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LECTURE 8 Mendel’s Experiments (Chapter 2). 2.1 MENDEL’S EXPERIMENTS Gregor Johann Mendel (1822-1884) is considered the father of genetics His success.

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Presentation on theme: "LECTURE 8 Mendel’s Experiments (Chapter 2). 2.1 MENDEL’S EXPERIMENTS Gregor Johann Mendel (1822-1884) is considered the father of genetics His success."— Presentation transcript:

1 LECTURE 8 Mendel’s Experiments (Chapter 2)

2 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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4 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

5 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

6 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

7 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

8 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

9 (a) Structure of a pea flower Petals Keel Stigma Anther Style Ovary Sepal Ovule Figure 2.2 Contain the pollen grains, where the male gametes (sperm) are produced Contain the female gametes (eggs) Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

10 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

11 Figure 2.3 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Remove anthers from purple flower. Anthers Transfer pollen from anthers of white flower to the stigma of a purple flower. Cross-pollinated flower produces seeds. Plant the seeds. White Purple Parental generation First- generation offspring

12 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

13 Figure 2.4 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. CHARACTERVARIANTS Flower color PurpleWhite Flower position AxialTerminal Height TallDwarf CHARACTERVARIANTS Seed color YellowGreen Seed shape RoundWrinkled Pod shape SmoothConstricted Pod color Green Yellow

14 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

15 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

16 Figure 2.5 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Experimental level P plants TallDwarf F 1 seeds Self- fertilization All tall Self- fertilization F 1 plants F 2 seeds F 2 plants Conceptual level TT + 2 Tt + tt All Tt Tt x x TTtt Note: The P cross produces seeds that are part of the F 1 generation. Tall Dwarf 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. 2. Collect many seeds. The following spring, plant the seeds and allow the plants to grow. These are the plants of the F 1 generation. 3. Allow the F 1 generation plants to self-fertilize. This produces seeds that are part of the F 2 generation. 4. Collect the seeds and plant them the following spring to obtain the F 2 generation plants. 5. Analyze the characteristics found in each generation.

17 P CrossF 1 generationF 2 generationRatio Tall X dwarf stem All tall787 tall, 277 dwarf 2.84:1 Round X wrinkled seeds All round5,474 round, 1,850 wrinkled 2.96:1 Yellow X Green seeds All yellow6,022 yellow, 2,001 green 3.01:1 Purple X white flowers All purple705 purple, 224 white 3.15:1 Axial X terminal flowers All axial651 axial, 207 terminal 3.14:1 Smooth X constricted pods All smooth882 smooth, 229 constricted 2.95:1 Green X yellow pods All green428 green, 152 yellow 2.82:1 DATA FROM MONOHYBRID CROSSES

18 Interpreting the Data For all seven characteristics studied –1. The F 1 generation showed only one of the two parental traits –2. The F 2 generation showed an ~ 3:1 ratio of the two parental traits These results refuted a “blending mechanism” of heredity previously proposed

19 Interpreting the Data Indeed, the data suggested a particulate theory of inheritance Mendel postulated the following:

20 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

21 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

22 Figure 2.6 P generation Segregation Self- fertilization Cross-fertilization F 2 generation Genotypes: (1 : 2 : 1) Phenotypes: (3 : 1) Gametes F 1 generation (all tall) Gametes t T t t TT t x Tall TT Tt Dwarf tt Tt TT tt Tall Dwarf T Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

23 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

24 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

25 3. Create an empty Punnett square Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Male gametes Female gametes T T t t

26 TTTt tt Male gametes Female gametes T T t t 4. Fill in the Punnett square with the possible genotypes of the offspring by combining the alleles of the gametes

27 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

28 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

29 Figure 2.7 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. P generation Haploid gametes RRYY RYx rryy RrYy F 1 generation Haploid gametes RY ry RRYY RYx rryy RrYy ry Ry ry rY RY (a) HYPOTHESIS: Linked assortment(b) HYPOTHESIS: Independent assortment 1/21/2 1/21/2 1/41/4 1/41/4 1/41/4 1/41/4

30 Mendel’s Experiments The experimental procedure for the dihybrid cross is shown in Figure 2-8

31 Figure 2.8 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1. Cross the two true-breeding plants to each other. This produces F 1 generation seeds. 2. Collect many seeds and record their phenotype. 3. F 1 seeds are planted and grown, and the F 1 plants are allowed to self-fertilize. This produces seeds that are part of the F 2 generation. 4. Analyze the characteristics found in the F 2 generation seeds. Seeds are planted Gametes formed Conceptual levelExperimental level RRYY RRYyRrYYRrYy RRYyRRyyRrYyRryy RrYYRrYyrrYYrrYy RrYyRryyrrYyrryy x RY rryy All RrYy RrYy RY Ry rY ry Ryry RrYy ry Cross- pollination F 1 generation seeds F 2 generation seeds True-breeding round, yellow seed True-breeding wrinkled, green seed x rY

32 P CrossF 1 generationF 2 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 DATA FROM DIHYBRID CROSSES

33 Interpreting the Data The F 2 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

34 P CrossF 1 generationF 2 generationRatio Round, Yellow seeds X wrinkled, green seeds All round, yellow315 round, yellow seeds 101 wrinkled, yellow seeds 108 round, green seeds 32 green, wrinkled seeds If the genes, on the other hand, assort independently –Then the predicted phenotypic ratio in the F 2 generation would be 9:3:3:1 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

35 Figure 2.9 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Four possible male gametes: Four possible female gametes: RY Ry ry rY Ry RY RRYY RRYyRrYY RrYyRRYyRRyy RrYyRryyRrYYRrYy rrYY RrYyRryyrrYy 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 Phenotypes: rrYy 9 round, yellow seeds 3 round, green seeds 3 wrinkled, yellow seeds 1 wrinkled, green seed

36 Independent assortment is also revealed by a dihybrid test- cross –TtYy X ttyy Thus, if the genes assort independently, the expected phenotypic ratio among the offspring is 1:1:1:1 TY ty TytYty Tall, yellowTall, green Dwarf, yellowDwarf, green TtYyTtyyttYyttyy Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

37 Figure 2.10 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cross: TtYy x TtYy TY Ty tY ty Genotypes: 1 TTYY : 2 TTYy : 4 TtYy : 2 TtYY : TTYYTTYy TtYYTtYy Tall, yellow Ttyy TtYy TTyy TTYy TtYY TtYy ttYY ttYy ttyyttYy Ttyy TtYy Tall, yellow Tall, green Dwarf, yellow Dwarf, green Phenotypes: 1 TTyy : 2 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 1 ttYY : 2 ttYy 1 ttyy Ty tYty

38 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)

39 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)

40 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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