Training Manual Application of Genetics and Biotechnology in ...

If the sample adequately reflects the variability present in the larger
population, then the statistic 'S' estimates the parameter o.
One of the important properties of the normal distribution is that
approximately 68% of the measurements will lie within plus or minus one standard
deviation from the mean ( pAa). Again approximately 95% of the measurements will
lie between 2 standard deviations of the mean (prt2o). More than 99% of the
measurements will be found within plus or minus 3 standard deviations of the mean
(!JA3o).
Coefficient of variation
It is observed that traits with relatively large average metric values generally
are expected to have corresponding large standard deviations than traits with
relatively small average metric values. Since different traits may be measured in
different units, the coefficients of variation are useful for comparing their relative
variabilities. If we divide the standard deviation by the mean, we get the coefficient
of variation which is independent of the unit of measurement.
CV = o / p for a population
CV = S /
for a sample
Variance
This is the square of the standard deviation. However, unlike the standard
deviation, the variance cannot be plotted on the normal curve and can only be
represented mathematically. Variance is used as an expression of variability because
of the additive nature of its components. Using a technique called the 'analysis of
variance', the total phenotypic variance (Vp) expressed by a given trait in a
population can be statistically partitioned into components of genetic variance (Vg),
non-genetic or environmental variance (Ve) and variance due to genotypeenvironment
interaction. (Vge)