Mendel Dreams The Beginning of Genetics Clyde Freeman

Mendel Dreams: The Beginning of Genetics Clyde Freeman Herreid Department of Biological Sciences University at Buffalo Paula P. Lemons Division of Biological Sciences University of Georgia

Learning Objectives • Describe some of the history of Mendel’s work on patterns of inheritance. • Explain the blending hypothesis. • Explain the law of the segregation of alleles. • Explain the law of independent assortment. • Set up a Punnett square correctly. • Draw conclusions about inheritance from genetic cross data. • Predict outcomes from genetic cross experiments. • Design genetic cross experiments to answer questions about inheritance and analyze genetic cross experiments to improve them. 2

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He was dying. And it was January. As was fitting for an old man with swollen feet, a disagreeable kidney, and an untenable position on the chessboard, he closed his eyes. 4

He dozed in an old man’s way, thinking not of yesterday’s battle with the city alderman over the infernal tax problems of the Abbey, but of his childhood days on the farm, his sister who had forfeited her dowry so he could go to school, and his parents. He pulled his cleric’s robe tighter around his too ample frame. He was cold. He waited. For Leos. The choirmaster had promised he would play at his funeral. He smiled and remembered him as a boy. 5

For sixteen years he had been the Abbot of the Abbey—a great honor. But it had drained his strength; he could no longer tend to his scientific studies. Still there were compensations. Yet at this moment, on this cold January 6 th, he had difficulty remembering what they were. 6

But mostly he remembered his beloved garden in the abbey where he worked so long ago… eight years with 29, 000 pea plants to tend. 7

How long ago was it? Twenty? Thirty years? “Here boys, listen. ” He was once again a teacher at Znaim. “Listen, boys. It is possible to cross breed plants just like animals. One has to take the pollen from one plant with a small brush and carefully, so carefully, place it on another. ” 8

“Here I take the pollen from the male and place it upon the carpel of another plant. Then the plant does the rest. ” 9

“But why would you do that, Father? ” “Because I want to know the rules. There must be rules—laws of breeding, just as there are laws of physics. “I want to find those rules. That is why, Master Pavel, I seek the answers. ” 10

“But how do you stop a bee bringing pollen from one plant to another to mix up your results, Father? ” “Try to guess. What should I do? ” “Possibly bring the plant inside, or cut the anther off before you pollinate the plant or cover the plant with a little sac? Or. . . maybe, Father, you just pray? ” 11

“Yes, I did most of those things. “Sometimes I did work in the greenhouse. When I worked in my garden, I covered the flower with a little cheesecloth bag. And I did have to remove the anther of the flower so that it would not fertilize itself. “And most assuredly I did pray. ” 12

“I was particularly interested in testing the old idea that the traits of the offspring were a simple blending of the characteristics of the two parents. You boys all know that you look like a combination of your two parents. “Pavel, you have your mother’s nose and sense of humor and your father’s eyes and his musical talent. “The same with all of us. “But I had long wondered what would happen if I looked carefully at individual traits. What then? ” 13

Clicker Question 1 “Here is one of my experiments. What do you think would happen when I breed a white-flowered plant with a purpleflowered plant, if the blending hypothesis is correct? ” A. All of the flowers in the next generation would be white. B. All of the flowers in the next generation would be purple. C. All of the flowers in the next generation would be light purple. D. Some of the flowers in the next generation would be white and some purple. E. The next generation would not flower at all. 14

“Here is what did happen. It is not at all what the plant scholars anticipated. ” 15

“How can that be, Father? Where did the white go? Purple must be stronger than white. I say it dominates the white. ” “I think so too, Pavel. I think so too, but not in the way you imagine. “Let me show you another cross, this time between a tall plant and a short plant. Notice, I am only watching one trait at a time. I could be watching the flower color too, but that would be too complicated. Let’s just look at the height of the plant. ” 16

