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Population Genetics

Population Genetics. What is Population Genetics?. The genetic study of the process of natural selection. (The study of the change of allele frequencies, genotype frequencies, and phenotype frequencies) . What is Natural Selection?. Natural selection causes changes in a population if

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Population Genetics

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  1. Population Genetics

  2. What is Population Genetics? • The genetic study of the process of natural selection. (The study of the change of allele frequencies, genotype frequencies, and phenotype frequencies) 

  3. What is Natural Selection? • Natural selection causes changes in a population if (1) There is variation in fitness (selection) (2) That variation can be passed from one generation to the next (inheritance) Hence the “survival of the fittest” which leads to changes within populations. This is the central insight of Darwin.

  4. Populations evolve genetically to survive. We know this happens. Look at the following clip. • Newt video clip http://www.pbs.org/wgbh/evolution/library/01/3/quicktime/l_013_07.html

  5. Who is Hardy and Weinberg? Hardy and Weinberg constructed a model of a population that does NOT change.

  6. Five factors necessary to remain in equilibrium • No mutations – no new alleles enter population. • No gene flow (i.e. no migration of individuals into, or out of, the population). • Random mating (i.e. individuals must pair by chance) • Population must be large so that no genetic drift (random chance) can cause the allele frequencies to change. • No selection so certain alleles are not selected for, or against.

  7. Why Hardy Weinberg? Due to the fact that this balance cannot work, scientists can use it to detect changes from generation to generation. Thus allowing a simplified method of determining that evolution is occurring.

  8. How? If we mate two individuals that are heterozygous Girl Bb Boy Bb • 25% BB • 50% Bb • 25% bb (not like their parents, express the recessive phenotype). This is what Mendel found when he crossed monohybrids

  9. But the frequency of two alleles in an entire population of organisms is unlikely to be exactly the same. The Hardy-Weinberg equation allowed geneticists to do the same thing Mendel did for individual families for entire populations.

  10. Allele Frequency Let’s talk about p and q • p = the frequency of the dominant allele • q = the frequency of the recessive allele For a population in genetic equilibrium:p + q = 1.0 (The sum of the frequencies of both alleles is 100%.)

  11. For example: • Imagine a 'swimming' pool of genes as shown in Figure 1. • Count the number of A and a inside the pool (not the ones on the outside of the pool). • A= 12 • a = 18

  12. Then take (A + a) or This shows the total population in the pool is what? 12 +18 = 30

  13. To determine the frequencies of A and a. • Dominant Gene (A)/total population to equal (p) Step 1: Frequency of A or f(A) = 12/30 = 0.4 so p= 0.4 • Recessive gene (a)/total population to equal (q) Step 2: Frequency of a or f(a) = 18/30 = 0.6 q = 0.6

  14. Then, take what you found for p + q= 1 p= 0.4 q = 0.6 Step 3: Example: 0.4(fA) + 0.6(fa) = 1

  15. Let’s try another one. For example: The population of squirrels near Harvard University has a mix of brown and black squirrels. The students surveyed the number of Brown and black squirrels. There are 200 Brown squirrels and 400 black squirrels. Black is dominant over grey squirrels. Brown is recessive.

  16. What is the number of black squirrels? A: 400 What is the number of Brown squirrels? A: 200 What is the total population? p + q = 1 A: 400 + 200 = 600

  17. To determine the frequencies of p and q. Dominant Gene A/total population to equal (p) Step 1: Frequency of A or f(A) = 400/600 p=0.7 Recessive gene (a)/total population to equal (q) Step 2: Frequency of a or f(a) = 200/600 = 0.6 q = 0.3

  18. Then, take what you found for p + q= 1 Step 3: 0.7(fA) + 0.3(fa) = 1 Work on problems #1 and 2 in homework if done.

  19. Let’s take both examples one step further. Genotype Frequency • The proportion of individuals in a group with a particular genotype. (Genotype can refer to one locus, two loci, or the whole genome, depending on the context

  20. Starting with the pool example If the Frequency of p= 0.4 And the Frequency of q = 0.6 Then determine the genotypic frequenciesof AA, Aa and aa. How? Using the following equations: p2 + 2pq + q2 (Same as AA + Aa + aa) Step 1: Example: p2 = (0.4 x 0.4) = .16 2pq = 2(0.4 x 0.6) = .48 q2 = (0.6 x 0.6) = .36 Which equals: .16 + .48 + .36 = 1

