Evolution Test 3 study guide
Lecture 13: Multiple Loci
The mathematics of multiple loci is the same as single loci but with more combinations
When many loci and alleles affect a trait, there will be hidden variation
There are so many possible combinations of alleles with multiple loci that variation becomes continuous and is best described statistically
Basic principles
Two loci in a population are in linkage equilibrium when the genotype of a chromosome at one locus is independent of its genotype at the other locus
Knowing the genotype of the chromosome at one locus is of no use at all in predicting genotype at the other
Polygenic variation (two or more loci affecting one trait)
Pleiotropy (one locus affecting more than one trait) genotype (e.g., A1B1/A2B2 or AB/ab)
Genotype frequencies in a two locus system : ( table should be filled out on handout)
Hidden variation
With 1 locus, 2 alleles, there are 3 genotypes and up to 3 phenotypes
With 2 loci, 2 alleles each, there are 9 genotypes but (if of equal effect) only 5 phenotypes
There may be more genetic variation possible than is expressed in a population: If n loci with 2 alleles, # genotypes = 3n, # phenotypes = 2n+1
Continuous traits and variance
When a trait is determined by a small number of genes/alleles, the phenotypes fall into a small number of discrete categories
With a large number of underlying genes, the variation becomes continuous
Quick review of statistics: mean ( x ), variance (V), and regression Variance (V = σ2)  population measure, not applicable to individuals The variance of a trait is the sum of the variances of underlying causes
Genetic variance key measure of evolutionary potential (the raw material for selection and drift to act upon) (look at formulas on handout)
Selection at two loci (that are epistatic for fitness) and its effect on linkage disequilibrium
Correcting the model of linkage disequilibrium to account for selection (i.e., for differences in the fitnesses of different haplotypes If specific interactions between alleles at different loci result in different fitnesses, there is "epistasis for fitness", and selection will affect haplotype frequencies
Other effects on linkage disequilibrium Note that genetic drift can also cause linkage disequilibrium (one allele combination may drift to excess or fixation, and founder populations are likely to have high |D| linked locus under strong selection will often show linkage disequilibrium Examples of selection and disequilibrium in natural populations:
Closely linked genes that contribute to the same character or function often show strong linkage disequilibrium (and coadaptation between particular alleles at these different loci may result)

For example: Batesian mimicry

Heterostyly
Selection on quantitative traits: heritability and response to selection
Heritability is the fraction of the phenotypic variance that is due to the additive component of genetic variation
Lecture 14: Quantitative genetics
Quantitative genetics provides a framework to study the evolution of polygenic traits
Quantitative traits distribution of phenotypes is continuous rather than discrete
Determined by: genotype at different loci and environment
Consistent with medelian genetics
Influenced by combined effects of the genotype at many loci
Phenotypic variation the sum of genetic and environmental variation
The selection differential and the selection gradient are two related ways to measure the strength of selection
The response to selection is the product of the strength of selection and the heritability
Observed rates of evolution (and strength of selection) can be very high, but long term evolution is usually much less
The complex relationship between genotype and phenotype (e.g., pleiotropy) results in correlations between characters
Genetic correlations can significantly affect the rate and direction of evolution
Why quantitative genetics?
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