Option D: Evolution D4: The Hardy- Weinberg Principle

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Transcript of Option D: Evolution D4: The Hardy- Weinberg Principle

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Slide 2 Option D: Evolution D4: The Hardy- Weinberg Principle Slide 3 Population Genetics = Foundation for studying evolution Darwins could not explain how inherited variations are maintained in populations - not trait blending A few years after Darwins Origin of Species, Gregor Mendel proposed his hypothesis of inheritance: Parents pass on discrete heritable units (genes) that retain their identities in offspring D 4.1 Explain how the Hardy-Weinberg equation is derived. Slide 4 Hardy-Weinberg Theorem: Frequencies of alleles & genotypes in a populations gene pool remain constant from generation to generation unless acted upon by agents other than sexual recombination (gene shuffling in meiosis) Equilibrium = allele and genotype frequencies remain constant D 4.1 Explain how the Hardy-Weinberg equation is derived. Slide 5 Hypothetical, non-evolving population preserves allele frequencies Serves as a model (null hypothesis) natural populations rarely in H-W equilibrium useful model to measure if forces are acting on a population measuring evolutionary change W. Weinberg physician G.H. Hardy mathematician D 4.1 Explain how the Hardy-Weinberg equation is derived. Hardy-Weinberg Theorem: Slide 6 Hardy-Weinberg theorem Counting Alleles assume 2 alleles = B, b frequency of dominant allele (B) = p frequency of recessive allele (b) = q frequencies must add to 1 (100%), so: p + q = 1 bbBbBB D 4.1 Explain how the Hardy-Weinberg equation is derived. Slide 7 Counting Individuals frequency of homozygous dominant: p x p = p 2 frequency of homozygous recessive: q x q = q 2 frequency of heterozygotes: (p x q) + (q x p) = 2pq frequencies of all individuals must add to 1 (100%), so: p 2 + 2pq + q 2 = 1 bbBbBB Hardy-Weinberg theorem D 4.1 Explain how the Hardy-Weinberg equation is derived. Slide 8 Alleles:p + q = 1 Individuals:p 2 + 2pq + q 2 = 1 bbBbBB BbBbbb Hardy-Weinberg theorem D 4.1 Explain how the Hardy-Weinberg equation is derived. Slide 9 What are the genotype frequencies? q 2 (bb): 16/100 =.16 0.4 q (b): .16 = 0.4 0.6 p (B): 1 - 0.4 = 0.6 q 2 (bb): 16/100 =.16 0.4 q (b): .16 = 0.4 0.6 p (B): 1 - 0.4 = 0.6 population: 100 cats 84 black, 16 white How many of each genotype? population: 100 cats 84 black, 16 white How many of each genotype? bbBbBB p 2 =.36 2pq=.48 q 2 =.16 Must assume population is in H-W equilibrium! D 4.2 Calculate allele, genotype and phenotype frequencies for two alleles of a gene, using the Hardy-Weinberg equation. Slide 10 bbBbBB p 2 =.36 2pq=.48 q 2 =.16 Assuming H-W equilibrium Sampled data bbBbBB p 2 =.74 2pq=.10 q 2 =.16 How do you explain the data? p 2 =.20 2pq=.64 q 2 =.16 How do you explain the data? Null hypothesis D 4.2 Calculate allele, genotype and phenotype frequencies for two alleles of a gene, using the Hardy-Weinberg equation. Slide 11 Using the calculated gene frequency to predict the EXPECTED genotypic frequencies in the NEXT generation OR to verify that the PRESENT population is in genetic equilibrium Slide 12 D 4.2 Calculate allele, genotype and phenotype frequencies for two alleles of a gene, using the Hardy-Weinberg equation. BB 0.18AB 0.25 AA 0.32 B 0.43 A 0.57 B 0.43A 0.57 Assuming all the individuals mate randomly SPERMS EGGS p*p= p 2 p*q q*q= q 2 Slide 13 D 4.2 Calculate allele, genotype and phenotype frequencies for two alleles of a gene, using the Hardy-Weinberg equation. Close enough for us to assume genetic equilibrium GenotypesExpected frequencies Observed frequencies AA p 2 = 0.32 233 747 = 0.31 AB 2pq =0.50 385 747 = 0.52 BB q 2 =0.18 129 747 = 0.17 Slide 14 Application of H-W principle Sickle cell anemia inherit a mutation in gene coding for hemoglobin oxygen-carrying blood protein recessive allele = H s H s normal allele = H b low oxygen levels causes RBC to sickle breakdown of RBC clogging small blood vessels damage to organs often lethal Slide 15 Sickle cell