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LECTURE 8 Mendel’s Experiments (Chapter 2)
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2.1 MENDEL’S EXPERIMENTS Gregor Johann Mendel ( ) is considered the father of genetics His success can be attributed, in part, to His boyhood experience in grafting trees This taught him the importance of precision and attention to detail His university experience in physics and natural history This taught him to view the world as an orderly place governed by natural laws These laws can be stated mathematically
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Mendel was an Austrian monk
He conducted his landmark studies in a small 115- by 23-foot plot in the garden of his monastery From , he performed thousands of crosses He kept meticulously accurate records that included quantitative analysis
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It was ignored for 34 years Probably because
His work, entitled “Experiments on Plant Hybrids” was published in 1866 It was ignored for 34 years Probably because The title did not capture the importance of the work Chromosome behavior had not yet been observed by light microscopy
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In 1900, Mendel’s work was rediscovered by three botanists working independently
Hugo de Vries of Holland Carl Correns of Germany Erich von Tschermak of Austria
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Mendel Chose Pea Plants as His Experimental Organism
Hybridization The mating or crossing between two individuals that are pure-breeding for specific phenotypes Purple-flowered plant X white-flowered plant We now know that “pure-breeding” = homozygous, e.g. PP x pp Hybrids The offspring that result from such a mating These are heterozygous – e.g. Pp
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Mendel Chose Pea Plants as His Experimental Organism
Mendel chose the garden pea (Pisum sativum) to study the natural laws governing plants hybrids The garden pea was advantageous because 1. It existed in several varieties with distinct characteristics 2. Its structure allowed for easy crosses where the choice of parental plants could be controlled
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Figure 2.2 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Petals Keel Sepal Stigma Anther Ovule Style Ovary Contain the pollen grains, where the male gametes (sperm) are produced (a) Structure of a pea flower Contain the female gametes (eggs)
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Mendel Chose Pea Plants as His Experimental Organism
Mendel carried out two types of crosses 1. Self-fertilization Pollen and egg are derived from the same plant Naturally occurs in peas because a modified petal isolates the reproductive structures 2. Cross-fertilization Pollen and egg are derived from different plants Required removing and manipulating anthers Refer to Figure 2.3
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Figure 2.3 White Remove anthers from purple flower. Anthers
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. White Remove anthers from purple flower. Anthers Transfer pollen from anthers of white flower to the stigma of a purple flower. Parental generation Purple Cross-pollinated flower produces seeds. Plant the seeds. First- generation offspring
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Mendel Studied Seven Characters That Bred True
The morphological characteristics of an organism are termed characters The term trait describes the specific properties of a character eye color is a character, blue eyes is a trait A variety that produces the same trait over several generations is termed a true-breeder The seven characters that Mendel studied are illustrated in Figure 2.4
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Figure 2.4 CHARACTER VARIANTS CHARACTER VARIANTS Seed color Yellow
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CHARACTER VARIANTS CHARACTER VARIANTS Seed color Yellow Green Height Seed shape Round Wrinkled Tall Dwarf Pod color Flower color Green Yellow Purple White Pod shape Smooth Constricted Flower position Axial Terminal
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Mendel’s Experiments Mendel did not have a hypothesis to explain the formation of hybrids Rather, he believed that a quantitative analysis of crosses may provide mathematical relationships that govern hereditary traits This is called an empirical approach This approach is used to deduce empirical laws
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Mendel’s Experiments Mendel studied seven characteristics
Each characteristic showed two variants found in the same species plant height variants were tall and dwarf His first experiments involved crossing two variants of the same characteristic This is termed a monohybrid cross A single characteristic is being observed The experimental procedure is shown in Figure 2.5
