Transcript Hardy-Weinberg

Population
Genetics
June 2, 2016
Sources of Genetic Variation
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What do you think are some sources of
genetic variation?
Population Genetics review
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Given a population of 300 plants…
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How many total height genes are there?
Given that 100 plants are short (recessive trait),
200 are tall, and 50 are homozygous tall, how
many are there of each genotype?
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How many T alleles are there in the gene pool?
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Homozygous recessive
Homozygous dominant
Heterozygous
What is this alleles’ frequency in the population?
How many t alleles are there in the gene pool?
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What is this alleles’ frequency in the population?
I. Relative Frequency of an Allele
A.
B.
The number of times an
allele occurs in the gene
pool, given as a
percentage
Relative frequency has
nothing to do with
dominant or recessive
1.
The recessive allele
can occur more
frequently
C. A note on sexual reproduction…
Sexual reproduction can produce many
different phenotypes
Sexual reproduction does NOT change
relative frequency of alleles in a population
Think about shuffling a deck of cards
1.
2.
3.
a.
Shuffling cards gives you different hands, but it
won’t change the number of kings in a deck
II. Population Genetics
A.
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
II. Population Genetics (cont.)
B. For every phenotype how many alleles do
you have???
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2
1.
1 from Mom and 1 from Dad
C. These scientists figured out an equation that
can be used to figure out the percentages of
alleles and genotypes that are in a
population.
III. Genetic Equilibrium: Hardy-Weinberg
Principle
A.
B.
Allele frequency in a population
will remain constant unless an
outside factor causes those
frequencies to change
When allele frequencies remain
constant, we call this genetic
equilibrium
III. Genetic Equilibrium (cont.)
C. In order for their equation to work the
population has to be in GENETIC
EQUILIBRIUM
1.
This means that there is no change in the gene
pool = no evolution
D. How do you achieve genetic
equilibrium?
1.
2.
Population size is large
No gene flow in the population
a.
3.
4.
No mutations
No environmental factors causing natural
selection
a.
5.
No new organisms introducing more alleles
No trait is favorable over another
Random mating must occur
E. The Hardy-Weinberg Equation
1.
p2 + 2pq + q2 = 1
a. p2 = frequency of the homozygous
dominant genotype
b. 2pq = frequency of the heterozygous
genotype
c. q2 = frequency of the homozygous
recessive genotype
D. Hardy-Weinberg (cont.)
d. p – frequency of the dominant allele
e. q – frequency of the recessive allele
f. 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%
1. p + q = 1
D. Hardy-Weinberg (cont.)
2. In reality, no population satisfies the HardyWeinberg equilibrium completely
3. However, in large populations with little
migration and little natural selection, it can
approximate gene frequencies
Hardy-Weinberg Example
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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.
Queens full of Jacks!
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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
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If this population is in equilibrium, we should
have the predicted % for our genotypes…
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We have…20 rr envelopes and 30RR
envelopes
Are we in equilibrium?
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What should happen?
If we are evolving…
If we are not…