mendelianGenetics

mendelianGenetics - Mendel’s Breakthrough Patterns,...

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Unformatted text preview: Mendel’s Breakthrough Patterns, Particles, and Principles of Heredity 1 Ancient Geneticists? 2 Outline of Mendelian Genetics The historical puzzle of inheritance and how Mendel’s experimental approach helped solve it Mendel’s approach to genetic analysis including his experiments and related analytic tools A comprehensive example of Mendelian inheritance in humans 3 Gregor Mendel (1822­1844) Fig. 2.2 4 Themes of Mendel’s work Variation is widespread in nature. Observable variation is essential for following genes. Variation is inherited according to genetic laws and not solely by chance. Mendel’s laws apply to all sexually reproducing organisms. 5 State of genetics in early 1800’s What is inherited? How is it inherited? What is the role of chance in heredity? 6 Historical theories of inheritance One parent contributes most features (e.g., homunculus, N. Hartsoiker, 1694). Blending inheritance – parental traits become mixed and forever changed in offspring 7 Fig.2.6 Keys to Mendel’s experiments The garden pea was an ideal organism. Vigorous growth Self fertilization Easy to cross fertilize Produced large number of offspring each generation 8 9 Monohybrid crosses reveal units of inheritance and Law of Segregation. Fig.2.9 10 10 Alternative forms of traits are alleles Each trait carries two copies of a unit of inheritance, one inherited from the mother and the other from the father. Alternative forms of traits are called alleles. 11 11 Law of Segregation Two alleles for each trait separate (segregate) during gamete formation, and then unite at random, one from each parent, at fertilization. Fig. 2.10 12 12 The Punnet Square Fig. 2.11 13 13 Rules of Probability Independent events - probability of two events occurring together What is the probability that both A and B will occur? Solution = determine probability of each and multiply them together. Mutually exclusive events - probability of one or another event occurring. What is the probability of A or B occurring? Solution = determine the probability of each and add them together. 14 14 Further crosses confirm predicted ratios. Fig. 2.12 15 15 Genotypes and Phenotypes Phenotype – observable characteristic of an organism Genotype – pair of alleles present in an individual Homozygous – two alleles of trait are the same (YY or yy) Heterozygous – two alleles of trait are different (Yy) 16 16 Genotypes versus phenotypes YK × Yy 1:2:1 YY:Yy:yy 3:1 yellow: green Fig. 2.13 17 17 Test cross reveals …? Fig. 2.14 18 18 Results of Mendel's dihybrid crosses F2 generation contained both parental types and recombinant types. Alleles of genes assort independently, and can thus appear in any combination in the offspring. 19 19 Dihybrid cross shows parental and recombinant types. Fig. 2.15 20 20 Dihybrid cross produces a predictable ratio of phenotypes. Fig. 2.15 21 21 The law of independent assortment During gamete formation different pairs of alleles segregate independently of each other. Fig. 2.16 22 22 Summary of Mendel's work Inheritance is particulate ­ not blending. There are two copies of each trait in a germ cell. Gametes contain one copy of the trait. Alleles (different forms of the trait) segregate randomly. Alleles are dominant or recessive ­ thus the difference between genotype and phenotype. Different traits assort independently. 23 23 Laws of probability for multiple genes P gametes F1 gametes RYTS RyTS rYTS ryTs RRYYTTSS X rryyttss RYTS RrYyTtSs RYTs RyTs rYTs rYtS RYtS RytS rYts rYts RYts Ryts rYTS ryts ryts X RrYyTtSs RYTS RyTS rYTS ryTs RYTs RyTs rYTs rYtS RYtS RytS rYts rYts RYts Ryts rYTS ryts F2 What is the ratio of different genotypes and phenotypes? 24 24 Punnet Square method - 24 = 16 possible gamete combinations for each parent Thus, a 16 × 16 Punnet Square with 256 genotypes That’s one big Punnet Square! Loci Assort Independently - So we can look at each locus independently to get the answer. 25 25 P F1 RRYYTTSS × rryyttss RrYyTtSs × RrYyTtSs What is the probability of obtaining the genotype RrYyTtss? Rr × Rr Yy X Yy Tt × Tt Ss × Ss 1SS:2Ss:1ss 1/4 ss 1RR:2Rr:1rr 1YY:2Yy:1yy 1TT:2Tt:1tt 2/4 Rr 2/4 Yy 2/4 Tt Probability of obtaining individual with Rr and Yy and Tt and ss. 2/4 × 2/4 × 2/4 × 1/4 = 8/256 (or 1/32) 26 26 P F1 RRYYTTSS × rryyttss RrYyTtSs × RrYyTtSs What is the probability of obtaining a completely homozygous genotype? Genotype could be RRYYTTSS or rryyttss Rr × Rr Yy × Yy Tt × Tt Ss × Ss 1SS:2Ss:1ss 1/4 SS 1/4 ss 1RR:2Rr:1rr 1YY:2Yy:1yy 1TT:2Tt:1tt 1/4 RR 1/4 rr 1/4 YY 1/4 yy 1/4 TT 1/4 tt (1/4 × 1/4 × 1/4 × 1/4) + (1/4 × 1/4 × 1/4 × 1/4) = 2/256 27 27 1900 ­ Carl Correns, Hugo deVries, and Erich von Tschermak rediscover and confirm Mendel’s laws. Fig. 2.19 28 28 Anatomy of a pedigree Fig. 2.20 29 29 30 30 A vertical pattern of inheritance indicates a rare dominant trait. Huntington’s disease: A rare dominant trait Assign the genotypes by working backward through the pedigree Fig. 2.21 31 31 A horizontal pattern of inheritance indicates a rare recessive trait. Cystic fibrosis: a recessive condition Assign the genotypes for each pedigree Fig.2.22 32 32 Cystic fibrosis or I II 1 1 2 2 2 1 1 2 3 4 3 2 5 3 4 2 3 4 4 III IV 1 1 V VI VII 1 2 33 33 Cystic fibrosis I II 1 2 2 2 1 1 2 3 4 3 2 5 3 4 3 4 4 1 2 III IV 1 1 V VI VII 1 2 34 34 Cystic fibrosis I II 1 2 2 2 1 1 2 3 4 3 2 5 3 4 3 4 4 1 2 III IV 1 1 V VI VII 1 2 35 35 Cystic fibrosis I II 1 2 2 2 1 1 2 3 4 3 2 5 3 4 3 4 4 1 2 III IV 1 1 V VI VII 1 2 36 36 Cystic fibrosis I II 1 2 2 2 1 1 2 3 4 3 2 5 3 4 3 4 4 1 2 III IV 1 1 V VI VII 1 2 37 37 Cystic fibrosis or I II 1 1 2 2 2 1 1 2 3 4 3 2 5 3 4 2 3 4 4 III IV 1 1 V VI VII 1 2 38 38 Cystic fibrosis I II I II 1 2 1 2 3 4 5 6 1 2 1 2 3 4 5 39 39 Cystic fibrosis I II I II 1 2 1 2 3 4 5 6 1 2 1 2 3 4 5 40 40 Cystic fibrosis I II I II 1 2 1 2 3 4 5 6 1 2 1 2 3 4 5 41 41 Cystic fibrosis I II I II 1 2 1 2 3 4 5 6 1 2 1 2 3 4 5 42 42 43 43 44 44 Homework: 16, 17, 29 and visit the www.mhhe.com/hartwell3 Go to Ch 2./ interactive web exercise Next time more on the family below! 45 45 ...
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This note was uploaded on 12/17/2009 for the course BIO 2354 taught by Professor Brockett during the Spring '08 term at Georgia Tech.

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