# BIOLOGY 3UU3 Chapter Notes - Chapter 6: Allele Frequency, Genotype Frequency, Genetic Drift

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Chapter 6 (Pg. 169 – 188 & 194 – 207)

Mendelian Genetics in Populations I: Selection and Mutation as Mechanisms of

Evolution

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

Simulation

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

conclusions:

oThe allele frequencies in a population will not change, generation after

generation

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

alleles.

•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

## Document Summary

Mendelian genetics in populations i: selection and mutation as mechanisms of. 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. Juveniles grow up to become the next generation of adults. 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: thus 625 in total carry allele a and 375 carry allele a, for a total of. 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.