Activity 23.1
A Quick Review of HardyWeinberg
Population Genetics
Part A.
Review Chapter 23 of
Biology,
7th edition. Then complete the discussion
by filling in the missing information.
If evolution can be defined as a change in gene (or more appropriately, allele)
frequencies, is it conversely true that a population not undergoing evolution should
maintain a stable gene frequency from generation to generation? This was the question
that Hardy and Weinberg answered independently.
1.
Definitions.
Complete these definitions or ideas that are central to understanding the
HardyWeinberg theorem.
a.
Population: An interbreeding group of individuals of the same
species
.
b.
Gene pool: All the alleles contained in the gametes of all the individuals in
the
population
.
c.
Genetic drift: Evolution (defined as a change in allele frequencies) that occurs
in
small
populations as a result of chance events.
2.
The HardyWeinberg theorem.
The HardyWeinberg theorem states that in a
population that
is not
(
is/is not
) evolving, the allele frequencies and genotype frequencies
remain constant from one generation to another.
3.
Assumptions.
The assumptions required for the theorem to be true are listed on
page 449 of
Biology,
6th edition, and are presented here in shortened form.
a.
The population is very
large
.
b.
There is no net
migration
of individuals into or out of the population.
c.
There is no net
mutation
; that is, the forward and backward mutation rates for alleles
are the same. For example, A goes to a as often as a goes to A
d.
Mating is at
random
for the trait/gene(s) in question.
e.
There is no
selection
. Offspring from all possible matings for the trait/gene are
equally likely to survive.
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The HardyWeinberg proof.
Consider a gene that has only two alleles,
R
(dominant)
and
r
(recessive). The sum total of all
R
plus all
r
alleles equals all the alleles at this gene
locus or 100% of all the alleles for that gene.
Let
p
=
the percentage or probability of all the
R
alleles in the population
Let
q
=
the percentage or probability of all the
r
alleles in the population
If all
R
+ all
r
alleles
=
100% of all the alleles, then
p
+
q
=
1 (or
p
=
1 –
q
or
q
=
1 –
p
)
[
Note:
Frequencies are stated as percentages (for example, 50%) and their associated
probabilities
are stated as decimal fractions (for example, 0.5).]
Assume that 50% of the alleles for fur color in a population of mice are
B
(black) and
50% are
b
(brown). The fur color gene is autosomal.
a.
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 Spring '10
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 Genetics, Population Genetics, Weinberg, Holstein, 2,500

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