Unformatted text preview: e population is in H-W equilibrium
– Ha: the population is not in H-W
– Use a χ2 test to determine if the
population is significantly different from
– If it is, one (or more) of the assumptions
must be violated! More uses of HWE
1) Genes with three alleles
2) Using HWE to estimate the allele
frequency for a recessive allele What if three alleles?
What HWE still works fine!
p = f(A1)
q = f(A2)
r = f(A3) p+q+r=1
(p + q + r)2 = 12
p2 +2pq + q2 + 2pr + 2qr + r2 = 1
f(A1A1) + f(A1A2) + f(A2A2) + f(A1A3) + f(A2A3) +
f(A3A3) = 1 Estimating Allele frequencies –
when dominance using HWE
Cannot allele count at a locus showing dominance:
- if we assume HWE we can still estimate the allele frequencies
at the locus. e.g. chondrodystrophic dwarfism in the California
condor. Caused by homozygosity for a recessive
allele. Out of 169 hatched eggs, 5 exhibited
What information does this give us??? Cannot allele count at a locus showing dominance: chondrodystrophic dwarfism in the California condor.
Caused by homozygosity for a recessive allele. Out of
169 hatched eggs, 5 exhibited chondrodystrophy. Estimate Frequency of Carriers
Frequency of carriers: Violations of H-W assumptions
We would expect frequency of deleterious disease to be low
because of Natural Selection:
How does Natural Selection influence HWE? Assumptions of H-W
6. Random mating – panmixia Infinitely large population
No gene flow – genes are not added from outside the population
No mutation – genes do not change from one allelic state to another
No natural selection – all individuals have equal probabilities of survival and reproduction
Discrete generations – offspring do not mate with previous generations What happens if the
assumptions are violated?
assumptions Evolution! Evolution (in terms of HW) is change in allele frequency over time. Allele frequencies do not change in a population in HW equilibrium, therefore: A population in HW equilibrium is not evolving. What causes deviations from HWE?
Genetic Drift – increase or decrease heterozygotes
Natural Selection – increase or decrease
Migration – Increase heterozygotes
Mutation – Increases heterozygotes
Inbreeding – reduced number of heterozygotes
Assortative mating – reduced number of
heterozygotes What causes deviations from HWE?
Lets revisit the primary microevolutionary processes:
Genetic Drift Natural Selection Empirical example with Drosophila • Two alleles for alcohol dehydrogenase locus
AdhF and AdhS Break down alcohol at different rates • Experiment maintained two populations of flies spiked with alcohol and two controls without alcohol
• At each generation took random sample of flies and determined their genotypes Selection
Selection Empirical example with Drosophila • Control populations appeared to be in Hardy
Weinberg Equilibrium Allele frequencies did not change • Populations under se...
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