Gene Pool:
All the genes of all themembers in a population.
Allele Frequency:
Percent of each allele inthe population.
Genetic Equilibrium:
Allele frequencies remainthe same from onegeneration to the next.
The Hardy-Weinberg Theorem
States the allele frequencies remain constant (Hardy-Weinberg Equilibrium) unless acted upon by outsideagents.
It best describes a nonevolving population.
Very useful to establish baselines in populations.
Godfrey Hardy
(1877-1947)
Wilhelm Weinberg
(1862 – 1937)
Hardy Weinberg Theorem
Populations remain stable under the following fiveassumptions.
1. Populations must be large
Genetic Drift: allele frequencies change as a result of random eventsor chance
2. No Migration
3. Random Mating
4. No Mutations
5. No Natural Selection
If one or more of these conditionsare not met, EVOLUTION occurs.
Tasters or Non-tasters??
A = tastersa = non-tasters
p+ q = 1
Phenotypes
Allele Frequency
Tasters
(p2 + 2pq )
Nontasters
(q2)
p
q
#
%
#
%
Class Population
N. AmericanPopulation
.55
.45
Hardy Weinberg Equation
To estimate the frequency of alleles in a population, we can use theHardy-Weinberg equation. According to this equation:
p = the frequency of the dominant allele (represented here by A)q = the frequency of the recessive allele (represented here by a)
For a population in genetic equilibrium:p + q = 1.0 (The sum of the frequencies of both alleles is 100%.)
(p + q)2 = 1
sop2 + 2pq + q2 = 1
The three terms of this binomial expansion indicate the frequencies ofthe three genotypes:
p2 = frequency of AA (homozygous dominant)2pq = frequency of Aa (heterozygous)q2 = frequency of aa (homozygous recessive)
Example
Screech owls are found in two different color patterns, redand gray. Scientists have raised and bred these owls incaptivity. The scientists mated red to red, red to gray andgray to gray. The red to gray mating usually gave equalnumbers of red and gray offspring but sometimes gave onlyred offspring.
A population survey done in Pennsylvania showed thepopulation to be composed of 51% red and 49% gray owls
a. Which gene is dominant?
b. Calculate the frequency of each allele.
c. Calculate the frequency of each genotype.
d. Suppose the frequency of each gene (allele) for redfeathers is .42 ten years later in the same population. Whatcould have caused the change in the gene frequency?