Lecture 2-Mendelian Genetics PDF

 Mendel’s Principles / Laws Law of Segregation Law of Independent Assortment (both of these “Laws” occur because of Meiosis I and Meiosis II [see next lecture]). Mendel knew nothing of role of Meiosis. Subsequent researchers (with access to microscopy) correlated movement of chromosomes w...

 Mendel’s Principles / Laws Law of Segregation Law of Independent Assortment (both of these “Laws” occur because of Meiosis I and Meiosis II [see next lecture]). Mendel knew nothing of role of Meiosis. Subsequent researchers (with access to microscopy) correlated movement of chromosomes with particulate inheritance.  Law of Segregation & Independent Assortment Visualized Segregation: each gamete carries only 1 allele because chromosomes segregate/separate during Meiosis I and II. Independent assortment: alleles of different genes assort / are distributed independently of each other. (during Meiosis I and II).  Review of Terms Gene: The fundamental physical unit of heredity (located on DNA, with exception of RNA viruses). Allele: One of a possible number of variants of a gene… A gene variant. Genotype: genetic makeup of an organism. Phenotype: characteristics of an organism that can be observed. Quantitative traits: Phenotype = Genotype + Environment + G X E. Today: characters not affected by environment: (Categorical, Mendelian)  Genotype: genetic makeup of an organism. Phenotype: characteristics of an organism that can be observed. Phenotype = Genotype + Environment + G X E. In Mendelian Traits, Phenotype = Genotype  Why Peas? Grow easily Produce a large number of seeds quickly (short generation time). Routinely self-fertilize (and outcross). Have a number of ‘categorical’ traits. How to cross a pea plant: Flowers are ‘perfect’. Mendel’s Experiments: looked at 1. 7 characters of pea plants. Mendel’s Experiments: 1. 7 characters of pea plants. 2. Isolated true-breeding strains Mendel’s Experiments: 1. 7 characters of pea plants. 2. Isolated true-breeding strains 3. Looked at each trait individually (initially) Mendel’s Experiments: 1. 7 characters of pea plants. 2. Isolated true-breeding strains 3. Looked at each trait individually (initially) 4. Conducted careful breeding experiments 5. Kept careful records 6. Had large sample sizes 7. Followed traits for multiple generations (we’ll look at 3 generations)  Mendel’s 1st Law: Principle of Segregation The 2 members of a gene pair (alleles) segregate (separate) from each other during gamete formation and are randomly distributed to the offspring. (Offspring receive 1 allele from each parent.) (Distinguish alleles versus genes)  Mendel’s Principle of Segregation (terminology) P Generation: Parental Generation F1 Generation: First Filial Generation F2 Generation: 2nd Filial Generation F3, F4, F5, etc. 3rd, 4th, 5th, etc. Generations  Mendel’s Principle of Segregation (terminology) P Generation: Parental Generation F1 Generation: First Filial Generation F2 Generation: 2nd Filial Generation F3, F4, F5, etc. 3rd, 4th, 5th, etc. Generations Monohybrid cross: true-breeding strains that differ in only ONE trait of interest. Terminology continued: Reciprocal Crosses Pollen Pollen  Principle of Uniformity of F1 F1 offspring of a monohybrid cross will exactly resemble only one parent (& each other). P F1 1 trait disappears in the F1 P F1 Traits Reappear in F2 P F1 We’ll talk in a minute about how 3 F2 1 he showed this.  Traits Reappear in F2 P Mendel reasoned (correctly): Information to create the trait was present in the F1 in the form of “factors” (now called genes). F1 (NOT BLENDING!) 3 F2 1  Traits Reappear in F2 P Mendel reasoned (correctly): Information to create the trait was present in the F1 in the form of “factors” (now called genes). F1 Genes exist in alternative forms (now called alleles) that control specific traits. 3 F2 1  Traits Reappear in F2 P F1 has one each. Since the progeny are not mixed there is one dominant and one recessive allele. F1 3 F2 1  Letters used to designate alleles Dominant = Capital (i.e. P) Recessive = Lowercase (i.e. p)  Heterozygous vs. Homozygous Homozygotes: have 2 copies of the same allele – PP – pp  Heterozygous vs. Homozygous Homozygotes: have 2 copies of the same allele – PP – pp Heterozygotes: have 1 copy of each allele – Pp Same Flower Example P F1 Self-pollinate  Punnett Square: Let’s make an F1 MOM : Gametes? PP P P p Pp Pp DAD: pp p Pp Pp ALL the F1s look like this!  Punnett Square: monohybrid cross (Crossing the F1 individuals now) MOM : Pp Gametes? DAD: Pp  Punnett Square: monohybrid cross MOM : Pp Gametes P p P DAD: Pp p  Punnett Square: monohybrid cross MOM : Pp Gametes P p P PP Pp DAD: Pp progeny p Pp pp  Punnett Square: monohybrid cross MOM : Pp Gametes P p P PP Pp DAD: Pp progeny p Pp pp Note 3:1 phenotypic ratio  IF ‘factors’ segregate in the F1 and are then randomly distributed among offspring, THEN the phenotypic ratios of the F2 offspring should be 3:1.  How did Mendel determine this? Were the ratios EXACTLY 3:1? NO!  How did Mendel determine this? Were the ratios exactly 3:1? No. Is this “close enough”?  REVIEW BINOMIAL DISTRIBUTION FROM LAB!!!  Question: how do we determine when a deviation from our expectations is due to chance and when it is due to some causative factor? Mendel’s Experiment: Pp x Pp 705 purple 224 white 929 Total ratio: 3.15 : 1 Are these results significantly different from expected 3:1? (different from random distribution?)  