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 F₁ 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 F₁ 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 F₁ 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 generation is more variable than F₁. 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 F₁ 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 F₁ generation. In Series 1, the mean of F₁ is 0.06, and that of F₂ is 0.24; in Series 2, the mean of F₁ is 1.00, and that of F₂ is 0.72.
It may also be seen from an examination of Table 50 that among the F₁ 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 F₁ individuals which are of high grade produce offspring of higher grade than do their low-grade brothers and sisters. This is exactly what has been observed in both uncrossed races.