Mendel Dreams: The Beginning of Genetics

1. Mendel Dreams: The Beginning of Genetics

3. 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 6th, 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.

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 purple-flowered plant, if the blending hypothesis is correct?” • All of the flowers in the next generation would be white. • All of the flowers in the next generation would be purple. • All of the flowers in the next generation would be light purple. • Some of the flowers in the next generation would be white and some purple. • 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 F1 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 F1 All Tall All Short F2 All Tall

18. “It is the same thing, Father. The short has disappeared. Where has it gone? Tall is stronger than short.” P Tall x Tall Short x Short F1 All Tall All Short F2 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. • Parents’ traits blend together in their offspring. • Parents’ traits do not blend together in their offspring. • It is not possible to predict what version of the trait will be present in the offspring. • One version of the trait is lost during reproduction. • 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 F2 generation tall plants together.” F1 All Tall All Short F2 Tall Short Tall x Tall ? 787 Tall 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. • This is additional evidence that traits do not blend together in the offspring of two parents. • It is not possible to predict what version of the trait will be present in the offspring. • One version of the trait is lost during reproduction. • 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? • All of their children will have glaucoma, but only some of their grandchildren will. • None of their children will have glaucoma, and some of their grandchildren will. • 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. • 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…”

27. “… and they were in almost the same ratio.” 3:1 Ratio 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, tT ¼ Short= tt 3:1 Phenotypic Ratio

33. Possible gamete combinations Tt x Tt = tT, 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. • Whether the man’s parents had glaucoma. • Whether the woman’s parents had glaucoma. • Whether the man is “pure” for glaucoma. • Whether the woman is “pure” for normal eyes.

35. GENETIC TERMS • Gene: a sequence of DNA that encodes for a certain trait • Allele: one of two (or more) alternative forms of a gene (a single letter) • Dominant Allele: an allele that dictates the expression of a trait (capital letter, ex: A) • Recessive Allele: an allele whose trait is masked by the presence of a dominant allele (lower case letter, ex: a)

36. TERMS Genotype: genetic make-up of an organism (letter combination) Phenotype: physical appearance of an organism (its outward appearance) Homozygous: both alleles in a gene pair code for the same trait (ex: AA or aa) Heterozygous: the two alleles in a gene pair that do not code for the same trait (ex: Aa)

37. TERMS Sex Chromosome: the chromosome that determines the sex of an organism (the X and Y chromosome) Autosome: any chromosome that is not a sex chromosome Punnett Square: a chart which shows all possible gene combinations in a cross of parents Monohybrid cross: a cross between two individuals for one trait (ex: Aa x Aa) Dihybrid cross: crossing two different characteristics at the same time (AaBb x AaBb)

38. TERMS Genotypic Ratio: the number of times each genotype appears in the offspring. Written from most dominant trait to the recessive. Phenotypic Ratio: the number of times each phenotype appears in the offspring. Written from the dominant trait to the recessive. Law of Segregation: factors that control a trait maintain a discrete identity when passed from parent to offspring.

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

40. 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?” • Breed two unknown plants together. • Breed the unknown with any tall plant. • Breed the unknown with any short plant. • Breed the unknown with a tall heterozygous plant. • Breed the unknown with a short heterozygous plant.

41. 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?”

42. Clicker Question 7 What are the predicted outcomes from this experiment? • Either all tall plants or approximately 75% tall plants and 25% short plants. • Only 75% tall and 25% short. • Only tall plants. • Either all tall plants or approximately 50% tall plants and 50% short plants. • Only 50% tall and 50% short.

43. “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, then…” Here are the possibilities: TT x tt or Tt x tt

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

45. 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.

46. Gregor Mendel knew he was dying. He had 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 would be forgiven.

47. 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?

48. 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

49. Dihybrid Crosses(Breeding experiments involving two traits) Seed shape = Round (R) or wrinkled (r) seeds Seed color = Yellow (Y) or green (y) seeds P RRYY x rryy F1 RrYy Genotype Round Yellow Phenotype

50. 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 RrYy x RrYy