A few notes on variance:

1
A few notes on variance   variance S(Xi - X)2
(A measure of the difference between individual
observations and the mean)   Imagine a locus for
adh (alcohol dehydrogenase).       Let allele A
have a phenotypic effect of 0, and let allele a
have a phenotypic effect of 1 (measured in units
of alcohol converted per second).   What is the
mean and variance in alcohol dehydrogenase
activity if f(A) p 0.5 in a population of 20
individuals.   Mean activity 10 (1) 10
(0) / 20 0.5 Variance 10 (0 - .5)2 10
(1 - .5)2 5.0      
2
If p 0.9 in a population of 20?   mean (18
0) (2 1) / 20 0.1 variance 18 (0-.1)2
2(1 - .1)2 .18 1.62 1.8   Genetic
variance is variance in some phenotype (fitness,
nose length, etc) attributed to genetic
differences. In a haploid system, it is at its
maximum when p 0.5. Graphically, we can look
at the frequency of each phenotype and how this
affects the mean.     
observed
mean
p 0.1
p 0.5
of individuals
of individuals
0
1
0
1
Genetic variance is maximized when p 0.5.