Monohybrid Crosses (Breeding experiments involving one trait) P Tall x Tall Short x Short F 1 All Tall All Short “Here I breed two tall parents with each other and they breed true. They produce all tall children. “And I breed two short plants and they breed true, yielding all short. ” 17

“But look what happens when I breed a tall and short together. ” P Tall x Tall Short x Short F 1 All Tall All Short F 2 All Tall 18

“It is the same thing, Father. The short has disappeared. Where has it gone? Tall is stronger than short. ” P F 1 F 2 Tall x Tall Short x Short All Tall All Short All Tall The tall trait is dominant 19

“The same thing happened with these traits too. The ones on top are dominant over those below. What could I conclude? ” 20

Clicker Question 2 Which of the following conclusions could Mendel draw from the breeding experiments we’ve examined to this point? There may be more than one good answer to this question. Choose the one you think is best, and be prepared to discuss each answer. A. Parents’ traits blend together in their offspring. B. Parents’ traits do not blend together in their offspring. C. It is not possible to predict what version of the trait will be present in the offspring. D. One version of the trait is lost during reproduction. E. Each of the different versions of that trait maintain a discrete identity in the offspring. 21

“I conclude that the traits do not blend together in the offspring of two parents. The blending hypothesis can be rejected!” The reason that everyone thought blending was right was because they never looked at individual traits. 22

“Now look what happens when I breed the two F 2 generation tall plants together. ” F 1 All Tall F 2 Tall All Short Tall 787 Tall x ? and 297 Short 23

Clicker Question 3 “With these new data, what should I conclude? ” Note that there may be more than one good answer to this question. Choose the one you think is best, and be prepared to discuss each answer. A. This is additional evidence that traits do not blend together in the offspring of two parents. B. It is not possible to predict what version of the trait will be present in the offspring. C. One version of the trait is lost during reproduction. D. Each of the different versions of that trait maintain a discrete identity in the offspring. 24

“I concluded that the factors that control a trait maintain a discrete identity when passed from parent to offspring. ” This became known as my Law of Segregation 25

Clicker Question 4 In humans the allele for glaucoma is dominant over the allele for normal eyes. Suppose a man with glaucoma marries a woman with normal eyes and they have children. If this trait is inherited like height in pea plants, which of the following would be true? A. All of their children will have glaucoma, but only some of their grandchildren will. B. None of their children will have glaucoma, and some of their grandchildren will. C. Some of their children may have glaucoma, and if the children with glaucoma reproduce with someone who also has glaucoma, most of those people will have glaucoma but some will have normal eyes. D. There is insufficient data to determine which, if any, of these answers is true. 26

Back to Mendel. . . “But I noticed something strange. The dominant traits were always more common than the recessive traits…” Trait Dominant vs. Recessive Phenotypes Dominant vs. Recessive Frequency Seed shape Round vs. Wrinkled 5474 vs. 1850 Seed color Yellow vs. Green 6022 vs. 2001 Pod shape Full vs. Constricted 882 vs. 299 Pod color Green vs. Yellow 428 vs. 152 Flower color Purple vs. White 705 vs. 224 Flower and position Axial vs. Terminal 651 vs. 207 Stem length Tall vs. Short 787 vs. 277 27

“… and they were in almost the same ratio. ” Dominant vs. Recessive Phenotypes Dominant vs. Recessive Round vs. wrinkled seed shape 5474 / 1850 = 2. 96 Yellow vs. green seed 6022 / 2001 = 3. 01 Full vs. constricted pod 882 / 299 = 2. 95 Green vs. yellow pod 428 / 152 = 2. 82 Purple vs. white flower 705 / 224 = 3. 15 Axial vs. terminal position 651 / 207 = 3. 14 Tall vs. short length 787 / 277 = 2. 84 75% show dominant traits 25% show recessive traits 28

“How could you explain this, Pater Gregor? ” “My mathematical training in the university came to my aid. My physics professor, Herr Doppler, always encouraged us to think mathematically. I asked myself could I explain this 3: 1 ratio simply? ” “Yes. Yes, and again, yes. ” 29

