+-------------------------------------+-------------------+------+------+ | |Standard deviation | | | | | of races crossed, | S. D.| S. D.| | | same generation. | F₁ | F₂ | +-------------------------------------+---------+---------+------+------+ |Plus-minus cross, series 1 (Table 50)| 0.49 | 0.50 | 0.71 | 1.01 | |Plus-minus cross, series 2 (Table 50)| .36 | .24 | .60 | .87 | |Mutant-minus cross, lower group | .25 | .24 | .77 | 1.17 | |Mutant-minus cross, upper group | .19 | .24 | .31 | .44 | |Mutant-plus cross, lower group | .25 | .36 | .24 | .35 | |Mutant-plus cross, upper group | .19 | .36 | .23 | .15 | +-------------------------------------+---------+---------+------+------+ The mutant-plus cross, it will be observed, shows no increase of variability either in F₁ or in F₂, but crosses involving the minus race show increase of variability both in F₁ and in F₂. Interpreted on a Mendelian basis, this means that the mutant and plus races on the one hand and the minus race on the other hand differ by more than a single factor. If they differed by only a single factor, then crosses between them should bring no increase of variability, either in F₁ or in F₂. This appears to be true as regards the mutant and plus races when crossed with each other. But if the races crossed differ by more than one factor, and if, further, neither parent is homozygous as regards the factors in which they differ, then we may expect an increase in variability both in F₁ and in F₂. This is exactly what we observe when the minus race is crossed with either the plus race or its derivative, the mutant race.