Fluent Essays

Transmission Genetics and the Sources of Genetic Variation B6 1 Mendel’s Laws. Contemporary with Darwin (but Darwin did not know of him). Phenotype: Genotype: 2 Mendel’s Laws. 2. The Law of Dominance. 3 3. The Law of Segregation. 4. The Law of Independent Assortment. 4. The Law of Independent Assortment... 9 yellow round : 3 yellow wrinkled : 3 green round : 1 green wrinkled 4. The Law of Independent Assortment. 5. Besides dominant and recessive alleles, alleles can be and… 5. … to each other. Gene at ‘organismal level’: Transmission Genetics. 10 Transmission Genetics. 11 The ‘Central Dogma’ of Molecular Biology. 12 a. DNA makes RNA: transcription b. RNA makes protein: translation -- mRNA codons specify the amino acid sequence in a growing protein... b. RNA makes protein: translation -- tRNA carries amino acids into a ribosome, binding their anticodon to the mRNA codon. -- Ribosomes contain rRNA. One of these is the ‘enzyme’ that catalyzes bonds between amino acids. 2. The Genetic Code is the code of mRNA codons for amino acid sequences. Proteins: are generally the builders of traits. Gene at ‘molecular level’: 3. Several attributes of the code indicate that it evolved by natural selection long ago. 19 Variation and Mutation. One of the challenges for Darwin was to solve this problem: 20 Variation and Mutation. 1... Natural selection selectively breeds for the ‘fit’ varieties in populations. But this should REDUCE the amount of variation! 21 2. Today, we know there are several means by which variation is regenerated, forming new genotypes: 22 2. a. Mutation. Errors in DNA replication Alternative bases Recombination, transposons… -------------------------------------- Radiation Chemicals 23 24 25 1 mm DNA Eukaryote Virus 26 b. Sexual reproduction (& ‘lateral gene transfer’) c. Recombination during meiosis. 28 d. (once again): independent assortment. # of possible allele combinations: = 2n 29 e. Chromosome rearrangements. 30 f. Polyploidy. (usually 4n, 8n, etc.) g. There are others: immigration, etc… 31 3. We have measured genetic diversity in populations, and found Darwin need not have worried: a. Most populations are pretty fit they way they are & are not under great selection pressure. So… 32 3. We have measured genetic diversity in populations, and found Darwin need not have worried: c. every gene is mutated or soon will be. b. most populations have accumulated an enormous store of untapped genetic diversity. 33 Effects of Mutations on Fitness. A common statement: ‘Mutations are random’. What should we mean by that? 2. It does not mean that DNA is mutated randomly (!) (since some DNA regions have a higher mutation rate than others). 34 Effects of Mutations on Fitness. 3. What we really mean is that mutations are random with respect to the needs of the organism. – they do not occur to increase fitness, nor do they occur (on purpose) to decrease it. A common statement: ‘Mutations are random’. What should we mean by that? 35 4. Before we knew this, we wondered: a. Does selection cause mutations to happen? b. Does selection somehow ‘direct’ the kind of mutations that happen? 5. In the case of bacteria, it once seemed that one or both could be true! They evolved resistance very quickly. Recommended viewing: Time lapse showing evolution of bacteria antibiotic resistance. 37 6. The Luria-Delbruck experiment (1943): Salvador Luria & Max Delbruck Some bacteria grow! these are resistant. 38 7. If mutations for resistance were ‘induced’ by viruses: Resistance forms at ~ same time after viruses. # of resistant colonies will be ~ constant from plate to plate. 8. If mutations are ‘random’, & present before virus exposure: Resistance appears before viruses at different times. # of resistant colonies varies from plate to plate. B7-1 41 By ~ 1900, Mendel was ‘discovered’, and a new science of genetics began in earnest. 2. But the initial integration between evolution & genetics science was not smooth at first. a. Geneticists saw that traits were generally ‘discontinuous’. Maybe evolution would be ‘jerky’? 43 b. Evolutionary biologists expected variations to be ‘small’  a ‘continuum’ of traits. There were examples … 3. Eventually (this took years, well into the 1900’s) the consensus was that both camps were right! Some traits are controlled ‘additively’ by multiple genes or multiple alleles. 45 b. Other traits are controlled by genes with few alleles  discrete phenotypes w/o intermediates. (= “polymorphisms”). 46 Individual versus Population-Level Thinking. Another area of confusion was that some thought natural populations ought to show Mendelian ratios. 47 Individual versus Population-Level Thinking. 