Transcript Population Genetics and the Hardy-Weinberg Equation

Review of
Natural
Selection Types
Effects of Selection
See Fig. 23.12
Coat color
Effects of Selection
Directional selection consistently favors
phenotypes at one extreme
See Fig. 23.12
Coat color
Coat color
Effects of Selection
Stabilizing selection favors
intermediate phenotypes
See Fig. 23.12
Coat color
Coat color
Effects of Selection
Diversifying (disruptive) selection
simultaneously favors both phenotypic
extremes
See Fig. 23.12
Coat color
Coat color
Effects of Selection
Directional, diversifying (disruptive),
and stabilizing selection
See Fig. 23.12
Coat color
Coat color
Coat color
Coat color
Population
Genetics
and the
Hardy-Weinberg
Equation
Population Genetics
• In the early 1900s these two men
discovered how the frequency of a
trait’s alleles in a population could be
described mathematically.
G H Hardy – British Mathematician
Wilhelm Weinberg – German Doctor
Population Genetics
• For every phenotype how many
alleles do you have???
–2
• 1 from Mom and 1 from Dad
• These scientists figured out an
equation that can be used to figure
out the percentages of alleles and
genotypes that are in a population.
Population Genetics
• In order for their equation to work
the population has to be in genetic
EQUILIBRIUM
– There is no change in the gene pool =
no evolution
Genetic Equilibrium
• 1.) Population size is large
• 2.) No gene flow in the population
• No new organisms introducing more alleles
• 3.) No mutations
• 4.) No environmental factors causing
natural selection
• No trait is favorable over another
• 5.) Random mating must occur
The Hardy-Weinberg
Equation
• p2 + 2pq + q2 = 1
• p2 = frequency of the homozygous
dominant genotype
• 2pq = frequency of the heterozygous
genotype
• q2 = frequency of the homozygous
recessive genotype
Hardy-Weinberg
• p – frequency of the dominant allele
• q – frequency of the recessive allele
• Because there are only 2 alleles, the
frequency of the dominant allele (p)
and the frequency of the recessive
allele (q) will add up to 1 or 100%
•p+q=1
Hardy-Weinberg Example
• In a population of 100 people 28 of them
were found to have freckles and 72 were
not. We learned in class during our
genetics unit that having freckles is a
recessive trait and not having them is
because of a dominant trait. If this
population is in genetic equilibrium then
solve for the allelic frequencies and the
variables in the hardy-weinberg equation:
Genetic Drift
Genetic Drift
Genetic Drift occurs when the frequency of alleles change
due to RANDOM PROCESSES! (NOT natural selection)
Bottleneck Effect
Bottleneck Effect
Founder Effect
Queens full of Jacks!
• Let’s Mate!
• red card=dominant allele=R
• black card=recessive allele=r
P2 + 2pq + q2
Prediction
1st gen.
2nd gen
3rd gen
RR
Rr
rr
36%
48%
16%
Predicted vs Actual
• If this population is in equilibrium,
we should have the predicted % for
our genotypes…
• We have…20 rr envelopes and 30RR
envelopes
• Are we in equilibrium?
What should happen?
If we are evolving…
If we are not…