Download Hardy -- Weinberg

Hardy -- Weinberg
When will a population evolve?
The Hardy-Weinberg principle states that the
frequencies of alleles and genotypes in a population’s
gene pool remain constant over the generations unless
acted upon by agents other than recombination from
sexual reproduction; populations do NOT evolve, unless
they are NOT in H-W equilibrium
H-W equilibrium conditions
and mechanisms of change
The Mechanisms of Change are
the agents of evolutionary change
Gene Flow
Mutation
Non-random mating
Selection
Genetic Drift
Populations & gene pools
 Concepts
 a population is a localized group of interbreeding
individuals
 The gene pool is collection of alleles in the population


remember difference between alleles & genes!
allele frequency is how common an allele is in the
population

Ex: 70% of individuals carry at least one dominant allele
Evolution of populations
 Evolution = change in allele frequencies in a
population


hypothetical: what conditions would cause allele
frequencies to not change?
Let’s imagine a non-evolving population
To stop evolution, REMOVE all agents of change
very large population size (no genetic drift)
2. no migration (no gene flow in or out)
3. no mutation (no genetic change)
4. random mating (no sexual selection)
5. no natural selection (everyone is equally fit)
*This is Hardy-Weinberg equilibrium*
1.
Hardy-Weinberg theorem
 Counting total Alleles (regardless of genotype)
 assume 2 alleles = B, b
 frequency of dominant allele = p
 frequency of recessive allele = q

frequencies must add to 1 (100%), since all individuals
have one or both so:
p+q=1
BB
Bb
bb
Hardy-Weinberg theorem
 Counting Individual genotypes
frequency of homozygous dominant genotype: p x p = p2
 frequency of homozygous recessive genotype: q x q = q2
 frequency of heterozygotes: (p x q) + (q x p) = 2pq


Again, frequencies of all individuals must add to 1 (100%), so:
p2 + 2pq + q2 = 1
BB
Bb
bb
NOTICE THE DIFFERENCE!!!
 Alleles:
p+q=1
B
 Genotypes:
p2 + 2pq + q2 = 1
BB
BB
b
Bb
Bb
bb
bb
Using Hardy-Weinberg equation
Population of 100 cats.
Black is dominant to
q2 (bb): 16/100 =
white in fur color. 84 are
.16
black and 16 are white.
q (b): √.16 = 0.4
Can I determine the
p (B): 1 - 0.4 = 0.6
number of each
genotype?
p2=.36
BB
2pq=.48
Bb
q2=.16
bb
Must assume population is in H-W
What
are the genotype frequencies?
equilibrium!
Online Practice
 http://bio1151.nicerweb.com/med/QUIZ/hw_q.html
 http://nhscience.lonestar.edu/biol/hwe.html
 http://www.hinsdale86.org/staff/kgabric/HWanswe
rs.htm
 http://natureboy.uta.edu/hwprobs.htm
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