A gene pool is the genetic make-up of a specific
population, and is the combination of all the alleles
for all traits members of the population exhibit.
For example, in a population of mice, the gene pool
consists of all the alleles of the genes for each
In the gene pool above, 60% of the alleles are
black (B) and 40% are white (b). The percent of
alleles in a pool is known as an allele frequency.
The sum of all alleles in any pool must be 100%.
The Hardy-Weinberg principle (sometimes called
the Castle-Hardy-Weinberg principle; it was named
after the scientists who discovered it) states that
the allele frequency for dominant and recessive
alleles remains the same over the generations in
any given population so long as certain conditions
exist. In other words, 60% of the alleles in the
sample population above will always be for black
coats and 40% of the alleles will always code for
white coats, even 100 years from now, so long as
nothing happens to the population. These five conditions
must be met in order for the principle to work:
1. No mutations
2. The population
must be large.
3. All mating
must be random (any male can mate with any female
or vice versa).
4. No migration
5. All genotypes
must be equal.
The Hardy-Weinberg principle is based on mathematical
laws of probability, and similar to Punnett Squares,
the results can be charted on a graph called a cross-multiplication
When two alleles are randomly drawn from the gene
pool (representing random mating), the resulting
probability is the product of the two individual
probabilities for drawing each allele. Hence, the
probability of producing a Bb child is:
B x b = Bb
60% x 40% = 24%
The probability of producing a BB child is:
B x B = BB
60% x 60% = 36%
On a cross multiplication table, the results look
Because a Bb mouse will still produce a black coated
mouse, the percentages can be added to find that
84% of the mice in the population will be black
(36% + 24% +24%) and 16% will be white. As long
as the required conditions are met, this population
will always be 84% black and 16% white.