  21. 16% of the population are AA 48% are Aa 36% are aa

  22. Let’s try with our squirrels. If the frequency of p=0.7 The frequency of q = 0.3 Then determine the genotypic frequenciesof BB, Bb and bb. How? Using the following equations: p2 + 2pq + q2 (Same as BB + Bb + bb) Step 1: Example: p2 = (0.7 x 0.7) 2pq = 2(0.7 x 0.3) q2 = (0.3 x 0.3) Which equals: .49 + .42 + .09 = 1

  23. 49% of the population are BB 42% are Bb 9% are bb

  24. Simplify • p = frequency of the dominant gene • q = frequency of the recessive gene • p2 = frequency of the homozygous dominant trait • q2 = frequency of the recessive trait

  25. Now, suppose more 'swimmers' dive in as shown in Figure 2. What will the gene and genotypic frequencies be? Apply the same thing we did for the first example to the additional members of the population to the swimming pool. Counting the a’s that were outside the pool now entering the pool. What is the frequency of A or p? What is the frequency of a or q? What is the genotypic frequencies?

  26. Solution: • f(A) = 12/34 = .35 = 35 % • f(a) = 21/34 = .65 = 65% Genotypic frequencies: f(AA) = .12, f(Aa) = .46 and f (aa) = .41 Which equals: .12 + .46 + .41 = 1

  27. Results: AA: 12% Aa: 46% Aa: 41% The results show that Hardy-Weinberg Equilibrium was not maintained. The migration of swimmers (genes) into the pool (population) resulted in a change in the population's gene frequencies

  28. Try problems #1-3 in your Homework packet.

  29. Part II Now that you know that Hardy-Weinberg Equilibrium is not naturally maintained. We can lead into the idea of natural selection and a result in the change in the population's gene frequencies.

  30. Natural Selection is due to Mutations How do Mutations happen? U.V. Rays Radiation chemicals Mistakes made during DNA replication (approximately 6 every time cells undergo mitosis)

  31. What affects do mutations have on organisms? • Lethal – deadly • Major: HOX genes cause rather unusual changes such as an extra limb, antennae, etc. • Small • None – same amino acid or in DNA not used • Beneficial – leads to a positive change that allows an organisms a better chance of survival.

  32. Examples of Natural Selection A. Sickle Cell Anemia Protects those individuals who are afflicted with this genetic disease from Malaria.

  33. B. Antibiotic Resistant Bacteria Bacteria reproduce fast so they can produce several generations in a very few hours and therefore evolve in a relatively short time. Most mutations lead to death but some survive Video on antibiotic resistance Part I http://www.youtube.com/watch?v=2L82V6VPJkQ&feature=related Part II http://www.youtube.com/watch?v=D0_FTJnhzXA&feature=related

  34. C. Peppered Moth Prior to industrial revolution (1850), most common phenotype was light colored After industrial revolution, dark phenotype became more common

  35. D. Viruses • Viruses also mutate quickly just as Bacteria do. • They cannot reproduce without a host

  36. How do they Trick immune system? • Viruses mutate to survive the immune system by changing or evolving to strike again Example: Avian flu Let’s look at the life cycle of a virus….

  37. Attachment 2. Penetration • 3. Un-Coating 4. Assembly • 5. Release

  38. PBS clip Immunity to HIV? http://www.pbs.org/wgbh/evolution/library/10/4/l_104_05.html

  39. Immune System I. Lymph vessels • Immune system pathway • return water to blood • Nodes = sites of immune system action, invaders destroyed

  40. Immunity • The system that gives the body the ability to resist disease. I. Active - the body produces its own antibodies to defend against a certain antigen.   II. Passive - is only for a short period of about one month because a person is given the antibodies required to defend against the antigen.

  41. First Line of Defense • Skin • Mucus • Hair • Ear Wax • Tear Drops • Sweat • Stomach Acid

  42. Second Line of Defense Inflammatory Response – response to tissue damage which increase white blood cell production and fever

  43. Third Line of Defense • Specific Immunity – immunity to specific pathogens by recognizing, attacking, destroying, and then remembers each foreign substance and pathogen that enters the body.  It does this by making specialized cells and antibodies that makes the pathogens useless.

  44. How? Attacks using WBC’s. Two types: • B Cells – produce antibodies • T Cells – killer cells Then produces Antibodies • Hold the pathogen so unable to infect other cells until T-cell destroys

  45. Vaccination • Give a piece of dead or weakened virus so body fights off and forms antibodies

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