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Figure 2.5 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Experimental level Conceptual level 1. For each of seven characters, Mendel cross-fertilized two different true-breeding lines. Keep in mind that each cross involved two plants that differed in regard to only one of the seven characters studied. The illustration at the right shows one cross between a tall and dwarf plant. This is called a P (parental) cross. P plants TT x tt x Tall Dwarf 2. Collect many seeds. The following spring, plant the seeds and allow the plants to grow. These are the plants of the F1 generation. Note: The P cross produces seeds that are part of the F1 generation. F1 seeds All Tt F1 plants Tt All tall Self- fertilization 3. Allow the F1 generation plants to self-fertilize. This produces seeds that are part of the F2 generation. Self- fertilization F2 seeds TT + 2 Tt + tt 4. Collect the seeds and plant them the following spring to obtain the F2 generation plants. F2 plants 5. Analyze the characteristics found in each generation. Tall Tall Dwarf Tall
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DATA FROM MONOHYBRID CROSSES
P Cross F1 generation F2 generation Ratio Tall X dwarf stem All tall 787 tall, dwarf 2.84:1 Round X wrinkled seeds All round 5,474 round, ,850 wrinkled 2.96:1 Yellow X Green seeds All yellow 6,022 yellow, 2,001 green 3.01:1 Purple X white flowers All purple 705 purple, white 3.15:1 Axial X terminal flowers All axial 651 axial, terminal 3.14:1 Smooth X constricted pods All smooth 882 smooth, constricted 2.95:1 Green X yellow pods All green 428 green, yellow 2.82:1
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Interpreting the Data For all seven characteristics studied
1. The F1 generation showed only one of the two parental traits 2. The F2 generation showed an ~ 3:1 ratio of the two parental traits These results refuted a “blending mechanism” of heredity previously proposed
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Interpreting the Data Indeed, the data suggested a particulate theory of inheritance Mendel postulated the following:
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2. The two factors may be identical or different
1. A pea plant contains two discrete hereditary factors for a given character, one from each parent 2. The two factors may be identical or different 3. When the two factors of a single character are different and present in the same plant One variant is dominant and its effect can be seen The other variant is recessive and is not seen 4. During gamete formation, the paired factors for a given character segregate randomly so that half of the gametes receive one factor and half of the gametes receive the other This is Mendel’s Law of Segregation Refer to Figure 2.6
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But first, let’s introduce a few terms
Mendelian factors are now called genes Alleles are different versions of the same gene An individual with two identical alleles is termed homozygous An individual with two different alleles, is termed heterozygous Genotype refers to the specific allelic composition of an individual Phenotype refers to the outward appearance of an individual
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Figure 2.6 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Tall Dwarf x P generation TT tt Segregation Gametes T T t t Cross-fertilization Tall F1 generation (all tall) Tt Segregation Gametes T t T t Self- fertilization F2 generation Genotypes: (1 : 2 : 1) TT Tt Tt tt Phenotypes: (3 : 1) Tall Tall Tall Dwarf
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Punnett Squares A Punnett square is a grid that enables one to predict the outcome of simple genetic crosses It was proposed by the English geneticist, Reginald Punnett We will illustrate the Punnett square approach using the cross of heterozygous tall plants as an example
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Punnett Squares 1. Write down the genotypes of both parents
Male parent = Tt Female parent = Tt 2. Write down the possible gametes each parent can make. Male gametes: T or t Female gametes: T or t
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Male gametes T t T Female gametes t 3. Create an empty Punnett square
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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4. Fill in the Punnett square with the possible genotypes of the offspring by combining the alleles of the gametes Male gametes T t T TT Tt Female gametes t Tt tt
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5. Determine the relative proportions of genotypes and phenotypes of the offspring
Genotypic ratio TT : Tt : tt 1 : 2 : 1 Phenotypic ratio Tall : dwarf 3 : 1
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Mendel’s Experiments Mendel also performed dihybrid crosses
Crossing individual plants that differ in two characters For example Character 1 = Seed texture (round vs. wrinkled) Character 2 = Seed color (yellow vs. green) There are two possible patterns of inheritance for these characters Refer to Figure 2.7