“Suppose that the traits were factors like particles. “We could list them this way: the dominant factor T = tall plant the recessive factor t = short plant. “Imagine that a pure bred tall plant would have 2 TT & a pure bred short plant would have 2 tt. ” 30

“Then when I bred them TT x tt = ? “If they would only pass on one of their traits via the pollen or the egg, then what? ” “I have it, Father. I have it! The answer would be Tt. ” 31

“But, Father, that would mean that the tall hybrids would be carrying a factor for shortness. ” “Exactly, exactly. But you cannot see the shortness. It is hidden. And what would happen when two hybrids were bred? Tt x Tt = ? ” “I know, Father, there would be 1 pure tall plant, 2 hybrid plants, and 1 short plant. ” “Exactly!” 32

MENDEL ANALYZES THE DATA Traits passed in the gametes Pollen (sperm) & eggs • One factor (allele) comes from the pollen • One factor (allele) comes from the egg. Note: ¾ Tall= TT, Tt, t. T ¼ Short= tt 3: 1 Phenotypic Ratio 33

Possible gamete combinations Tt x Tt = t. T, Tt, TT tt TT & Tt T Notice the 3: 1 ratio 34

Clicker Question 5 Look at the glaucoma question again. What information would be most helpful to you in predicting with more certainty whether the offspring of these parents will have glaucoma or normal eyes: a man with glaucoma (dominant) marries a woman with normal eyes (recessive) and they have children. A. Whether the man’s parents had glaucoma. B. Whether the woman’s parents had glaucoma. C. Whether the man is “pure” for glaucoma. D. Whether the woman is “pure” for normal eyes. 35

Using a Punnett Square Alleles= Alternative forms of the same gene (T or t) Homozygous dominant Pollen T Eggs t T TT Tt t t. T tt Heterozygous Suppose Tt X Tt Heterozygous Homozygous recessive 36

Terms and Principles • • Dominant (e. g. , Tall) Recessive (e. g. , Short) Monohybrid crosses involve following 1 trait Dihybrid crosses involve following 2 traits Phenotype (visible characteristics) Genotype (genetic characteristics) Particulate theory of inheritance = 2 particles (factors) determine a trait 37

Terms and Principles • • • Alleles Homozygous dominant Homozygous recessive Heterozygous Punnett square 38

Clicker Question 6 “Now students, suppose I wanted to test my particulate hypothesis of inheritance. Let’s say you gave me a tall plant, and asked me to tell you what factors [alleles] it carried? How might I do it? ” A. Breed two unknown plants together. B. Breed the unknown with any tall plant. C. Breed the unknown with any short plant. D. Breed the unknown with a tall heterozygous plant. E. Breed the unknown with a short heterozygous plant. 39

Answer Slide “Here is what I did: “If the unknown plant is tall, it must be either TT or Tt. “If I breed it with a pure short plant, tt, what are the predicted outcomes? ” 40

Clicker Question 7 What are the predicted outcomes from this experiment? A. Either all tall plants or approximately 75% tall plants and 25% short plants. B. Only 75% tall and 25% short. C. Only tall plants. D. Either all tall plants or approximately 50% tall plants and 50% short plants. E. Only 50% tall and 50% short. 41

“Here is what I did: “If the unknown plant is tall, it must be either TT or Tt. “If I breed it with a pure short plant, then…” Here are the possibilities: TT x tt or Tt x tt 42

Clicker Question 8 “Now Leos, suppose the results were 57 tall and 43 short, what would you conclude? ” A. The unknown was a pure tall. B. The unknown was heterozygous for height. C. There was an error with the experiment because these results are nothing like I predicted. 43

Gregor Mendel opened his eyes and saw Leos quietly putting coal on the fire. The monk reached over to his cold cigar. Holding it to a candle, he lit it, puffing deeply. He knew it was bad for his heart— his kidneys—his everything. A disgusting habit, he thought. He reached for his old notes, and paged along. 44

