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Tracing Human Evolution with Genetics

Tracing Human Evolution with Genetics SELECTION June 9-17, 2007 SNPs Single Nucleotide Polymorphisms May or may not be in coding regions May or may not cause phenotypic changes Frequency of SNP distribution varies Seq 1 ATCGG AT C CA TG T AT CGATT Seq 2 AT G GG ATGCA TG T AT CGATT

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Tracing Human Evolution with Genetics

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  1. Tracing Human Evolution with Genetics SELECTION June 9-17, 2007

  2. SNPs • Single Nucleotide Polymorphisms • May or may not be in coding regions • May or may not cause phenotypic changes • Frequency of SNP distribution varies Seq 1 ATCGG ATCCA TGTAT CGATT Seq 2 ATGGG ATGCA TGTAT CGATT Seq 3 ATCGG ATGCA TGAAA CGATT

  3. Haplotype • Refers to either: • Genetic makeup of one set of chromosomes • An area of a chromosome defined by a set of associated SNPs • Based on statistical analysis and measurement of linkage disequilibrium (LD) • Sources of LD • Recombination • Genetic linkage • Random drift • Non-random mating • Interactions between genes • Population structure

  4. Important points… • Correlation of a SNP and a phenotype is just that – a correlation, not necessarily a cause. • Haplotypes often identify genes involved in polygenic traits. • No single site controls the phenotype. • Quantitative trait loci (QTLs) are genetic areas involved in modulating expression of polygenic traits.

  5. Haplotypes and Evolution • Recent human evolution is visible in the genome as “selective sweeps”. • Selective sweeps are identified based on LD and haplotypes. • Articles: • Localizing Recent Adaptive Evolution in the Human Genome • Convergent adaptation of human lactase persistence in Africa and Europe

  6. Lactase Persistence and Pastoralism • Positive selective pressure • Liquid • Protein • Subsequent migration and spread of phenotype • Northern Europeans to North America • Southern migrations through Africa

  7. Lactase Persistence • Lactose malabsorption • Lactose tolerance • Lactase Persistence • Continued expression of lactase-phlorizin hydrolase (LPH) in mammals past weaning

  8. Global Distribution of Lactase Persistence • Europeans • High levels in Scandinavians • Decreasing levels further south in Europe • Asian • Generally low levels in tested populations • High in Khazaks • African • High in Tutsi and Fulani • Low in other groups

  9. Genetics of Lactase Persistence • Northern Europeans • SNP identified • Enhanced expression of lactase gene • Africans • Not the same SNP

  10. Lactose Tolerance Test • Fast for 8-12 hours • Ingest 50g lactose • Take blood samples for two hours and test for a rise in blood glucose levels • Caveats: • Fasting? • Field conditions: Used finger pricks and strips for monitoring diabetes

  11. Evolutionary Medicine • Many common medical issues are polygenic • Traditionally required a large affected family to identify candidate genes • Genome Wide Association (GWA) Articles • Genome-wide association study of 14,000 cases of seven common diseases and 3,000 shared controls. • Guilt by association

  12. Case • A medical researcher is interested in the underlying causes of type II diabetes. Specifically, why do different people have different tendencies to develop diabetes? Obviously current lifestyle will have a major impact, but lifestyle is not a complete explanation. What about genetic history? Is there a way to use tools such as the HapMap or genome-wide association surveys to predict risk for populations and individuals? • How might this be useful for helping an American of mixed ancestry understand their risk for developing diabetes? • Would it be useful for a Han Chinese person? • What are the ethical considerations of collecting and using this kind of information?

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