Benjamin A. Pierce Genetics Essentials Concepts and Connections SECOND EDITION CHAPTER 3 Basic Principles of Heredity CHAPTER 3 OUTLINE 3.1 Gregor Mendel Discovered the Basic Principles of Heredity, 44 3.2 Monohybrid Crosses Reveal the Principle of Segregation and the Concept of Dominance, 47 3.3 Dihybrid Crosses Reveal the Principle of Independent Assortment, 55 3.4 Observed Ratios of Progeny May Deviate from Expected Ratios by Chance, 60 2013 W. H. Freeman and Company The Case of the Red Hair Did you read the Chapter 3? What do you think, what is the case? Did you find any new research on the subject? Are you ready to present? Mendel and His Study of Heredity His success was result of his choice of model organism Pisum sativum Rapid growth Many offspring Large number of varieties with known genetic makeup 7 characteristics he investigated And his mathematical knowledge Before we Continue 3.2 MONOHYBRID CROSSES REVEAL THE PRINCIPLE OF SEGREGATION AND THE CONCEPT OF DOMINANCE Monohybrid cross: cross between two parents that differ in a single characteristic. Conclusion 1: one character is encoded by two genetic factors. Conclusion 2: two genetic factors (alleles) separate when gametes are formed. Conclusion 3: The concept of dominant and recessive traits. Conclusion 4: Two alleles separate with equal probability into the gametes. Fig. 3.3 1
Experimental Outline Mendel used this approach for all studied characteristics Observe Count Analyze The Monohybrid Cross Conclusion 1: one character is encoded by two genetic factors. Cross between parents different in a single characteristic (in this example seed shape; round or wrinkled) Parental (P) generation Pure breed (homozygous) Filial 1 (F1) generation Uniform only one characteristic Filial 2 (F2) generation Both characteristics appear in 3:1 ratio IN MENDEL S PEA PLANTS, HOW DID HE KNOW THAT EACH F1 GENERATION CONTAINED TWO ALLELES ENCODING DIFFERENT CHARACTERISTICS? a. The F1 generation had a blended phenotype of the two parental phenotypes. b. Both parental phenotypes reappeared in the F2 generation. c. Each F1 plant had a different phenotype. d. The F1 plants did not have two alleles. e. None of the above. The Labeling of Factors (Alleles) Conclusion 3: The concept of dominant and recessive traits. Remember meiosis and gamete formation? Each gamete and representative allele is labeled with a letter that corresponds to the characteristic studied R for round (dominant) r for wrinkled (recessive) The Factors (Alleles) in the Experiment Conclusion 2: two genetic factors (alleles) separate when gametes are formed. What happens with the alleles in the cross of the P generation? F1 generation is heterozygous Rr F1 generation form gametes Each plant two different gametes The Alleles in the Next Generation Conclusion 4: Two alleles separate with equal probability into the gametes. Self fertilization of F1 plants The two gametes combine and fertilize randomly F2 generation Two phenotypes (3:1 ratio) Three genotypes RR Rr rr How will that be determined? 2
Yet another generation: F3 3.2 MONOHYBRID CROSSES REVEAL THE PRINCIPLE OF SEGREGATION AND THE CONCEPT OF DOMINANCE Principle of segregation: (Mendel s first law) Each individual diploid organism possesses two alleles for any particular characteristic. These two alleles segregate when gametes are formed, and one allele goes into each gamete. The concept of dominance: when two different alleles are present in a genotype, only the trait encoded by one of them the dominant allele is observed in the phenotype. The Crosses and the Meiosis Meiosis I- Crossing Over Introduction to chromosome theory of heredity Meiosis II- the Gametes and the Alleles Predicting the Outcomes of Genetic Crosses- The Punnett Square The example of backcross on tall and short plants (T and t alleles) F1 heterozygous (Tt) plant with homozygous recessive (tt) parental variety Generate possible gametes from each parent Align the gametes from one parent vertically on the left Alight the gametes of the other parent horizontally on top MIX and count 3
Predicting the Outcomes of Genetic Crosses- Multiplication Rule Two or more independent events taking place together Key word is and Probability to roll 4 is 1/6 What is the probability to obtain 4 in the first and the second try? 1/6 in the first and 1/6 in the second 1/6 X 1/6 = 1/36 Independent means that the first does not influence the second 3.3 DIHYBRID CROSSES REVEAL THE PRINCIPLE OF THE INDEPENDENT ASSORTMENT Dihybrid Crosses The principle of independent assortment Relating the principle of independent assortment to meiosis Applying probability and the branch diagram to dihybrid crosses The Dihybrid testcross Dihybrid Cross Cross between parents different in two characteristic (in this example seed shape and color; round/wrinkled and yellow/green) Parental (P) generation Pure breed (homozygous for two characteristics) Filial 1 (F1) generation Uniform round/yellow Filial 2 (F2) generation Various combinations of the two characteristics appear in 9:3:3:1 ratio The Factors (Alleles) in the Experiment Conclusion: genetic factors (alleles) of the first trait separate independently from the alleles of the second trait What happens with the alleles in the cross of the P generation? F1 generation is heterozygous for both traits RrYy F1 generation form gametes Each sex forms four different gametes 4
The Alleles in the Next Generation Conclusion: genetic factors (alleles) of the first trait separate independently from the alleles of the second trait The Meiotic Behavior of Chromosomes Explains the Independent Assortment Self fertilization of F1 plants The four gametes fertilize randomly F2 generation Four phenotypes 16 genotypes in 9:3:3:1 ratio Crosses and Probabilities- Branched Diagrams-Monohybrid Crosses and Probabilities- Branched Diagrams-Dihybrid Crosses and Probabilities- Branched Diagrams-Dihybrid Test Cross Human Traits- Pedigrees ½ Rr ½ Yy ½ yy ¼ Rr Yy ¼ Rr yy ½ rr ½ Yy ½ yy ¼ rr Yy ¼ rr yy 5
Pedigrees- Autosomal Dominant Waardenburg Syndrome- deafness might have fair skin, white forelock and visual problems Pedigrees- Autosomal Recessive consanguinity 6