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.
o A population is a group of interbreeding individuals and their offspring
o The 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.
o This 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.
o This 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.
o Assuming 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.
o Thus 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.
o Blind luck causing populations to evolve unpredictably is an important
result of population genetics.
o This mechanism of evolution is called genetic drift.
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
• This is known as the Hardy-Weinberg Equilibrium Principle. It is based on two
o The allele frequencies in a population will not change, generation after
o If the allele frequencies in a populat