Mendel's Breakthrough

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Mendel's Breakthrough

Powerpoint to accompanyGenetics: From Genes to GenomesSecond EditionHartwell ● Hood ● Goldberg ● Reynolds ● Silver ● VeresChapter2Mendel’sBreakthroughPatterns, Particles, and Principlesof HeredityCopyright © The McGraw-Hill Companies, Inc. Permission required to reproduce or displayOutline of Mendelian GeneticsGregor Mendel (1822-1844)1844)• The historical puzzle of inheritance and howMendel’s s experimental approach helped solve it• Mendel’s s approach to genetic analysis includinghis experiments and related analytic tools• A comprehensive example of Mendelianinheritance in humansffFig. 2.2Themes of Mendel’s s work• Variation is widespread in nature• Observable variation is essential for followinggenes• Variation is inherited according to genetic lawsand not solely by chance• Mendel’s s laws apply to all sexually reproducingorganisms.The historical puzzle of inheritance• Artificial selection has been an important practicesince before recorded history• Domestication of animals• Selective breeding of plants• 19 th century – precise techniques for controlledmatings in plants and animals to produce desiredtraits in many of offspring• Breeders could not explain why traits wouldsometimes disappear and then reappear insubsequent generations.1

State of genetics in early 1800’sMendel’s s workplaceWhat is inherited?How is it inherited?What is the role of chance inheredity?Fig. 2.5Historical theories of inheritanceFig.2.6• One parent contributesmost features (e.g.,homunculus, N.Hartsoiker, , 1694)• Blending inheritance –parental traits becomemixed and foreverchanged in offspringKeys to Mendel’s s experiments• The garden pea was an ideal organism• Vigorous growth• Self fertilization• Easy to cross fertilize• Produced large number of offspring each generation• Mendel analyzed traits with discrete alternative forms• purple vs. white flowers• yellow vs. green peas• round vs. wrinkled seeds• long vs. short stem length• Mendel established pure breeding lines to conduct hisexperimentsMonohybrid crosses reveal units ofinheritance and Law of SegregationTraits have dominant and recessiveforms• Disappearance of traits in F1 generation andreappearance in the F2 generation disproves thehypothesis that traits blend• Trait must have two forms that can each breedtrue• One form must be hidden when plants witheach trait are interbred• Trait that appears in F1 is dominant• Trait that is hidden in F1 is recessiveFig.2.92

Alternative forms of traits are alleles• Each trait carries two copies of a unit ofinheritance, one inherited from the mother andthe other from the father• Alternative forms of traits are called allelesLaw of Segregation• Two alleles for eachtrait separate(segregate) duringgamete formation,and then unite atrandom, one fromeach parent, atfertilizationFig. 2.10The Punnet SquareRules of ProbabilityIndependent events - probability of two events occurring togetherWhat is the probability that both A and B will occur?Solution = determine probability of each and multiplythem together.Mutually exclusive events - probability of one or another eventoccurring.Fig. 2.11What is the probability of A or B occurring?Solution = determine the probability of each and addthem together.Probability and Mendel’s s Results• Cross Yy xYy pea plants.• Chance of Y sperm uniting with a Y egg• ½ chance of sperm with Y allele• ½ chance of egg with Y allele• Chance of Y and Y uniting = ½ x ½ = ¼• Chance of Yy offpsring• ½ chance of sperm with y allele and egg with Y allele• ½ chance of sperm with Y allele and egg with y allele• Chance of Yy – (½ x ½) ) + (½(x ½) ) = 2/4, or 1/2Further crosses confirm predictedratiosFig. 2.123

Genotypes and Phenotypes• Phenotype – observable characteristic of anorganism• Genotype – pair of alleles present in andindividual• Homozygous – two alleles of trait are the same(YY or yy)• Heterozygous – two alleles of trait are different(Yy)Genotypes versus phenotpyesYy × Yy1:2:1YY:Yy:yy3:1yellow: greenFig. 2.13Test cross reveals unkown genotpyeFig. 2.14Dihybrid crosses reveal the law ofindependent assortment• A dihybrid is an individual that is heterozygousat two genes• Mendel designed experiments to determine iftwo genes segregate independently of oneanother in dihybrids• First constructed true breeding lines for bothtraits, crossed them to produce dihybridoffspring, and examined the F2 for parental orrecombinant types (new combinations notpresent in the parents)Results of Mendels dihybrid crosses• F2 generation contained both parental types andrecombinant types• Alleles of genes assort independently, and canthus appear in any combination in the offspringDihybrid cross shows parental andrecombinant typesFig. 2.15 top4