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Figure 2.7 P generation RRYY rryy RRYY rryy Haploid gametes RY x ry RY
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. P generation RRYY rryy RRYY rryy Haploid gametes RY x ry RY x ry F1 generation RrYy RrYy Haploid gametes 1/2 RY 1/2 ry Haploid gametes 1/4 RY 1/4 Ry 1/4 rY 1/4 ry (a) HYPOTHESIS: Linked assortment (b) HYPOTHESIS: Independent assortment
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Mendel’s Experiments The experimental procedure for the dihybrid cross is shown in Figure 2-8
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Figure 2.8 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Experimental level Conceptual level True-breeding round, yellow seed True-breeding wrinkled, green seed 1. Cross the two true-breeding plants to each other. This produces F1 generation seeds. RRYY rryy Seeds are planted Gametes formed RY x ry Cross- pollination 2. Collect many seeds and record their phenotype. F1 generation seeds All RrYy 3. F1 seeds are planted and grown, and the F1 plants are allowed to self-fertilize. This produces seeds that are part of the F2 generation. RrYy x RrYy RrYy RY Ry rY ry RY RRYY RRYy RrYY RrYy F2 generation seeds Ry RRYy RRyy RrYy Rryy 4. Analyze the characteristics found in the F2 generation seeds. RrYy rY RrYY RrYy rrYY rrYy ry RrYy Rryy rrYy rryy
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DATA FROM DIHYBRID CROSSES
P Cross F1 generation F2 generation Round, Yellow seeds X wrinkled, green seeds All round, yellow 315 round, yellow seeds 101 wrinkled, yellow seeds 108 round, green seeds 32 green, wrinkled seeds
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Interpreting the Data The F2 generation contains seeds with novel combinations (i.e.: not found in the parentals) Round and Green Wrinkled and Yellow These are called nonparentals Their occurrence contradicts the linkage model Refer to Figure 2.7a
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If the genes, on the other hand, assort independently
Then the predicted phenotypic ratio in the F2 generation would be 9:3:3:1 P Cross F1 generation F2 generation Ratio Round, Yellow seeds X wrinkled, green seeds All round, yellow 315 round, yellow seeds 101 wrinkled, yellow seeds 108 round, green seeds 32 green, wrinkled seeds 9.8 3.2 3.4 1.0 Mendel’s data was very close to segregation expectations Thus, he proposed the law of Independent assortment During gamete formation, the segregation of any pair of hereditary determinants is independent of the segregation of other pairs
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Figure 2.9 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. RY Ry rY ry Four possible male gametes: Four possible female gametes: RY Ry rY ry RRYY RRYy RrYY RrYy RRYy RRyy RrYy Rryy RrYY RrYy rrYY rrYy RrYy Rryy rrYy rryy By randomly combining male and female gametes, 16 combinations are possible. Totals: 1 RRYY : 2 RRYy : 4 RrYy : 2 RrYY : 1 RRyy : 2 Rryy : 1 rrYY : 2 rrYy : 1 rryy 9 round, yellow seeds 3 round, green seeds 3 wrinkled, yellow seeds 1 wrinkled, green seed Phenotypes:
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Independent assortment is also revealed by a dihybrid test-cross
TtYy X ttyy Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. TY Ty tY ty TtYy Ttyy ttYy ttyy ty Tall, yellow Tall, green Dwarf, yellow Dwarf, green Thus, if the genes assort independently, the expected phenotypic ratio among the offspring is 1:1:1:1
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Figure 2.10 Cross: TtYy x TtYy TTYY TTYy TtYY TtYy Tall, yellow
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cross: TtYy x TtYy TY Ty tY ty TTYY TTYy TtYY TtYy TY Tall, yellow Tall, yellow Tall, yellow Tall, yellow TTYy TTyy TtYy Ttyy Ty Tall, yellow Tall, green Tall, yellow Tall, green TtYY TtYy ttYY ttYy tY Tall, yellow Tall, yellow Dwarf, yellow Dwarf, yellow TtYy Ttyy ttYy ttyy ty Tall, yellow Tall, green Dwarf, yellow Dwarf, green Genotypes: 1 TTYY : 2 TTYy : 4 TtYy : 2 TtYY : 1 TTyy : 2 Ttyy 1 ttYY : 2 ttYy 1 ttyy 9 tall plants with yellow seeds 3 tall plants with green seeds 3 dwarf plants with yellow seeds 1 dwarf plant with green seeds Phenotypes:
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Linkage Problem In unicorns, horn length is under the control of a single gene where L =long, l = short; horn color is under the control of a second gene where G = gold, g = silver LlGg is test-crossed to determine whether the genes assort independently or are linked Data 1: 34 long silver horns (Lg) 31 short silver horns (lg) 33 short gold horns (lG 29 long gold horns (LG)
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Data 2: 22 long silver horns (Lg)
62 short silver horns (lg) 18 short gold horns (lG) 58 long gold horns (LG) Data 3: 45 long silver horns (Lg) 14 short silver horns (lg) 54 short gold horns (lG) 12 long gold horns (LG)
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Data 4: 5 long silver horns (Lg)
92 short silver horns (lg) 89 short gold horns (lG) 9 long gold horns (LG)
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