Gregor Mendel knew he was dying. He had a good life. He communicated with scientists and worshiped with God. Even now in his latter years when the business of the Abbey was so irritating, he still had his music and chess and his cigars. Always his twenty cigars a day. A small vice that God would forgive. 45

He no longer could breed mice, for they escaped too often and stunk too much. But he had his honey bees and his meteorological reports—his records. He always kept meticulous records. Records were essential, otherwise his pea experiments would have been impossible. How else could he have been able to keep track of two traits or more when he was breeding? 46

He found the section in his notes, called Dihybrid Crosses, where he analyzed 2 traits at the same time. Given that he wanted to breed a pure round yellow seed plant (both were dominant traits) with a pure wrinkled green seed (both were recessive traits), he decided that he would write it this way: RRYY x rryy 47

Dihybrid Crosses (Breeding experiments involving two traits) Seed shape = Round (R) or wrinkled (r) seeds Seed color = Yellow (Y) or green (y) seeds P F 1 RRYY x Rr. Yy Round Yellow rryy Genotype Phenotype 48

Record keeping was essential. Mendel thought the traits for seed shape and color were inherited completely separate from one another. Suppose he was correct. Set up a Punnett square for Rr. Yy x Rr. Yy Fill in the headings for the squares Fill in 49

Clicker Question 9 Suppose someone set the Punnett square up this way for the following cross: Rr. Yy x Rr. Yy. What would you say? RR rr YY yy A. Correct because the eggs and the sperm are written the same way. B. Correct because the gametes carry two alleles for each trait. C. Correct but you should understand that there are many ways to set up a Punnett square for RRYY x rryy. D. Not correct because the gametes should carry only one allele for each trait and each gamete should have an allele for both traits. 50

Dihybrid Cross Rr. Yy x Rr. Yy RY Ry r. Y ry RY RRYy Rr. YY Rr. Yy Ry RRYy RRyy Rr. Yy Rryy r. Y Rr. Yy rr. YY rr. Yy ry Rr. Yy Rryy rr. Yy rryy Find the combinations where there are round yellow seeds. 51

How many round yellow seeds will there be out of 16 possibilities? RY Ry r. Y ry RY RRYy Rr. YY Rr. Yy Ry RRYy RRyy Rr. Yy Rryy r. Y Rr. Yy rr. YY rr. Yy ry Rr. Yy Rryy rr. Yy rryy • 9 out of 16 52

Possibilities Rr. Yy x Rr. Yy RY Ry r. Y ry RY RRYy Rr. YY Rr. Yy Ry RRYy RRyy Rr. Yy Rryy r. Y Rr. Yy rr. YY rr. Yy ry Rr. Yy Rryy rr. Yy rryy 9: Round Yellow 3: Round Green 3: Wrinkled Yellow 1: Wrinkled Green 53

RATIO 9: 3: 1 Round Yellow Round Green Wrinkled Yellow Predicted 56 19 19 6 Actual 55 19 17 4 Wrinkled Green • Consistent with the predicted 9: 3: 3: 1 ratio. 54

Mendel’s Data for the Dihybrid Cross • Consistent with the predicted 9: 3: 3: 1 ratio. • Mendel called this condition independent assortment – Factors (genes) sort independently of one another when passed from parent to offspring. 55

Clicker Question 10 In humans, a gene controls formation of muscles in the tongue that allow people with those muscles to roll their tongues, while people who lack those muscles cannot roll their tongues. The ability to roll one’s tongue is dominant over non-rolling. The ability to taste certain substances is also genetically controlled. For example, there is a substance called phenylthiocarbamate (PTC for short), which some people can taste (the dominant trait), while others cannot (the recessive trait). Suppose a woman who is both a homozygous tongue-roller and a non-PTCtaster marries a man who is heterozygous for tongue-rolling and PTC tasting. Let R represent tongue-rolling, r represent a non-roller, T represent ability to taste PTC, and t represent non-tasting. What is the predicted phenotypic ratio of tongue-rolling and PTC-tasting among their children? 56

