Textbook Notes (270,000)
CA (160,000)
UTSG (10,000)
Chapter 6

BCH447H1 Chapter Notes - Chapter 6: Allele Frequency, Genotype Frequency, Genetic Drift

Course Code
Peter Hyland

of 5
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.
The math on these pages prove that any allele frequency can remain constant
and at equilibrium for numerous generations without external interference
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
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
Selection happens when individuals with particular phenotypes survive to
reproductive age at higher rates than individuals with other phenotypes, or
when individuals with particular phenotypes produce more offspring during
reproduction than individuals with other phenotypes.
It can lead to evolution when the phenotypes that exhibit difference in
reproductive success are heritable.
Adding Selection to the Hardy-Weinberg Analysis: Changes in Allele Frequencies
Read pages 183 to 184. It shows how selection and survival rates can affect the allele
frequency among generations using math.
Adding Selection to the Hardy-Weinberg Analysis: The Calculation of Genotype
Read pages 187-188. It exemplifies what the pages above have done except it is
dealing with the second conclusion of the Hardy-Weinberg Principle using math.
(Pages 194 – 207)
6.3. Patterns of Selection: Testing Predictions of Population Genetic Theory
Selection on Recessive and Dominant Alleles
This chapter covers the mathematics behind the evolution of populations. Read
the book because I can’t explain numbers in words.
Chapter 6 (Pages 210 – 218)
6.4 Mutation
Adding Mutation to the Hardy Weinberg Analysis: Mutation as an Evolutionary
Mutation by itself is generally not a rapid mechanism of evolution.
oFor example in a population with a frequency of 0.9 for A alleles and a
frequency of 0.1 for a alleles, a mutation rate of 1 per 10 000 (a high
rate of mutation) would result in frequency of 0.89991 for the A allele
and frequency of 0.10009 for the a allele.
This number is very similar to the original frequencies of alleles
within that given population.
Therefore, it would take 1000 generations for the allele frequency
for A to change from 0.9 to 0.81.
Mutation can cause substantial change in allele frequencies but it
does slowly.
See Page 211 for clarification and the math shown
Mutation and Selection
It is not correct to assume that mutation is unimportant because of its
unappreciable changes in the allele frequencies of a population
Mutation combine with selection can become a crucial piece of the
evolutionary process.
oRead experiment on page 212 and 213.
oThe experiment shows that while mutation itself is only a weak
mechanism of evolution, it nonetheless supplies the raw material on
which natural selection acts.
oMutation is the ultimate source of genetic variation.
Mutation-Selection Balance
Most mutations are deleterious.