1921-22.] 
On the Dominance Ratio. 
331 
mutations sufficient only to be in equilibrium with a smaller number B 
factors, we may put 
or, 
so that if 
^ = Ipir cot ^p7T, 
tt B ^ 7T 
, = T’ 
tan a A 2 
and 
p = - 
7T 
k=. 
irn 
Similarly, if B>A, the rate of increase in variance may be calculated from 
the equations 
a B 
tanh a A 
k = 
irn 
The rate of decrease, therefore, cannot, in the absence of selection, 
exceed the value indicated by = no such limit can be assigned to the 
rate of increase. 
6. Uniform Genetic Selection. 
In section 1 we have seen that the effects of selection on any 
Mendelian factor may be expressed by the triple ratio a : b : c representing 
the relative fitness of the three phases. Only when b exceeds both a and c 
is there a condition of stable equilibrium ; when b is less than both a and c 
there is a condition of unstable equilibrium ; and such factors will tend 
rapidly to disappear from the stock. Generally, however, we may expect 
that either b will be intermediate, or equal to a, the value for the dominant 
homozygote. Two hypothetical cases may, therefore, be considered: (1), 
in which b is the geometric mean of a and c, and the selection merely affects 
the proportion of the allelomorphic genes ; we may call this uniform genetic 
selection ; and (2), in which b is equal to a, which we may call uniform 
genotypic selection. 
In uniform genetic selection the genetic ratio will be altered in a con- 
stant ratio r in each generation, so that after n generations of selection 
we have 
<2 % 
evidently section 1. 
