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Short Introduction to Genetic Variation
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)
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
natural selection, genetic drift, Mutation, Recombination, gene flow/migration,
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
Most of the processes that cause changes in allele and/or genotype frequencies in populations
require genetic variation to have any affect:
natural selection, genetic drift, recombination, gene flow/migration, Inbreeding/assortative
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.
Seeks to understand the structure and dynamics of naturally occurring genetic variation both
within and among populations/species
Gene – a sequence of DNA that codes for a protein
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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.
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.