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CSS 650 Advanced Plant Breeding Module 3: Changes in gene frequency due to selection Systematic changes in gene frequency • Predictable in both magnitude and direction – Migration – Mutation – Selection • Quantities of interest – Δq = q1 – q0 – qt – q at equilibrium (when Δq = 0) Falconer & Mackay, Chapter 2 Selection Individuals differ in their contribution of genes to the next generation fitness = adaptive value = selective value Types of selection • Viability - the probability of survival through each reproductive stage – Darwin’s natural selection • Fertility – number of offspring produced – may work in opposition to survival – Darwin’s sexual selection Components of fitness Zygote viability (survival) Adult Artificial selection mating success fertility (capability to have offspring) fecundity (#gametes) Gamete viability Zygote • fitness is defined in the context of the environment in which individuals live, mate, and reproduce Relative fitness • • Absolute fitness refers to the number of gametes transmitted by a zygote, but this is seldom observed directly We generally make observations on zygotes, so we measure relative fitness in comparison to either the average for the population or the best genotype Genotype Fitness A1 A1 A 1A 2 A 2A 2 W11 1 W11 W12 1 hs W11 W22 1 s W11 s = coefficient of selection Selection • To quantify fitness, we have to consider the degree of dominance with respect to fitness – may not be the same as the dominance with respect to the expression of the primary trait controlled by the gene Degree of Dominance for Fitness A2A2 A1A2 A1A1 1-s 1-(1/2)s 1 no dominance A2A2 A1A2 A1A1 1-s 1-hs 1 partial dominance complete dominance A2A2 A1A2 A1A1 1-s 1 A2A2 A1A1 1-s2 1-s1 A1A2 overdominance fitness 1 Selection against a recessive gene Genotype Initial frequencies Fitness From gametes A 1A 1 p2 1 p2 A 1A 2 2pq 1 2pq A 2A 2 q2 1-s q2(1-s) Total 1 1-sq2 q 2 1 s pq q 2 1 s 1 q q q sq 2 q1 2 2 1 sq 1 sq 1 sq 2 q sq 2 sq 2 (1 q ) q q1 q q 2 1 sq 1 sq 2 Genetic load = proportion of the population that does not reproduce due to presence of a gene i.e., the reduction in fitness Selection against a recessive lethal - example • What will the frequency of the recessive allele be after t generations? s=1 q0 q sq 2 q q 2 q(1 q ) q1 2 2 1 sq 1 q (1 q )(1 q ) 1 q0 q0 qt 1 tq0 1 1 t qt q0 1 1 to go from q=0.20 0.01 t 105 generations 0.01 0.20 What could a breeder do to speed up the process? Change in gene frequency with selection no dominance partial dominance complete dominance of A1 selection against A2 complete dominance of A1 selection against A1 overdominance Falconer & Mackay, pg 28 Change of gene frequency due to selection Complete Dominance s=0.20 (-)=selection against A2 (+)=selection for A2 Falconer & Mackay, pg 30 q Direction of selection http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/E/Evolution.html#Fitness