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Option D: Evolution D4: The Hardy- Weinberg Principle.

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Presentation on theme: "Option D: Evolution D4: The Hardy- Weinberg Principle."— Presentation transcript:

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2 Option D: Evolution D4: The Hardy- Weinberg Principle

3 Population Genetics = Foundation for studying evolution Darwin’s could not explain how inherited variations are maintained in populations - not “trait blending” A few years after Darwin’s “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.

4 Hardy-Weinberg Theorem: Frequencies of alleles & genotypes in a population’s 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.

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:

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.

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.

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.

9 What are the genotype frequencies? q 2 (bb): 16/100 = q (b): √.16 = p (B): = 0.6 q 2 (bb): 16/100 = q (b): √.16 = p (B): = 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.

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.

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

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

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 =  747 = 0.31 AB 2pq =  747 = 0.52 BB q 2 =  747 = 0.17

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

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…

16 Malaria Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells 1 2 3

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

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.

19 If any of the Hardy-Weinberg conditions are not met  microevolution occurs Microevolution = generation to generation change in a population’s allele frequencies D 4.3 State the assumptions made when the Hardy-Weinberg equation is used.

20 Main Causes of Microevolution Mutation Gene Flow Genetic DriftSelection Non-random mating

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

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

23 Human evolution today Gene flow in human populations is increasing today ▫transferring alleles between populations Are we moving towards a blended world?

24 3. Non-random mating Sexual selection

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

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

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

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

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

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


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