Lecture 9 - Learning Objectives 1 1 2 3 4 5 6 Explain the...

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Learning Objectives 1. Explain the difference between linkage equilibrium and linkage disequilibrium 2. Determine if a population is in linkage disequilibrium given allele and haplotype frequencies 3. Explain the role of sex in restoring linkage equilibrium 4. Explain why an asexual mutant in a sexual population should, theoretically, have a reproductive advantage 5. Explain how sex speeds up evolution and why this might be favourable 6. Explain the Muller’s ratchet hypothesis for the disadvantage of asexuality 1
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Lecture 9: Linkage and Sex Suggested reading: 8.1 Evolution at two loci: Linkage equilibrium and linkage disequilibrium 8.3 The adaptive significance of sex Friday, Jan.24 2
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We have taken a first pass at Evolution (by mutation , selection , migration, inbreeding and drift ) Δ genotype freq. allows selection on recessive alleles Δ allele freq. and so… Δ genotype freq Δ allele freq. Δ genotype freq. Δ allele freq. Δ genotype freq. (at least initially) Slow change Magnifies mutations 3
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It’s time to scale up to more than one locus! Evolution at two loci: Linkage disequilibrium and HWE Linkage and sex Slight detour, WHY SEX? Evolution at multiple loci: Quantitative genetics TERMS: LINKAGE EQUILIBRIUM- Loci are inherited independently LINKAGE DISEQUILIBRIUM- Loci are inherited together (as a group) 4
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parent 2n = 4 Consider meiosis in a diploid cell with 2 pairs of chromosomes A B a b gametes n = 2 From a single cell, each combination is equally likely. 5
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A B a b Consider a population with the allele frequencies: ( f )A = 0.7 ( f )B = 0.8 ( f )a = 0.3 ( f )b = 0.2 If HWE, ( f )AB = ( f )A x ( f )B The frequency of each combination (haplotype) in the population depends on the frequency of alleles at each locus ( f )AB + ( f )Ab + ( f )aB + ( f )ab = 1 ( f )AB = ( f )Ab = ( f )aB = ( f )ab = 6
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But what if those two alleles are on the same chromosome and are close together? They are more likely to be inherited as a pair Haplotypes are allele combinations (e.g., AB or ab) We now consider 2 situations for the relationship between haplotype frequencies and allele frequencies A B a b 7
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Consider a population with the allele frequencies: ( f )A = 0.6 ( f )B = 0.8 ( f )a = 0.4 ( f )b = 0.2 ( f )AB = ( f )Ab = ( f )ab = ( f )aB = And chromosome frequencies: ( f )AB = ( f )A x ( f )B ( f )Ab = ( f )A x ( f )b ( f )aB = ( f )a x ( f )B ( f )ab = ( f )a x ( f )b Situation #1: The alleles are in LINKAGE EQUILIBRIUM See Fig. 8.2 (8.2 & 8.3 in 5th ed.) 8
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