Assume that dark hair is dominant
over light hair; if there are 5
students in a room of 20 students
with light hair what can you tell me
about the population of students
using this equation:
p² + 2pq + q² = 1
Hardy Weinberg
REVIEW
Evolution is simply a change in
frequencies of alleles in the gene
pool of a population over time.
GENETICS AND EVOLUTION
• Genes determine most of an individual’s
features, such as tooth shape or flower color.
• If an organism has a feature that is poorly
adapted to its environment, the organism
may be unable to survive and reproduce.
Picture all of the alleles of a population as being together
in a large pool called a gene pool.
The percentage of any specific
allele in the gene pool is called
the allelic frequency.
What is the gene pool for this
frog population???
What is the allelic frequency
of green frogs? Red frogs?
Purple frogs?
Genetic Drift
• Genetic Drift – the random fluctuation in allele
frequencies over time, due to chance occurrences
alone
• It is more significant in smaller populations
• It increases the chance of any given allele
becoming more or less prevalent when the
number of individuals is small
Gene Flow
• Genes move with individuals when they move
(emigrate or immigrate) into and out of a
population…and it changes the gene pool
GENETICS AND EVOLUTION
• Evolution occurs as a population’s genes and their
frequencies change over time.
•This can take millions of years for a species to change
GENETICS AND EVOLUTION
•A population in which the frequency of alleles
remains the same over generations is in genetic
equilibrium.
• A population that is in genetic equilibrium is NOT
evolving or changing.
According to Hardy-Weinberg, Evolution is …simply a
change in frequencies of alleles in the gene pool of a
population.
• Alleles:
One of two or more forms of a gene that
code for different versions of the same trait.
•Gene Pool:
All possible genes and genetic
combinations in a population.
• Population:
A group of organisms from the same
species and the geographic location.
Hardy-Weinberg came up with five basic reasons
why a population would stay at genetic equilibrium:
1. the population is large, and genetic drift is not an issue
(People won’t separate into cliques, and disease, predation, or any
other catastrophe will not occur).
2. there is no gene flow, or migration in or out of the population
3. no mutations occur in any individuals DNA within this population.
4. all mating is totally random (No artificial selection or sexual selection)
5. natural selection is not occurring
Under these conditions it is obvious that
evolution would NOT occur.
There are no mechanisms of evolution acting on the
population, so the process cannot happen--the gene
pool frequencies will remain unchanged.
Hardy and Weinberg developed an equation we
can use to calculate how many individuals in
a population are:
BB (Homozygous Dom.)
Bb (Heterozygous)
bb (Homozygous Rec.)
This is used to track allelic
frequencies from generation
to generation in a population
to monitor evolution OR
changes in the gene pool.
This is the Hardy-Weinberg equilibrium equation.
p² + 2pq + q² = 1
p is defined as the frequency of the dominant
allele
p = B
q is defined as the frequency of the recessive
allele
q = b
Because there are only two alleles in this
case, “B” and “b” the frequency of one plus
the other must equal 100%, so…
p + q = 1 (or 100%)
p² + 2pq + q² = 1
p = B
q = b
In this equation:
p² = homozygous dominant (BB) organisms in a
population.
2pq = heterozygous (Bb) organisms
q² = homozygous recessive (bb) ones
Albinism is a rare genetically inherited trait that is only
expressed homozygous recessive individuals (aa).
The most characteristic symptom is
a marked deficiency in the skin and
hair pigment melanin.
This condition can occur among
any human group as well as
among other animal species.
The average human frequency of
albinism in North America is only
about 1 in 20,000.
The Hardy-Weinberg equation (p² + 2pq + q² = 1),
and the frequency of homozygous recessive
individuals (aa) in a population is q². Therefore,
in North America the following must be true for
albinism: q² = 1/20,000 = .00005
By taking the square root of both sides of this equation, we get:
q = .007 (rounded)
Knowing one of the two variables
(q) in the Hardy-Weinberg equation,
it is easy to solve for the other (p).
p=1–q
p = 1 - .007
p = .993
The frequency of the dominant, normal allele (A) is,
therefore, .99293 or about 99 in 100.
The next step is to plug the frequencies of p and q into
the Hardy-Weinberg equation:
p² + 2pq + q² = 1
(.993)² + 2 (.993)(.007) + (.007)² = 1
.986 + .014 + .00005 = 1
This gives us the frequencies for each of the three genotypes for
this trait in the population:
p² = AA = .986 = 98.6%
2pq = Aa = .014 = 1.4%
q² = aa = .00005 = .005%
1. If 98 out of 200 individuals in a population express the
recessive phenotype, what percent of the population are
homozygous dominant?
Original problem source unknown
2. Brown hair (B) is dominant to blond hair (b). If
there are 168 individuals with brown hair in a
population of 200:
What is the predicted frequency of heterozygotes?
A. 16%
B. 40%
Original problem source unknown
C. 60%
D. 48%
E. 84%
Brown hair (B) is dominant to blond hair (b). If
there are 168 individuals with brown hair in a
population of 200:
3. What is the predicted frequency of homozygous
dominant genotype?
A. 16%
B. 36%
Original problem source unknown
C. 60%
D. 48%
E. 84%
4. What are the allele frequencies in an isolated field of 382
pink, 355 white, and 103 red snapdragon plants?
Original problem source unknown
5. A population of turtles (in Hardy Weinberg equilibrium) is variable for
the length of their tail. Long tails are dominant to short tails. 25 have long
tails and 75 have short tails. If 400 baby turtles are born in the population,
predict how many of the offspring will have long tails and how will have
short tails.
Original problem source unknown
6. A population of 1000 individuals has 49 people that are lefthanded. Assume left-handedness is homozygous recessive
and that this population is in Hardy Weinberg equilibrium.
Calculate p and q. What is the frequency of homozygous
dominants, heterozygotes, and homozygous recessives? How
many individuals in the population carry the allele for lefthandedness, but are not left-handed?
Original problem source unknown
7. Assume 1 in 10,000 babies born
in the US have PKU (which is
recessive). What is the frequency
of carriers for this allele?
8. 30,000 individuals in the United States have
cystic fibrosis. Assuming the population is in
Hardy Weinberg equilibrium how many
individuals are carrier of the disease? Assume
the US population is 316,000,000.
Bell Ringer
• In your journal...