Presentation on theme: "Mendel’s Breakthrough"— Presentation transcript:
1Mendel’s Breakthrough Patterns, Particles, and Principles of Heredity
2Outline of Mendelian Genetics The historical puzzle of inheritance and how Mendel’s experimental approach helped solve itMendel’s approach to genetic analysis including his experiments and related analytic toolsA comprehensive example of Mendelian inheritance in humans
4Themes of Mendel’s work Variation is widespread in natureObservable variation is essential for following genesVariation is inherited according to genetic laws and not solely by chanceMendel’s laws apply to all sexually reproducing organisms.
5The historical puzzle of inheritance Artificial selection has been an important practice since before recorded historyDomestication of animalsSelective breeding of plants19th century – precise techniques for controlled matings in plants and animals to produce desired traits in many of offspringBreeders could not explain why traits would sometimes disappear and then reappear in subsequent generations.
6State of genetics in early 1800’s What is inherited?How is it inherited?What is the role of chance in heredity?
8Historical theories of inheritance One parent contributes most features (e.g., homunculus, N. Hartsoiker, 1694)Blending inheritance – parental traits become mixed and forever changed in offspringFigure 2.6Fig.2.6
9Keys to Mendel’s experiments The garden pea was an ideal organismVigorous growthSelf fertilizationEasy to cross fertilizeProduced large number of offspring each generationMendel analyzed traits with discrete alternative formspurple vs. white flowersyellow vs. green peasround vs. wrinkled seedslong vs. short stem lengthMendel established pure breeding lines to conduct his experiments
10Monohybrid crosses reveal units of inheritance and Law of Segregation Figure 2.9Fig.2.9
11Traits have dominant and recessive forms Disappearance of traits in F1 generation and reappearance in the F2 generation disproves the hypothesis that traits blendTrait must have two forms that can each breed trueOne form must be hidden when plants with each trait are interbredTrait that appears in F1 is dominantTrait that is hidden in F1 is recessive
12Alternative 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 fatherAlternative forms of traits are called alleles
13Law of SegregationTwo alleles for each trait separate (segregate) during gamete formation, and then unite at random, one from each parent, at fertilizationFigure 2.10Fig. 2.10
15Rules 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.What is the probability of A or B occurring?Solution = determine the probability of each and add
16Probability and Mendel’s Results Cross Yy xYy pea plants.Chance of Y sperm uniting with a Y egg½ chance of sperm with Y allele½ chance of egg with Y alleleChance of Y and Y uniting = ½ x ½ = ¼Chance of Yy offpsring½ chance of sperm with y allele and egg with Y allele½ chance of sperm with Y allele and egg with y alleleChance of Yy – (½ x ½) + (½ x ½) = 2/4, or 1/2
18Genotypes and Phenotypes Phenotype – observable characteristic of an organismGenotype – pair of alleles present in and individualHomozygous – two alleles of trait are the same (YY or yy)Heterozygous – two alleles of trait are different (Yy)
19Genotypes versus phenotpyes Yy Yy1:2:1YY:Yy:yy3:1yellow: greenFigure 2.13Fig. 2.13
21Dihybrid crosses reveal the law of independent assortment A dihybrid is an individual that is heterozygous at two genesMendel designed experiments to determine if two genes segregate independently of one another in dihybridsFirst constructed true breeding lines for both traits, crossed them to produce dihybrid offspring, and examined the F2 for parental or recombinant types (new combinations not present in the parents)
22Results of Mendels dihybrid crosses F2 generation contained both parental types and recombinant typesAlleles of genes assort independently, and can thus appear in any combination in the offspring
23Dihybrid cross shows parental and recombinant types Figure 2.15 topFig top
24Dihybrid cross produces a predictable ratio of phenotypes Figure 2.15 bottomFig bottom
25The law of independent assortment During gamete formation different pairs of alleles segregate independently of each otherFigure 2.16Fig. 2.16
26Summary of Mendel's work Inheritance is particulate - not blendingThere are two copies of each trait in a germ cellGametes contain one copy of the traitAlleles (different forms of the trait) segregate randomlyAlleles are dominant or recessive - thus the difference between genotype and phenotypeDifferent traits assort independently
27Laws of probability for multiple genes P RRYYTTSS X rryyttssF RrYyTtSs X RrYyTtSsF2 What is the ratio of different genotypesand phenotypes?gametesRYTSrytsRyTSrYTSRYTsRyTsrYTsRYtSRytSrYtsRytsRYtsryTsrYtS
28Punnet Square method - 24 = 16 possible gamete combinations for each parentThus, 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.
29What is the probability of obtaining the genotype RrYyTtss? F RrYyTtSs RrYyTtSsWhat is the probability of obtaining the genotype RrYyTtss?P RRYYTTSS rryyttssRr Rr1RR:2Rr:1rr2/4 RrYy X Yy1YY:2Yy:1yy2/4 YyTt Tt1TT:2Tt:1tt2/4 TtSs Ss1SS:2Ss:1ss1/4 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)
30F RrYyTtSs RrYyTtSsP RRYYTTSS rryyttssWhat is the probability of obtaining a completely homozygousgenotype?Genotype could be RRYYTTSS or rryyttssRr Rr1RR:2Rr:1rr1/4 RR1/4 rrYy Yy1YY:2Yy:1yy1/4 YY1/4 yyTt Tt1TT:2Tt:1tt1/4 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) = 2/256
31Rediscovery of MendelMendel’s work was unappreciated and remained dormant for 34 yearsEven Darwin’s theories were viewed with skepticism in the late 1800’s because he could not explain the mode of inheritance of variationIn 1900, 16 years after Mendel died, four scientists rediscovered and acknowledged Mendel’s work, giving birth to the science of genetics
321900 - Carl Correns, Hugo deVries, and Erich von Tschermak rediscover and confirm Mendel’s laws Figure 2.19Fig. 2.19
33Mendelian inheritance in humans Most traits in humans are due to the interaction of multiple genes and do not show a simple Mendelian pattern of inheritance.A few traits represent single-genes. Examples include sickle-cell anemia, cystic fibrosis, Tay-Sachs disease, and Huntington’s disease (see Table 2.1 in text)Because we can not do breeding experiments on humans, we use model organisms.
34In humans we must use pedigrees to study inheritance Pedigrees are an orderly diagram of a families relevant genetic features extending through multiple generationsPedigrees help us infer if a trait is from a single gene and if the trait is dominant or recessive