Understanding Gregor Mendel's Work in Genetics

1. Chapter 9: Introduction to Genetics Section 1: The Work of Gregor Mendel

2. The Work of Gregor Mendel • Biological inheritance, or heredity, is the key to differences between species • Heredity is much more than the way in which a few characteristics are passed from one generation to another • Heredity is at the very center of what makes each species unique, as well as what makes us human • The branch of biology that studies heredity is called genetics

3. Early Ideas About Heredity • Until the 19th century, the most common explanation for family resemblances was the theory of blending inheritance • Because both male and female were involved in producing offspring, each parent contributed factors that were “blended” in their offspring • But, in the last century biologists began to look at the details of heredity • They began to develop a very different view • The work of the Austrian monk Gregor Mendel was particularly important in changing people’s views about how characteristics are passed from one generation to the next

4. Gregor Mendel • Born in 1822 to peasant parents in what is now the Czech Republic • Entered a monastery at the age of 21 • Four years later he was ordained a priest • In 1851, Mendel was sent to the University of Vienna to study science and mathematics • He returned 2 years later and spent the next 14 years teaching high school • In addition to his duties, Mendel was in charge of the monastery garden • This is where he did his work that revolutionized biological science

5. Gregor Mendel • From his studies, Mendel had gained an understanding of the sexual mechanisms of pea plants • Pea flowers have both male and female parts • Normally, pollen from the male part of the pea flower fertilizes the female egg cells of the very same flower • Self-pollination • Seeds produced by self-pollination inherit all of their characteristics from the single plant that bore them

7. Gregor Mendel • Mendel learned that self-pollination could be prevented • He was able to pollinate the two plants by dusting the pollen from one plant onto the flowers of another plant • Cross-pollination • Produces seeds that are the offspring of two different plants • Mendel was able to cross plants with different characteristics

9. Gregor Mendel • Mendel started his studies with peas that were purebred • If they were allowed to self-pollinate, the purebred peas would produce offspring that were identical to themselves • These purebred plants were the basis of Mendel’s experiments

10. Gregor Mendel • In many respects, the most important decision Mendel made was to study just a few isolated traits, or characteristics, that could be easily observed • He chose seven different traits to study • By deciding to restrict his observations to just a few traits, Mendel made his job of measuring the effects of heredity much easier

12. Genes and Dominance • Mendel decided to see what would happen if he crossed pea plants with different characters for the same trait • A character is a form of a trait • For example, the plant height trait has two characters: tall and short • Mendel crossed the tall plants with the short ones • From these crosses, Mendel obtained seeds that he then grew into plants • These plants were hybrids, or organisms produced by crossing parents with different characters

13. Genes and Dominance • What were those hybrid plants like? • Did the characters of the parent plants blend in the offspring? • To Mendel’s surprise, the plants were not half tall • Instead, all of the offspring had the character of only one of the parents (They were all tall) • The other characteristic had apparently disappeared

14. Genes and Dominance • From this set of experiments, Mendel was able to draw two conclusions • Individual factors, which do not blend with one another, control each trait in a living thing • Merkmal – German for character • Today, the factors that control traits are called genes • Each of the traits Mendel studied was controlled by one gene that occurred in two contrasting forms • The different forms of a gene are now called alleles 2. Principle of dominance • Some alleles are dominant, whereas others are recessive

15. Segregation • Mendel did not stop his experimentation at this point • What happened to the recessive characters? • To answer this question, he allowed all seven kinds of hybrid plants to reproduce by self-pollination • P Generation • Purebred parental plants • F1 Generation • First filial generation • F2 Generation • Second filial generation

16. The F1 Cross • The results of the F1 cross were remarkable • The recessive characters reappeared in the F2 generation • This proved that the alleles responsible for the recessive characters had not disappeared • Why did the recessive alleles disappear in the F1 generation and reappear in the F2?

17. Explaining the F1 Cross • Mendel assumed that the presence of the dominant tall allele had masked the recessive short allele in the F1 generation • But the fact that the recessive allele was not masked in some of the F2 plants indicated that the short allele had managed to get away from the tall allele • Segregation • During the formation of the reproductive cells, the tall and short alleles in the F1 plants were segregated from each other

18. Explaining the F1 Cross • The possible gene combinations in the offspring that result from a cross can be determined by drawing a diagram known as a Punnett square • Represent a particular allele by using a symbol • Dominant = capital letter • Recessive = lowercase letter • Punnett squares show the type of reproductive cells, or gametes, produced by each parent • Punnett square results are often expressed as ratios

19. Explaining the F1 Cross • Phenotype • Physical characteristic • Genotype • Genetic makeup • Homozygous • Two identical alleles for a trait • Purebred • Heterozygous • Two different alleles for a trait • Hybrid

22. Independent Assortment • After establishing that alleles segregate during the formation of gametes (reproductive cells), Mendel began to explore the question of whether they do so independently • In other words, does the segregation of one pair of alleles affect the segregation of another pair of alleles? • For example, does the gene that determines whether a seed is round or wrinkled in shape have anything to do with the gene for seed color?

