Allele Frequencies and
Genetic Equilibrium
(Population Genetics)
Allele Frequencies
How common certain alleles are in a
population of organisms
Example:
– The frequencies of the allele for blood types
A, B, and O differ from group to group, in
some groups, A is the most common, in
others, B is…and so on.
Changes in Allele Frequencies
1. Natural Selection: the increase or
decrease in allele frequencies due to the
impact of the environment…but there are
others
2. Mutations: introduce new alleles that
may provide selective advantages
(although most mutations are deleterious)
Changes in Allele Frequencies
3. Gene Flow: describes the introduction
or removal of alleles from the population
when individuals leave (emigration) or
enter (immigration) the population
4. Genetic Drift: a random increase or
decrease of alleles (by chance alone),
especially in small populations (see next
slide)
Genetic Drift
Analogy: flipping a coin
– Out of 100 tosses: about 50 will be heads
– Out of 5 tosses: ??? All could be tails by chance
Founder Effect: occurs when allele
frequencies in a group of migrating
individuals are, by chance, not the same as
that of their population of origin
– Example: polydactylism among Amish
Genetic Drift
Bottleneck: occurs when the population
undergoes a dramatic decrease in size,
the small population becomes severely
vulnerable to genetic drift depending on
the genetic diversity of the survivors
– Example: Cheetahs
Changes in Allele Frequencies
5. Nonrandom Mating: occurs when
individuals choose mates based upon their
particular traits
Examples:
– Inbreeding: individuals mate with relatives
– Sexual Selection: females choose mates
based upon their attractive appearance or
behavior or their ability to defeat other males
in contests
Genetic Equilibrium
When allele frequencies in a population
remain constant from generation to
generation, the population is said to be in
genetic equilibrium…or
Hardy-Weinberg equilibrium
At equilibrium, no evolution occurs
No changes in allele frequencies
Genetic Equilibrium
Conditions (or assumptions):
1. All traits are selectively neutral (no natural
selection)
2. Mutations do not occur
3. The population must be isolated from other
populations (no gene flow)
4. The population is large (no genetic drift)
5. Mating is random
Genetic Equilibrium
Allele frequencies for each allele (p, q)
Frequency of homozygotes (p2, q2)
Frequency of heterozygotes (pq + qp = 2pq)
p+q=1
(all alleles sum to 100%)
p2 + 2pq + q2 = 1
(all individuals sum to 100%)
Genetic Equilibrium
Example: suppose a plant population where
84% have red flowers and 16% have white
flowers. Assume the red allele (R) is dominant
to white (r):
q2 = 0.16 = white flowered plants (rr)
p2 + 2pq = 0.84 = red flowered plants (RR & Rr)
To determine the frequency of the white flower
allele, find q. Hint: square root of q2
Genetic Equilibrium
q = √0.16 = 0.4
since p + q = 1 ; p must equal 0.6
You can also determine the frequency (or
percentages) of individuals with the
homozygous dominant and heterozygous
condition:
Genetic Equilibrium
Heterozygotes:
2pq = (2)(0.6) (0.4) = 0.48 or 48%
Homozygous Dominant:
p2 = (0.6)(0.6) = 0.36 or 36%
Practice
A population of deer mice consists of
25% of individuals with long tails, which
is the dominant condition (T), and 75% of
individuals with short tails (tt).
a. What is the allele frequency of the short tail
allele?
b. What is the allele frequency of the long tail
allele?
c. What is the frequency of individuals with the
homozygous and heterozygous conditions?
(Both)
Reality Check
In most natural populations, the conditions
of Hardy-Weinberg equilibrium are not
obeyed. What does this tell us?
However, these calculations can serve as
a starting point to reveal how allele
frequencies are changing, and which
driving forces are changing them.