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Lecture 4


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Howard Rundle

find more resources at Short Introduction to Genetic Variation Learning outcomes  By the end of this section you should: o Understand why genetic variation is important o Be comfortable with simple terminology (e.g. loci, gene, allele, mono and polymorphic, identify by state and descent) o Be familiar with two different ways of quantifying genetic variation (prop. of polymorphic loci and average heterozygosity) o Be able to calculate genotype frequencies from absolute numbers of different genotypes, and allele frequencies from genotype frequencies. Population genetics considers the processes that cause changes in allele and/or genotype frequencies in populations:  natural selection, genetic drift, Mutation, Recombination, gene flow/migration, inbreeding/assortative mating o Aside: what is a population?  consists of members of the same species  are usually geographically continuous  any individual can potentially mate with any other individual in the population  can be studied as a unit;  for example, you can study the size of a population, or the frequency of a particular allele in that population Genetic Variation  Most of the processes that cause changes in allele and/or genotype frequencies in populations require genetic variation to have any affect: o natural selection, genetic drift, recombination, gene flow/migration, Inbreeding/assortative mating.  Example: natural selection o Natural selection in the differential survival and/or reproductive success of individuals differing in phenotype, and is the only evolution process that produces adaptation. o Imagine a phenotypically homogeneous population.  Can natural selection occur in this population?  Natural selection works with DNA changes that are already present. Population Genetics  Seeks to understand the structure and dynamics of naturally occurring genetic variation both within and among populations/species Definitions:  Gene – a sequence of DNA that codes for a protein find more resources at find more resources at  Locus – location on a chromosome, may be coding or noncoding.  Allele – the alternative form(s) of DNA at a particular locus  Monomorphic locus – only one allele for this locus in the population  Polymorphic/segregating locus – a locus with multiple alleles in the population Two ways in which alleles at the same locus can differ: 1. By descent a. If alleles were inherited from a common ancestor in the recent past (i.e. they are copies of a single ancestral allele), they are termed autozygous b. If alleles were NOT inherited from a common ancestor in the recent past, they are termed allozygous ▯▯allo▯ ▯ea▯s ▯other▯▯ 2. By state a. Alleles with different DNA (protein) sequences differ in state. b. c. d. Genotype and allele frequencies  Let▯s call our two alleles 1 and A 2 o Three possible (combinations) genotypes exist, A A , 1 A1, 1nd2A A . 2 2  Of course, A 1 2nd A A 2r1 the same genotype.
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