Dihybrid cross produces apredictable ratio of phenotypesThe law of independent assortment• During gamete formation different pairs of allelessegregate independently of each otherFig. 2.15 bottomFig. 2.16Summary 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) segregaterandomly• Alleles are dominant or recessive - thus thedifference between genotype and phenotype• Different traits assort independentlyLaws of probability formultiple genesF2PRRYYTTSS X rryyttssgametes RYTS rytsgametesF1 RrYyTtSs X RrYyTtSsRYTSRyTSrYTSRYTsRyTsrYTsRYtSRytSrYtsRYtsRytsrYTSryTs rYtS rYts rytsRYTSRyTSrYTSRYTsRyTsrYTsRYtSRytSrYtsWhat is the ratio of different genotypesand phenotypes?RYtsRytsrYTSryTs rYtS rYts rytsPunnet Square method - 2 4 = 16 possible gametecombinations for each parentPF1RRYYTTSS × rryyttssRrYyTtSs × RrYyTtSsThus, a 16 × 16 Punnet Square with 256 genotypesThat’s one big Punnet Square!Loci Assort Independently - So we can look at each locusindependently to get the answer.What is the probability of obtaining the genotype RrYyTtss?Rr × Rr Yy X Yy Tt × Tt Ss × Ss1RR:2Rr:1rr 1YY:2Yy:1yy 1TT:2Tt:1tt 1SS:2Ss:1ss2/4 Rr 2/4 Yy 2/4 Tt 1/4 ssProbability of obtaining individual with Rr and Yy and Tt and ss.2/4 × 2/4 × 2/4 × 1/4 = 8/256 (or 1/32)5

PF1RRYYTTSS × rryyttssRrYyTtSs × RrYyTtSsWhat is the probability of obtaining a completely homozygousgenotype?Genotype could be RRYYTTSS or rryyttssRr × Rr1RR:2Rr:1rr1/4 RR1/4 rrYy × Yy1YY:2Yy:1yy 1TT:2Tt:1tt1/4 YY1/4 yyTt × Tt1/4 TT1/4 ttSs × Ss1SS:2Ss:1ss1/4 SS1/4 ss(1/4 × 1/4 × 1/4 × 1/4) + (1/4 × 1/4 × 1/4 × 1/4) = 2/256Rediscovery of Mendel• Mendel’s s work was unappreciated and remaineddormant for 34 years• Even Darwin’s s theories were viewed withskepticism in the late 1800’s s because he couldnot explain the mode of inheritance of variation• In 1900, 16 years after Mendel died, fourscientists rediscovered and acknowledgedMendel’s s work, giving birth to the science ofgenetics1900 - Carl Correns, , Hugo deVries,and Erich von Tschermak rediscoverand confirm Mendel’s s lawsMendelian inheritance in humans• Most traits in humans are due to the interactionof multiple genes and do not show a simpleMendelian pattern of inheritance.• A few traits represent single-genes. genes. Examplesinclude sickle-cell cell anemia, cystic fibrosis, Tay-Sachs disease, and Huntington’s s disease (seeTable 2.1 in text)• Because we can not do breeding experiments onhumans,.Fig. 2.19In humans we must use pedigrees tostudy inheritanceAnatomy of a pedigree• Pedigrees are an orderly diagram of a familiesrelevant genetic features extending throughmultiple generations• Pedigrees help us infer if a trait is from a singlegene and if the trait is dominant or recessiveFig. 2.206

A vertical pattern of inheritanceindicates a rare dominant traitA horizontal pattern of inheritanceindicates a rare recessive traitFig. 2.20Hunitington’s disease: A rare dominant traitAssign the genotypes by working backward through the pedigreeCystic fibrosis: a recessive conditionAssign the genotypes for each pedigreeFig.2.217

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