Chapter 17-Population Genetics Notes from 2/16/2009 Darwin?s dilemma ? ?reversion to the average? Variation becomes less longer a population goes on Even if phenotypic variation was reduced, genetic variation was not necessarily lost during reproduction Triumph of Mendel :Variation was preserved Can dominance form a frequency change? A dominant allele will still be in same frequency in next generation with nothing affecting it. (so will recessive) Hardy and Weinberg showed that gene expression does not affect gene frequencies in populations Hardy-Weinberg Equilibrium frequencies p^2 : 2pq : q^2 Genotypic ratio p^2 + 2pq + q^2 = 1 Genotypic frequencies p + q = 1 Allele frequencies Ratios stayed no matter how many alleles Null hypothesis Verify equal segregation and/or random assortment If not, find what is causing the deviations to help determine reason of phenomena HW theorem shows that when frequency of one allele is really rare, then it is usually found only in heterozygotes HW prediction of a population at equilibrium Pop is large Mating is random No inbreeding No pop division No natural selection No Mutation No Migration Two types of non-random mating Within populations Inbreeding Positive assortative mating = mating with like partners Negative assortative mating = mating with unlike partners Assortative mating usually connected to one or a few traits Outside populations Notes from 2/18/2009 Inbreeding increases chances of getting rare homozygous diseases Out breeding chance of getting a disease is one in a million in inbreeding chance is 1/16 if parent was a carrier Heterozygotes will decrease in frequency each generation, variation is lost over time Deficit of heterozygotes is called inbreeding coefficient, F Causes evolution but not the population genetics meaning of evolution? Evolution = a change in allele frequency Allele frequencies do not change but genotypic do Causes phenotypic variation to be released Rare alleles have higher probability of being brought together New combinations are also becoming apparent No new variation, just hidden variation Mutation point mutation = one base pair substitution mutations in population of N individuals are rare weak force that influences allele frequencies Population genetics is concerned with what happens after a mutation Variation will not be lost unless some force is acting on it Mutation ? weak force Inbreeding ? changes genotypic frequencies not allele Assortative mating ? changes genotypic frequencies but only at loci associated with + or ? assortative mating Variation from recombination 1. Mutation introduces new allele After generations second mutation occurs 2. Mutation introduces new allele at linked locus With recombination new gamete type forms 3. Recombination results in new haplotype Produces variation much faster than mutation Recombination eventually randomize the association of a new mutation with alleles at other gene loci Linkage equilibrium genes appear randomly associated takes a long time to reach Linkage disequilibrium genes non-randomly associated tightly linked genes may be found together more often than expected for a long time Migration Any form of the introduction of genes from one population into another Homogenizes allele frequencies
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