BIS 2B Fall 2011
STUDY GUIDE
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This study guide has two major parts.
PART I
is a primer on the use of the HardyWeinberg model to calculate genotype frequencies
from allelic frequencies, and allelic frequencies from genotype frequencies.
It also highlights
some of the pitfalls of assuming that a population is in HW equilibrium, when it is not.
Note
especially that the mere fact that allelic or genotypic frequencies sum to 1 does NOT mean a
population is in HW equilibrium, and that you can readily convert from allelic to genotypic
frequencies (and vice versa) using the HW model.
Part I also contains a series of questions that
test your mastery of population genetics.
PART II
contains a series of general questions on population genetics and natural selection.
Some of the questions are quite tough, so don’t be easily daunted.
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PART I:
A PRIMER ON HARDYWEINBERG EQUILIBRIUM & BASIC
POPULATION GENETICS
I. SOME BASIC ESSENTIALS
•
The five conditions required for a HardyWeinberg equilibrium are:
1.
No mutation
2.
Random Mating
3.
No gene flow (
i.e
. no immigration or emigration)
4.
Large population size (genetic drift does not affect allelic frequencies)
5.
No selection (
., no advantage to individuals carrying particular alleles or genotypes at a
locus)
!
If these conditions are satisfied, then there should be no change in genotype frequencies or
allele frequencies across generations.
This means that if we have information about allele or
genotype frequencies for one generation, we can determine the expected frequencies for future
generations under HardyWeinberg equilibrium.
A first step is to compare expected genotypic frequencies (assuming equilibrium) with observed
genotypic frequencies: if they match we assume that the 5 conditions are satisfied and we can go
on.
If they don't match, then we CANNOT use the HW formula to calculate genotype frequencies
from observed allelic frequencies.
(We may be able to determine which condition is violated.)
Keep in mind that if you know the frequencies of all of the genotypes, then you can always
calculate allelic frequencies just by counting the number of copies of each allele in each
genotype.
But, if genotype frequencies are NOT in HW equilibrium, you CANNOT calculate the
allelic frequencies by taking the square root of the frequencies of one or the other homozygote
class.
This is a classic “trick” that will be played on you, so beware!
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There are several different ways to use information about genotype and allele frequencies,
depending on what you know at the beginning and what you want to find out.
II. HOW TO ESTIMATE GENOTYPE FREQUENCIES IF YOU KNOW ALLELE FREQUENCIES:
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 Spring '11
 xi
 Macroeconomics, Genetics, Evolution

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