frequency High frequency of heterozygotes 1 in 5 in Central Africans = H b H s unusual for allele with severe detrimental effects in homozygotes 1 in 100 = H s H s usually die before reproductive age Why is the H s allele maintained at such high levels in African populations? Suggests some selective advantage of being heterozygous Slide 16 Malaria Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells 1 2 3 Slide 17 Heterozygote Advantage In tropical Africa, where malaria is common: homozygous dominant (normal) die or reduced reproduction from malaria: H b H b homozygous recessive die or reduced reproduction from sickle cell anemia: H s H s heterozygote carriers are relatively free of both: H b H s survive & reproduce more, more common in population Hypothesis: In malaria-infected cells, the O 2 level is lowered enough to cause sickling which kills the cell & destroys the parasite. Hypothesis: In malaria-infected cells, the O 2 level is lowered enough to cause sickling which kills the cell & destroys the parasite. Frequency of sickle cell allele & distribution of malaria Slide 18 Conditions for Hardy-Weinberg Equilibrium: Hardy-Weinberg Theorem describes a non-evolving population. 1.Extremely large population size (no genetic drift). 2.No gene flow (isolation from other populations). 3.No mutations. 4.Random mating (no sexual selection). 5.No natural selection. D 4.3 State the assumptions made when the Hardy-Weinberg equation is used. Slide 19 If any of the Hardy-Weinberg conditions are not met microevolution occurs Microevolution = generation to generation change in a populations allele frequencies D 4.3 State the assumptions made when the Hardy-Weinberg equation is used. Slide 20 Main Causes of Microevolution Mutation Gene Flow Genetic DriftSelection Non-random mating Slide 21 1. Mutation & Variation Mutation creates variation new mutations are constantly appearing Mutation changes DNA sequence changes amino acid sequence? changes protein? changes structure? changes function? changes in protein may change phenotype & therefore change fitness Slide 22 2. Gene Flow Movement of individuals & alleles in & out of populations seed & pollen distribution by wind & insect migration of animals sub-populations may have different allele frequencies causes genetic mixing across regions reduce differences between populations Slide 23 Human evolution today Gene flow in human populations is increasing today transferring alleles between populations Are we moving towards a blended world? Slide 24 3. Non-random mating Sexual selection Slide 25 Warbler finch Tree finches Ground finches 4. Genetic drift Effect of chance events founder effect small group splinters off & starts a new colony bottleneck some factor (disaster) reduces population to small number & then population recovers & expands again Slide 26 Founder effect When a new population is started by only a few individuals some rare alleles may be at high frequency; others may be missing skew the gene pool of new population human populations that started from small group of colonists example: colonization of New World Slide 27 Bottleneck effect When large population is drastically reduced by a disaster famine, natural disaster, loss of habitat loss of variation by chance event alleles lost from gene pool not due to fitness narrows the gene pool Slide 28 Cheetahs All cheetahs share a small number of alleles less than 1% diversity as if all cheetahs are identical twins 2 bottlenecks 10,000 years ago Ice Age last 100 years poaching & loss of habitat Slide 29 Conservation issues Bottlenecking is an important concept in conservation biology of endangered species loss of alleles from gene pool reduces variation reduces adaptability Breeding programs must consciously outcross Peregrine Falcon Golden Lion Tamarin Slide 30 5. Natural selection Differential survival & reproduction due to changing environmental conditions climate change food source availability predators, parasites, diseases toxins combinations of alleles that provide fitness increase in the population adaptive evolutionary change