08-27 - BIO 5099: Molecular Biology for Computer Scientists...

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BIO 5099: Molecular Biology for Computer Scientists (et al) Lecture 3: More Evolution, Geologic time, and Mass Extinctions http://compbio.uchsc.edu/hunter/bio5099 Larry.Hunter@uchsc.edu Housekeeping We will delay answers and discussion of the problem set until Thursday so we can get a sense of how everyone did. The controversy about the Peppered Moth story turns out to be about the proposed mechanism of selection (differential predation by birds), not about the selection itself. More detail in a link on the course web site.
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More Housekeeping (and a new term) In the example of balanced polymorphism (sickle cell & malaria resistance), someone asked if sickle cell was really recessive, since it has an effect on phenotype in heterozygotes Answer is a new term: pleiotropy . A pleiotropic gene is one that effects more than one phenotype. In this case the two traits are sickling (recessive) and malaria resistance (dominant). Population Genetics One of the few places where math models were important in pre-molecular biology. What are the dynamics of allele frequencies in a population? Great mathematicians of statistical / population genetics: Haldane (effect of selection on allele frequencies) Fisher (key to the Modern Synthesis, invented much of modern statistics for his studies of plant breeding) Sewell Wright (genetic drift, fitness landscape)
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Hardy-Weinberg Equations Allele frequency equilibrium (no change over time) is reached when: random mating of an infinite population, no migration no mutation no natural selection Let p f(A) and q f(a), with p+q=1, then f(AA) = p 2 , f(Aa)=2pq and f(aa)=q 2 genotype frequency tells us allele frequency p & q are constants; rare genotypes don't die out! f A =1 ± f aa What happens when those assumptions fail? The rest of population genetics Finite populations Genetic Drift: frequencies change due to finite sampling Founder effect & bottlenecks: extreme examples of genetic drift where a population begins with a small number of individuals + inbreeding among descendents Non-random mating: Inbreeding Assortive mating (genotype influences mate selection) Gene Flow and Mutation Selection pressure
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Inbreeding Mating of close relatives: a form of non-random mating. Allele frequencies don't change(without other pressures), but genotype frequencies do F probability that a two alleles in one individual are identical by descent. F=1 is completely homozygous; F = 0 is complete heterozygous Extreme example: self-fertilization. Self fertilization
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08-27 - BIO 5099: Molecular Biology for Computer Scientists...

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