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Lecture3_2009 - Why do we care about variation Variation...

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Variation & Inheritance BIO 370 S. Hedtke September 3 2009 Why do we care about variation? For a population to evolve, it must have variation in inherited traits. If every individual is identical for a particular trait, then the population isn ! t going to change from generation to generation! **Population: a group of interbreeding individuals and their offspring Inheritance in Darwin ! s Day “Blended inheritance” (or, more informally, “just plain wrong” inheritance): Organisms inherit half of a trait value from mom and half from dad. The values for each trait get BLENDED together in the offspring. Expectation is that over time, there will be no variation. But we OBSERVE variation in nature. Darwin spends an entire CHAPTER on it. Some of this variation is heritable, because the trait is coded for by genes. Variation at the protein level Polymorphisms detected by electrophoresis Proteins separated by charge and size. ** Polymorphism : when more than one type (allele, phenotype, etc.) is found within a population.
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15-60 % of all genetic loci in a typical population are polymorphic (2 or more alleles/locus) 4%-20% of individuals are heterozygotes ** Heterozygote : at a particular locus, an individual has two different genetic variants. ** Homozygote: at a particular locus, an individual has only one genetic variant. Variation is the norm Variation at the level of DNA Natural history of Drosophila and alcohol Drosophila melanogaster , the common fruit fly, is found throughout the world and feeds on rotten fruit There is genetic variation for the efficiency of alcohol metabolism in Drosophila Alcohol metabolism in the fly is controlled by many genes, of which one is Alcohol dehydrogenase Marty Kreitman Adh, Drosophila melanogaster Positions of Adh nucleotide variation in 11 samples 26/1857 variable sites in introns (non-transcribed) 2.4% 14/765 variable sites in exons (coding regions) 1.8% 13/14 changes were synomymous Variation is the norm: In coding regions: ~1% of the nucleotides differ between randomly chosen individuals Most are silent (synonymous) changes In non-coding regions: 2-5 times more variation BIG Picture: there is usually more variation at DNA sites that do not affect the phenotype Human genetic variation: Most humans differ by about 1 in 1000 bp » 6 x 10 6 bp that are different Mitochondrial DNA, Y chromosome, nuclear DNA 85% of the variation among individuals 15% among “races” or populations on earth
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Human genetic variation: How is human genetic variation partitioned? The data: 1056 individuals sampled from 52 populations 377 autosomal microsatellite loci analyzed Results: ~4% of variation corresponds to major geographical “races” (Americas, Africa, Oceania, Mideast, Europe) ~96% of all variation found within each “race” We are genetically very homogeneous and form a single evolutionary lineage.
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