Chapter 25__upload_Genetics of Populations

Frequencies p q we ended generation 2 with the same

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Unformatted text preview: e population is in H-W equilibrium – Ha: the population is not in H-W equilibrium – Use a χ2 test to determine if the population is significantly different from H-W expectations – If it is, one (or more) of the assumptions must be violated! More uses of HWE More 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 chondrodystrophy. 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 Estimate 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 Equilibrium Equilibrium 1. 2. 3. 4. 5. 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 heterozygotes Migration – Increase heterozygotes Mutation – Increases heterozygotes NonRandom mating: Inbreeding – reduced number of heterozygotes Assortative mating – reduced number of heterozygotes What causes deviations from HWE? Lets revisit the primary microevolutionary processes: Natural Selection Mutation Migration 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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This document was uploaded on 09/17/2013.

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