22 PIEBALD RATS AND SELECTION. 



CROSSES OF THE PLUS RACE WITH THE MINUS RACE. 



When animals of the plus selection series are crossed with animals of 

 the minus selection series, an Fi generation of offspring is obtained 

 which varies about a mean intermediate between those of the respective 

 uncrossed races. Thus, from an examination of Table 50 it will be seen 

 that when —2 animals of generation 6, minus series, were crossed with 

 +3.50 or +3.75 animals of generation 5, plus series, an Fi generation 

 (Series 1) was obtained consisting of 93 animals of mean grade +0.06. 

 This generation is rather more variable than either uncrossed race, 

 its standard deviation being 0.71. The same is true of a second set 

 (Series 2) of crosses made between a male of grade —3.25, generation 

 10, minus series, and females of grade +3.75, generation 10, plus series. 

 The 14 Fi offspring are of mean grade +1 and have a standard deviation 

 of 0.60. (See Table 50, Series 2.) 



In both the series of crosses summarized in Table 50, the F2 genera- 

 tion is more variable than Fi. In Series 1, 305 F2 animals are recorded, 

 having a standard deviation of 1.01 as compared with 0.71, the standard 

 deviation of the Fi generation. In Series 2, the F2 offspring number 73 

 and have a standard deviation of 0.87, that of the previous generation 

 being 0.60. 



The mean of the F2 generation is very similar to that of the Fi genera- 

 tion. In Series 1, the mean of Fi is 0.06, and that of F2 is 0.24; in Series 

 2, the mean of Fi is 1.00, and that of F2 is 0.72. 



It may also be seen from an examination of Table 50 that among the 

 Fi offspring produced by crossing the plus and the minus series there are 

 differences in transmission, as there are in the expression of the hooded 

 pattern. In general those Fi individuals which are of high grade pro- 

 duce offspring of higher grade than do their low-grade brothers and 

 sisters . This is exactly what has been observed in both uncrossed races. 



SUMMARY OF RESULTS. 



The experiments which have been described in the foregoing pages 

 have shown that : 



1. The hooded pattern of rats behaves as a simple Mendelian char- 

 acter in crosses with either the Irish pattern or the wholly pigmented 

 condition of wild rats. 



2. Though behaving as a imit, the hooded pattern fluctuates — that 

 is, it is subject to plus and minus variations. 



3. Selection, plus or minus, changes the position of the mean and 

 mode about which variation occurs. 



4. The results of such plus or minus selections are permanent, for 

 return selection is not more effective than the original selection, and 

 during return selection regression occurs away from the original mode, 

 that is, toward the mode established by selection. 