Testing Mendel’s first law Law of Segregation: The 2 members of a gene pair (alleles) segregate (separate) from each other during gamete formation and are randomly distributed to the offspring. (Offspring receive 1 allele from each parent.)  Punnett Square: monohybrid cross MOM : Pp Gametes P p P PP Pp DAD: Pp progeny p Pp pp Note 3:1 phenotypic ratio  Be able to do this I give you… (Mendel’s actual results) 705 purple 224 white Are these results significantly different from our expected (predicted) 3:1? χ2 Test Statistic (“Chi-Square”) Used to compare data with the predicted values according to a hypothesis. -if χ2 exceeds some critical value, reject the null hypothesis (our null hypothesis = 3:1) - if χ2 < critical value, we cannot reject the hypothesis. (We conclude that the deviation of observed values from predicted is due to chance) Null Hypothesis H0: there is no difference between observed and expected values Cross: Pp x Pp F2 Phenotype Observed Expected Purple Flowers 705 ? White Flowers 224 ? Total (observed - expected)^2/expected Cross: Pp x Pp F2 Phenotype Observed Expected Purple Flowers 705 0.75*929 = 697 White Flowers 224 0.25*929 = 232 Total 929 929 Cross: Pp x Pp F2 Phenotype Observed Expected Purple Flowers 705 0.75*929 = 697 White Flowers 224 0.25*929 = 232 Total 929 929 χ2 χ2 = 0.39 Degrees of Freedom = #classes of data - 1 = 2 - 1 = 1df χ2 Table Critical χ2 Values (p. 288 in Russell)  χ2 Table Critical χ2 Values 1. Find df row df = 1 2. Find χ2 value χ2 = 0.37  χ2 Table Critical χ2 Values Cannot reject H0 Reject H0 P-values tell us the probability of obtaining a result equal to or "more extreme" than what was actually observed, when the null hypothesis is true. In our case, P = 0.54 means that 54% of the time we could expect to get results with the deviation we saw just by chance. Or, if you repeated the experiment 1000 times, 540 of those experiments would result in a deviation as great or greater than what we observed.  Remember… Mendel falsified idea of blending inheritance…  Mendel’s first results/conclusions Results of reciprocal crosses are always the same. The F1 resembled only one of the parents. The trait missing in the F1 reappeared in about 1⁄4 of the F2 individuals. (exactly as you would predict if… Alleles (factors) segregated / separated (into gametes) and reunited into offspring (randomly). (Principle of Segregation) Mendel’s 2nd Law: Principle of Independent Assortment: The alleles of different genes assort independently of each other (i.e. all allele combinations are equally likely). Mendel’s 2nd Law: Principle of Independent Assortment: The alleles of different genes assort independently of each other (i.e. all allele combinations are equally likely). -Principle of segregation… just looked at one “gene”. Alleles separated / segregated and came back together. Mendel’s 2nd Law: Principle of Independent Assortment: The alleles of different genes assort independently of each other (i.e. all allele combinations are equally likely). -Principle of segregation… just looked at one “gene”. Alleles separated / segregated and came back together. -Principle of independent assortment… looking at 2 (or more) genes / “factors”. Mendel’s 2nd Law: Principle of Independent Assortment: The alleles of different genes assort independently of each other (i.e. all allele combinations are equally likely). Example: Flower color and Plant Height (2 genes) Dihybrid Cross PpTt X PpTt What are the possible gametes? Dihybrid Cross PpTt X PpTt Mendel’s 2nd Law: Principle of Independent Assortment: The alleles of different genes assort independently of each other (i.e. all allele combinations are equally likely). Frequency of Gametes: 1/4 1/4 1/4 1/4 Mendel’s 2nd Law: Principle of Independent Assortment: The alleles of different genes assort independently of each other (i.e. all allele combinations are equally likely). Frequency of Gametes: 1/4 1/4 1/4 1/4 9:3:3:1 ratio Dihybrid Cross PpTt X PpTt Note: you only see this ratio IF alleles at the 2 genes are assorting randomly with respect to each other. (independent assortment) Mendel’s REAL Data from Dihybrid Cross Genotypes Phenotypes Observed P- T- purple, tall 372 P- tt purple, short 113 pp T- white, tall 126 pp tt white short 29 640 Mendel’s Data from Dihybrid Cross (differ from exp?) Genotypes Phenotypes Observed Expected P- T- purple, tall 372 P- tt purple, short 113 pp T- white, tall 126 pp tt white short 29 640 Mendel’s Data from Dihybrid Cross Genotypes Phenotypes Observed Expected P- T- purple, tall 372 9/16 x 640= 360 P- tt purple, short 113 3/16 x 640= 120 pp T- white, tall 126 3/16 x 640= 120 pp tt white short 29 1/16 x 640= 40 640 Mendel’s Data from Dihybrid Cross Genotypes Phenotypes Observed Expected P- T- purple, tall 372 9/16 x 640= 360 P- tt purple, short 113 3/16 x 640= 120 pp T- white, tall 126 3/16 x 640= 120 pp tt white short 29 1/16 x 640= 40 640 = 4.13 Mendel’s Data from Dihybrid Cross Genotypes Phenotypes Observed Expected P- T- purple, tall 372 9/16 x 640= 360 P- tt purple, short 113 3/16 x 640= 120 pp T- white, tall 126 3/16 x 640= 120 pp tt white short 29 1/16 x 640= 40 640 df = 4-1 = 3  χ2 Table Critical χ2 Values 4.13 p = 0.25 (roughly) NOT SIGNIFICANTLY DIFFERENT IF alleles are assorting independently, you’d expect to see this 9:3:3:1 ratio. Mendels results did not differ from this. Implications of Mendel’s Laws: Falsified idea of Blending Inheritance - Genes / “factors” controlled inheritance.