the data of the second broods 

 from tables 1 and 2 was made for comparison with the results 

 of the first broods (table 4). 



TABLE 4 

 Comparison of firsts and seconds {purple and vestigial) with balanced 



viability 



If we take 11.8 units as the standard amount of crossing-over, 

 the fall of 3.8 units is 32 per cent of this original amount of 

 crossing-over. This fall of a third in amount, in connection 

 with the fall in thirteen out of fourteen cases, shows that a real 

 variation in linkage has occurred. 



In this case the second broods gave a trifle over half of the 

 10,589 flies, showing that the egg-laying powers of the females 

 had not diminished. 



The case upon which there are the fullest data (16,873 flies 

 in back-cross experiments) is that of black and curved. The 

 first experiment involves 'coupling' (table 5). 



Of the broods later than the firsts, four showed a decrease 

 and two a smaller rise. In the totals the fall of 1.8 units is not 

 large enough to be significant in itself. In culture 29 the female 

 was carried through four broods and showed very little decrease 

 in the number of ofl"spring. Likewise the mother of 31 main- 

 tained her output. It is interesting that the fourth brood of 

 29 showed more crossing-over than did the first. Perhaps the 

 fall reaches a maximum in the second broods. This question 



