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Chapter 6

BIO342H5 Chapter Notes - Chapter 6: Genetic Drift, Cystic Fibrosis, Genotype Frequency

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Don Mc Kenzie

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Chapter 6 (Pg. 169 – 188 & 194 – 207)
Mendelian Genetics in Populations I: Selection and Mutation as Mechanisms of
6.1. Mendelian Genetics in Populations: The Hardy-Weinberg Equilibrium Principle
Population genetics begins with a model of what happens to allele and
genotype frequencies in an idealized population. Once we know how
Mendelian genes behave in the idealized population, we will be able to
explore how they behave in real populations.
oA population is a group of interbreeding individuals and their offspring
oThe crucial events in the life cycle of a population are:
The adults produce gametes
The gametes combine to make zygotes
The zygotes develop into juveniles
Juveniles grow up to become the next generation of adults
This chapter will be simulating a population of mice to explain the material
All the eggs and sperm produced by all the adults in the population are
dumped together in a barrel and stirred.
oThis barrel is known as the gene pool.
Imagine that 60% if the eggs and sperm received a copy of allele A and 40%
received allele a.
oThis means the frequency of allele A is 0.6 and of a is 0.4.
Using a simulation, 34 mice had genotype AA, 57 had Aa and 9 had aa.
oAssuming that each mouse donates 10 gametes to the gene pool:
The 34 AA adults together make a total of 340 games: 340 carry
allele A and none carry allele a.
The 57 Aa adults together make a total of 570 gametes: 285
carry allele A and 285 carry allele a.
The 9 aa adults together make a total of 90 gametes: none carry
allele A and 90 carry allele a.
oThus 625 in total carry allele A and 375 carry allele a, for a total of
1000 gametes. The frequency of gametes in the new gene pool is
0.625 for allele A and 0.375 for allele a.
In simulated populations allele frequencies change somewhat across
generations. This is evolution resulting from blind luck.
oBlind luck causing populations to evolve unpredictably is an important
result of population genetics.
oThis mechanism of evolution is called genetic drift.
Numerical Calculation
Read pages 174 – 176 for visual reference.
Numerical examples show that when blind luck plays no role, allele
frequencies remain constant from one generation to the next.
The General Case
Read pages 177 – 179 for visual reference.
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The math on these pages prove that any allele frequency can remain
constant and at equilibrium for numerous generations without external
This is known as the Hardy-Weinberg Equilibrium Principle. It is based on two
oThe allele frequencies in a population will not change, generation after
oIf the allele frequencies in a population are given by p and q, the
genotype frequencies will be given by p2, 2pq and q2
What Use Is the Hardy-Weinberg Equilibrium Principle?
What makes it useful is that it rests on a specific set of simple assumptions. When
one or more of these assumptions is violated, the Hardy-Weinberg conclusions no
longer hold.
There is no selection
oAll members of the model pop. survived at equal rates and contributed
equal number of gametes to the gene pool. When this assumption is
violated (some survive better than others), the frequencies of alleles
may change from one generation to the next.
There is no mutation
oIn the model population, no copies of existing alleles were converted
by mutation into copies of other existing alleles, and no new alleles
were created. When this assumption is violated, allele frequencies may
change from one generation to the next
There is no migration
oNo individuals moved into or out of the model population. When this
assumption is violated, individuals carrying some alleles move into or
out of the population at higher rates than individuals carrying other
There are no chance events
oBlind luck plays no role
oWhen this assumption is violated, and by chance some individuals
contribute more alleles to the next generation than others, allele
frequencies may change from one generation to the next. This is
known as genetic drift.
Individuals choose their mates at random
oUnlike the first four assumptions, if this assumption is violated—
species choose to mate those of the same genotype—allele
frequencies do not change from one generation to the next but rather
genotype frequencies do.
oShifts in combination w/ violation of the other assumptions lead to
evolutionary change.
When any of these five assumptions are violated, it is an indication that the
population is heading towards evolution.
Changes in the Frequency of the CCR5-∆32 Allele
Read page 182.
6.2. Selection
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