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Chapter 25

BIOLOGY 1M03 Chapter Notes - Chapter 25: Null Hypothesis, Stabilizing Selection, Egg Cell

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Dr. Dushoff

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CHAPTER 25: Evolutionary Processes
The Hardy-Weinberg Principle a null hypothesis that projects what genotype frequencies
should be given through known allele frequencies
five assumptions/conditions are met with respect to the gene in question:
1. no natural selection it produces adaptations by causing increases or decreases in the
frequency of certain alleles
2. no genetic drift - it causes allele frequencies to change randomly
3. no gene flow (migration) it introduces new alleles and change frequencies between
4. no mutations it introduces new alleles
5. no biased mating inbreeding and sexual selection change frequencies
HWE was devised as a mathematical model to analyze allele frequencies
To do this they imagined that all of the gametes produced in each generation go into a single
group called a gene pool and then combine
Their calculations predict the genotypes of the offspring that the population would produce as
well as the frequency of each genotype
HWE (Hardy-Weinberg Equilibrium) Marbles Example
Only two alleles A1 and A2
The frequency of A1 is represented by p and the frequency of A2 is represented by q because
there are only two alleles p + q = 1
In this situation three genotypes are possible A1A1, A1A2, A2A2

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A1A1 homozygous A1 o 0.7x0.7=0.49
o PxP=p^2
A2A2 homozygous A2 o 0.3x0.3=0.09
o Q x Q = q^2
A1A2 (A2A1 also counts, but it produces the same genotype) - heterozygous
o 0.7x0.3=0.21 o P x Q = PQ
o 0.3x0.7=0.21 o Q x P = QP
o 0.21+0.21=0.42 o PQ + QP = 2 PQ
P^2 + 2 PQ + Q^2 = 1
These are the genotype frequencies only if the population is in HWE
Population not in HWE
A1A1 46.7%, A1A2 26.7%, A2A2 26.7%
Since the population is not in HWE you cannot assume that p = sqr root of (p^2)
If the population is in HWE then you can assume that p = sqr root of (p^2)
In a population at HWE, the frequency of A1A1 will be P^2, the frequency of A1A2 will be 2PQ
and the frequency of A2A2 will be Q^2
The sum of the three frequencies must equal 1 (100% of the population) < regardless of if it is
HWE or not
P^2 + 2 PQ + Q^2 = 1 < ONLY for populations in HWE
If the frequencies of A1 and A2 are p and q then the equation is valid generation to generation
When alleles are transmitted according to the rules of Mendel inheritance the frequencies do
not change
^ Two Statements of HWE
1) Are the MN Blood Types in Humans in HWE?
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