48 2. The truth is that populations will keep whatever allele ‘frequency’ they start with unless something changes it. 49 50 The Hardy-Weinberg Model: A ‘Null’ Model for Population Genetics. Hardy-Weinberg Equilibrium G.H. Hardy and W. Weinberg independently realized that allele frequencies do not change unless something changes it. This condition is … 51 2. The central points of H.W.E. are: The frequencies of alleles (A1 and A2) will not change over time unless evolutionary processes act upon them. A definition of evolution: A change in the frequency of genes (alleles) in a population. 52 2. The central points of H.W.E. are: b. Given allele frequencies, we can predict genotype frequencies (A1A1, A1A2, A2A2) c. If values are not at predicted frequencies, they will ‘move’ to predicted frequencies in 1 generation of random mating. 53 3. The H.W. model provides conditions for when a population is not evolving – gene / allele frequencies are not changing. 54 4. Conditions for H.W.E. : a. No selection. No alleles provide greater or lesser ‘fitness’. b. Random mating. No individuals ‘prefer’ certain other individuals. c. No mutation. No new alleles. d. No migration. No one enters / no one leaves. e. No genetic drift. No random fluctuations. The pop is considered infinite in size. 55 5. A population is a group of sexually interbreeding individuals. These share… -- a Gene pool… -- Allele frequencies Let’s make sure you are aware of some basics: -- Genotype frequencies 56 In mice, the allele for Agouti (A) is dominant to the allele for black (a). So Agouti mice are AA or Aa. Black mice are aa. The Worksheet: 57 1. Lets suppose that in one population of mice, 80\% of the genes at this locus are A, and 20\% are a. Express their allele frequencies as decimals. The Worksheet: 58 2. Assume that these mice mate randomly. What are the expected frequencies of the genotypes AA, Aa, and aa? … Frequency of allele A: p = 0.8 Frequency of allele a: q = 0.2… 59 A A a a Freq. of A = p.. Freq of a = q.. 60 p = q = p = q = 2. p2 = Frequency of AA 2pq = Frequency of Aa q2 = Frequency of aa 61 p = frequency of allele 1 q = frequency of allele 2 p + q = 1 Basic Hardy-Weinberg Terminology & Equations. In a population with alleles A and a, & genotypes AA, Aa, and aa… 62 Basic Hardy-Weinberg Terminology & Equations. In a population with alleles A and a, & genotypes AA, Aa, and aa… p2 + 2pq + q2 = 1 It is also at times handy to remember that... q = q2 … p = p2 + ½ (2pq) q = q2 + ½ (2pq) WARNING: This only applies if we assume H.W.E. (you will see why later). Basic Hardy-Weinberg Terminology & Equations. All above statements are always true, even for pops. not in H.W.E. 64 Lets try another one: Suppose a population of Agouti & black mice were stranded on an island, with no predators and lots of food… 65 The initial population is 400 mice, with 196 AA, 168 Aa, and 36 aa… 3. What are the frequencies of AA, Aa, and aa? Lets try another one: … 66 4. What are the frequencies of A, and a? You can calculate this from the # of gene copies… The initial population is 400 mice, with 196 AA, 168 Aa, and 36 aa. 67 You can also calculate this from the genotype frequencies… 4. What are the frequencies of A, and a? The initial population is 400 mice, with 196 AA, 168 Aa, and 36 aa. 5. Some common mistakes: a. Not understanding that a heterozygote frequency IS ‘2pq’ b. Not understanding that homozygote frequencies ARE ‘p2’ and ‘q2’ 69 Now, p + q will always = 1, and p2 + 2pq + q2 will always = 1, but… 5. Some common mistakes: c. Using the q = q2 (or p = p2 ) when it is not right to assume H.W.E. [Use instead: q = q2 + ½ (2pq), & so on.] 70 …when these values are linked together, then the population is in H.W.E.! … 71 …when these values are linked together, then the population is in H.W.E.! 72 …when these values are linked together, then the population is in H.W.E.! q2=.09 p2=.49 2pq=.42 73 6. Is our last population in H.W.E.? (196 AA, 168 Aa, and 36 aa.) Observed: Expected: p2 = p = p2 = 2pq = q = 2pq = q2 = q2 = 7. One in 1700 Caucasians in the US will come down with cystic fibrosis, an autosomal recessive mutation effecting chloride ion transport. a. Individuals with cf are genotype cc. Unaffected people are either CC or Cc. 7. One in 1700 Caucasians in the US will come down with cystic fibrosis, an autosomal recessive mutation effecting chloride ion transport. b. What is the frequency of people who are Cc ‘carriers’? Assume H.W.E. c. In a population of 10,000,000, how many will be carriers? 7. One in 1700 Caucasians in the US will come down with cystic fibrosis, an autosomal recessive mutation effecting chloride ion transport. ADDITIONAL HWE problems are in the Moodle web site. Be able to do them, and you should be ok.