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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 l.jpg

Tracing Human Evolution with Genetics

SELECTION

June 9-17, 2007


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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


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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


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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.


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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


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Lactase Persistence and Pastoralism

  • Positive selective pressure

    • Liquid

    • Protein

  • Subsequent migration and spread of phenotype

    • Northern Europeans to North America

    • Southern migrations through Africa


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Lactase Persistence

  • Lactose malabsorption

  • Lactose tolerance

  • Lactase Persistence

    • Continued expression of lactase-phlorizin hydrolase (LPH) in mammals past weaning


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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


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Genetics of Lactase Persistence

  • Northern Europeans

    • SNP identified

    • Enhanced expression of lactase gene

  • Africans

    • Not the same SNP


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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


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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


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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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