Hardy-Weinberg Principle
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Transcript Hardy-Weinberg Principle
SBI3U
Change of the gene pool of a population
over time
In
large populations where only random chance
is at work, allele frequencies are expected to
remain constant from generation to generation.
Remaining constant = EQUILIBRIUM
However,
there are ‘disturbing factors’ that
cause allele frequencies to change, which lead
to evolutionary change
To
be in Hardy-Weinberg Equilibrium none of
these disturbing factors can be present in the
population
Natural
Selection- favours the passing on of
some alleles over others
Small Population Size- limited allele
diversity
Mutation- introduce new alleles to a
population
Immigration/ Emigration- introduce or
remove alleles in a population
Horizontal gene transfer- gaining of new
alleles from a different species
In
reality, the conditions for HardyWeinberg equilibrium (no disturbing
factors, therefore only random chance)
are NEVER met
There are ALWAYS disturbing factors
HW can be approximated in the lab
It has usefulness as a model for studying real
populations
p + q = 1
p2 + 2pq + q2 = 1
A
gene has two alleles, A and a
The frequency of allele A is represented by p
The frequency of allele a is represented by q
The frequency of genotype AA = p2
The frequency of genotype aa = q2
The frequency of genotype Aa = 2pq
Assume a population in which 36% of the
population are homozygous for a certain
recessive allele, a. Assume the population is
at equilibrium.
Question 1: What is the frequency of the
recessive allele, a, in this population?
q2 = 0.36
q = 0.36
q = 0.60
Assume a population in which 36% of the
population are homozygous for a certain
recessive allele, a. Assume the population is
at equilibrium.
Question 2: What is the frequency of the
dominant allele, A, in this population?
q = 0.60
p + 0.60 = 1
p = 0.40
Assume a population in which 36% of the
population are homozygous for a certain
recessive allele, a. Assume the population is at
equilibrium.
Question 3: What percentage of the population
are homozygous for the dominant allele, A?
p = 0.40
p2 = 0.402
p2 = 0.16 = 16 %
Assume a population in which 36% of the
population are homozygous for a certain
recessive allele, a. Assume the population is
at equilibrium.
Question 4: What percentage of the
population are heterozygous for this trait?
2pq = 2(0.40)(0.60)
2pq = 0.48 = 48 %
Assume a population in which 36% of the
population are homozygous for a certain
recessive allele, a. Assume the population is at
equilibrium.
Question 5: Why do we have to start the problem
with the percentage of the homozygous recessive
in the population?
It is not possible to tell the homozygous
dominant (AA) from the heterozygous (Aa) by
examining the phenotype.
Trolls are mythical creatures that live beneath
flower gardens.
Of the 100 trolls in a population, 91 have pink
hair(T) and 9 have orange hair(t).
Assuming genetic equilibrium:
What are the gene frequencies of T and t?
What are the genotypic frequencies?