Clicker Question 10 What is the predicted phenotypic ratio of tonguerolling and PTC-tasting among their children? A. 9 rolling, tasting: 3 rolling, non-tasting: 3 non-rolling, tasting: 1 non-rolling, non-tasting. B. 1 rolling, tasting: 1 rolling, non-tasting. C. 1 rolling, tasting: 1 rolling, non-tasting: 1 non-rolling, non-tasting. D. 3 rolling, tasting: 1 rolling, non-tasting E. All rolling, tasting. 57

Gregor Mendel died on January 6, 1884, of chronic nephritis. He entered the Abbey at the age of 21 and was ordained at the age of 25. He worked in his garden studying peas from 1856 to 1863 studying 29, 000 plants and published his landmark paper, but no one took serious notice. It was too mathematical and went against the prevailing view of blending characteristics. But he was always fond of saying, “My time will come. ” 58

Mendel is credited with three major ideas: 1)The Particle Theory of Inheritance – which states that hereditary traits act like particles, units, or factors as they are passed from generation to generation. 2) His Law of Segregation – which states that his hereditary factors do not blend but remain distinct during breeding— thus, disproving the blending theory. 3) His Law of Independent Assortment – which states that character traits are not connected but are inherited independent of one another. This law was soon to be modified. 59

When Mendel died on that January 6, 1884, at the age of 61, the great composer, Leos Janacek, played the organ at his funeral as promised. To his friends’ sorrow his papers were burned by the abbot who succeeded him. 60

Twenty-five years later his work was rediscovered by three botanists and his work became famous. His time had come. 61

Image Credits Slide 1: Description: Fresh green peas, Fotolia image #34423680. Source: http: //us. fotolia. com/id/34423680 Clearance: © vadim yerofeyev - Fotolia. com. Licensed royalty free image. Slide 3, Slide 62: Description: Group photograph of the Augustinian friars at the Abbey of St Thomas. Source: Mendelianum, Maravian Museum, Brno http: //www. mendel-museum. com/eng/1 online/popup. php 3? id=r 1_a_1 Slide 4: Description: Snowy day photo Source: http: //www. publicdomainpictures. net/view-image. php? image=3439&picture=snowy-day Clearance: Public domain Slide 6: Description: Mendel. Source: http: //commons. wikimedia. org/wiki/File: Mendel_Gregor_1822 -1884. jpg, from The History of Biology de Erik Nordenskiöld, Ed. Knopf, 1928. Clearance: Expired copyright 62

Slide 7: Description: Augustinian Monastery and Basilica of the Assumption of Our Lady, Brno, Czech Republic. Source: http: //commons. wikimedia. org/wiki/File: St. Thomas. Abbey. Brno. jpg, by Gareth Simkins (also known as User: Parmesan) in April 2005, scanned and uploaded in November 2005. Clearance: Used in accordance with the Creative Commons Attribution-Share Alike 3. 0 Unported license. Slide 9: Description: Depiction of artificial pollination. Source: University of Waikato, http: //www. biotechlearn. org. nz/themes/mendel_and_inheritance/images/cross_pollination_of_pea_pl ants Clearance: Used in accordance with educational usage guidelines as described at http: //www. biotechlearn. org. nz/about_this_site/copyright_and_privacy Slide 20: Description: Diagram showing the seven "characters" observed by Mendel Source: Modified from an image by Mariana Ruiz, available at Wikimedia Commons, http: //commons. wikimedia. org/wiki/File: Mendel_seven_characters. svg Clearance: Public domain. Slide 60: Description: Leoš Janáček in 1881. Source: Cropped version of a photo from http: //commons. wikimedia. org/wiki/File: Janacek_with_wife. jpg, identified as coming "from the archive of the Department of Music History of the Moravian Regional Museum in Brno. ” Clearance: Expired copyright. 63