23. The Two Factor Cross: F1 • In this cross, the two kinds of plants would be symbolized like this: • Round yellow seeds • RRYY • Wrinkled green seeds • rryy

25. The Two Factor Cross: F1 • Because two traits are involved in this experiment, it is called a two-factor cross • The plant that bears round yellow seeds produces gametes that contain the alleles R and Y, or RY gametes • The plant that bears wrinkled green seeds produces ry gametes • An RY gamete and an ry gamete combine to form a fertilized egg with the genotype RrYy

26. The Two Factor Cross: F1 • Thus, only one kind of plant will show up in the F1 generation – plants that are heterozygous, or hybrid, for both traits • Remember that the concept of dominance tells us that the dominant traits will show up in a hybrid, whereas the recessive traits will seem to disappear

27. The Two Factor Cross: F1 • This cross does not indicate whether genes assort, or segregate independently • However, it provides the hybrid plants needed for the next cross – the cross of F1 plants to produce the F2 generation • The seeds from the F2 plants will show whether the genes for seed shape and seed color have anything to do with one another

28. The Two Factor Cross: F2 • What will happen when F1 plants are crossed with each other? • If the genes are not connected, then they should segregate independently, or undergo independent assortment • This produces four types of gametes RY, Ry, rY, and ry • Mendel actually carried out this exact experiment • Concluded that genes could segregate independently during the formation of gametes • In other words, genes could undergo independent assortment

29. A Summary of Mendel’s Work • Mendel’s work on the genetics of peas can be summarized in four basic statements: • The factors that control heredity are individual units known as genes. In organisms that reproduce sexually, genes are inherited from each parent. • In cases in which two or more forms of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive. • The two forms of each gene are segregated during the formation of reproductive cells. • The genes for different traits may assort independently of one another.

30. Chapter 9: Introduction to Genetics Section 2: Applying Mendel’s Principles

31. Genetics and Probability • Mendel applied the mathematical concept of probability to biology • Probability is the likelihood that a particular event will occur • Probability = the number of time a particular event occurs divided by the number of opportunities for the event to occur

32. Genetics and Probability • Flipping a coin • One of two possible events can occur • Heads up or tails up • The probability of the coin coming up heads is ½, or 1:1 • The larger the number of trials, the closer you get to the expected ratios

33. Using the Punnett Square • The Punnett square is a handy device for analyzing the results of an experimental cross

34. One-Factor Cross • In pea plants, tall (T) is dominant over short (t) • You have a tall plant • Design a cross to see if this plant is homozygous (TT) or heterozygous (Tt)

35. One-Factor Cross • Solution: • Cross your tall plant with a short plant • The cross of an organism of unknown genotype and a homozygous individual is called a test cross • As you can see in the Punnett squares, if any of the offspring resulting from a test cross shows the recessive phenotype, then the unknown parent must be heterzygous

36. TT X tt Tt X tt T = tall t = short

37. Two-Factor Cross • In pea plants, green pods (G) are dominant over yellow pods (g), and smooth pods (N) are dominant over constricted pods (n) • A plant heterozygous for both traits (GgNn) is crossed with a plant that has yellow constricted pods (ggnn)

38. GgNn X ggnn G = green g = yellow N = smooth n = constricted

39. Chapter 9:Introduction to Genetics Section 3: Meiosis

40. Meiosis • How are gametes formed? • In Chapter 8 you learned abut mitosis, a process that involves the separation of chromosomes and the formation of new cells • Could gametes be formed by mitosis? • The answer to this question is no • If gametes were formed by mitosis, when sperm and egg fuse during fertilization, the number of chromosomes would double in each generation • Before long the cells would contain a very large number of chromosomes

41. Chromosome Number • The number of chromosomes in a cell is different from organism to organism • However, it is true that each cell has an equal number of chromosomes from each parent • Each chromosome in the male set has a corresponding chromosome in the female set • These corresponding chromosomes are said to be homologous

42. Chromosome Number • A cell that contains both sets of homologous chromosomes (one set from each parent) is said to be diploid • The diploid number is sometimes represented by the symbol 2N • Contains two complete sets of chromosomes and two complete sets of genes • The gametes of organisms that reproduce sexually contain a single set of chromosomes (and genes) • Haploid (N) • In order for gametes to be produced, there must be a process that divides the diploid number of chromosomes in half

43. The Phases of Meiosis • Haploid gametes are produced from diploid cells by the process of meiosis • Meiosis is a process of reduction division in which the number of chromosomes per cell is cut in half and the homologous chromosomes that exist in a diploid cell are separated • Meiosis takes place in two stages • Meiosis I • Meiosis II

44. Interphase • Chromosome replicate during S phase • Each replicated chromosome consists of two genetically identical sister chromatids connected at the centromere

45. Prophase I • Typically occupies more that 90% of the time required for meiosis • Chromosomes begin to condense • Homologous chromosomes loosely pair along their lengths, precisely aligned gene by gene • Tetrad consists of 4 chromatids • In crossing over, the DNA molecules in nonsister chromatids break at corresponding places and then rejoin to the other’s DNA

46. Metaphase I • The pairs of homologous chromosomes, in the form of tetrads, are now arranged on the equator of the cell • Homologous chromosomes are attached to spindle fibers

47. Anaphase I • The chromosomes move toward the poles, guided by the spindles • Sister chromatids remain attached at the centromere and move as a single unit toward the same pole • Homologous chromosomes, each composed of two sister chromatids, move toward opposite poles

48. Telophase I and Cytokinesis • At the beginning of telophase I, each half of the cell has a complete haploid set of chromosomes, but each chromosome is still composed of two sister chromatids • Cytokinesis usually occurs simultaneously with telophase I, forming two haploid daughter cells • No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II, as the chromosomes are already replicated

49. Prophase II • A spindle forms • In late prophase II, chromosomes, each still composed of two chromatids, start to move toward the middle of the cell

50. Metaphase II • The chromosomes are positioned on the equator as in mitosis • Because of crossing over in meiosis I, the two sister chromatids of each chromosome are NOT genetically identical