MENDELIAN INHERITANCE

1. MENDELIAN INHERITANCE Chapter 2

2. 2 INTRODUCTION The concept of heredity is ancient Dates back to at least 400 b.c.a. Our understanding of genetics is rather recent Gregor Mendel�s work began only 150 years ago Many inaccurate views of heredity were held prior to Mendel�s time

3. 3 INTRODUCTION Hippocrates ca. 400 b.c.a. Greek physician First to attempt to explain transmission of hereditary traits �Pangenesis� �Seeds� are produced by all parts of the body Collected and transmitted at time of conception These �seeds� caused certain traits of offspring to resemble their parents

4. 4 INTRODUCTION Spermists Microscopes invented in the late 1600s Sperm viewed through microscopes Some imagined a tiny creature inside each sperm �Homunculus� Hypothesized to be a miniature human waiting to develop within the womb of its mother Only the father was responsible for creating future generations Any resemblance to mother was due to influences �within the womb�

5. 5 INTRODUCTION Ovists Views in opposition to those of the spermists Considered the egg to be solely responsible for human characteristics Role of sperm was simply to stimulate the egg on its path of development

6. 6 INTRODUCTION Joseph Kolreuter First to conduct systematic studies of genetic crosses 1761 � 1766 Crossed different strains of tobacco plants Found that offspring were generally intermediate in appearance between the two parents Both parents make equal genetic contributions to the offspring Consistent with the �blending theory of inheritance�

7. 7 INTRODUCTION Blending theory of inheritance Factors that dictate genetic traits can blend together from generation to generation Blended traits are passed to the next generation

8. 8 INTRODUCTION Popular views before the 1960s Combined notions of pangenesis and the blending theory of inheritance Hereditary traits were rather malleable Could change and blend over one or two generations Mendel�s work was crucial in refuting these archaic views of heredity

9. 9 GREGOR JOHANN MENDEL 1822 � 1884 Father of genetics Augustinian monk Austria, now Czech Republic Training in Agriculture Scientific method Mathematics Statistical analysis Studied inheritance in garden peas Pisum sativum

10. 10 GARDEN PEAS Several properties made Pisum sativum a superb model organism for genetic analysis Available in many varieties Easy to maintain Control over mating Short generation time Numerous offspring No major ethical issues Findings applicable to other organisms

11. 11 FLOWER STRUCTURE Most flowers are simultaneously both male and female Carpels are female structures (?) Produce eggs Stamen are male structures (?) Produce sperm

12. 12 FLOWER STRUCTURE Pollination involves the transmission of sperm-containing pollen to a (female) stigma Fertilization follows successful pollination Self-pollination and cross-pollination are both possible

13. 13 FLOWER STRUCTURE Structure of a pea flower Produces both pollen and egg cells Reproductive structures enclosed by a modified petal �Keel� Self-pollination is the rule, not the exception

14. 14 FLOWER STRUCTURE Pollination Pollen grain lands on stigma Pollen grain sends out long pollen tube Sperm travel toward ovules (and eggs) One sperm fertilizes an egg to form zygote One sperm fuses with two polar nuclei to form the nutrient-rich endosperm Storage material for developing embryo

15. 15 MENDEL�S EXPERIMENTS Mendel obtained several varieties of peas Differences between varieties were confined to one (or more) of seven different traits e.g., Purple or white flowers e.g., Yellow or green seeds

16. 16 MENDEL�S EXPERIMENTS

17. 17 MENDEL�S EXPERIMENTS Mendel began his experiments with true-breeding lines Traits did not vary in appearance between generations e.g., White-flowered lines that produced only white-flowered offspring for many generations

18. 18 MENDEL�S EXPERIMENTS Mendel�s first crosses involved a single trait Two variants existed for each trait e.g., Purple and white are two forms of the flower color trait Purple and white are two phenotypes for flower color �Single-factor cross�

19. 19 MENDEL�S EXPERIMENTS True-breeding purple x true-breeding white Parental generation Stamen (?) removed from �female� Pollen transfer Cross-fertilization Seeds are offspring F1 generation Single-trait hybrids �Monohybrids�

20. 20 MENDEL�S EXPERIMENTS True-breeding purple x true-breeding white Parental generation Stamen (?) removed from �female� Pollen transfer Cross-fertilization Seeds are offspring F1 generation Single-trait hybrids �Monohybrids�

21. 21 MENDEL�S EXPERIMENTS True-breeding purple x true-breeding white F1 monohybrids are produced F1 monohybrids all possess purple flowers White flowers are absent

22. 22 MENDEL�S EXPERIMENTS F1 monohybrids are allowed to self-pollinate �Monohybrid cross� How did Mendel facilitate this pollination? Explain why this is sexual reproduction F2 generation is produced Both phenotypes are present in the F2 generation

23. 23 MENDEL�S EXPERIMENTS Mendel obtained similar results for each of the seven traits he studied One phenotype disappeared in the F1 generation This recessive phenotype reappeared in approximately � of the F2 individuals 3:1 phenotypic ratio

24. 24 MENDEL�S CONCLUSIONS Mendel�s results argued strongly against a blending mechanism of heredity F1 individuals have the characteristics of one parent, not intermediate characteristics Units of heredity are discrete units Now called genes Mendel�s Law of Segregation explained these results Described the particulate nature of inheritance

25. 25 LAW OF SEGREGATION Alternative versions of genes account for variations in inherited characteristics These alternative versions of genes are termed alleles The flower color gene exists in two forms Purple allele White allele

26. 26 LAW OF SEGREGATION For each character, an organism inherits two alleles, one from each parent Diploid organisms possess two copies of each chromosome Genes reside upon chromosomes A gene�s position on a chromosome is called its locus Diploid organisms possess two copies of each gene Paired chromosomes ? paired alleles Each parent donates one copy of each chromosome Each parent donates one copy of each gene

27. 27 LAW OF SEGREGATION For each character, an organism inherits two alleles, one from each parent These alleles may be either identical or non-identical

28. 28 LAW OF SEGREGATION If these two alleles differ The allele that is visibly apparent is termed dominant The allele that is masked is termed recessive P = purple allele (dominant) p = white allele (recessive) PP ? purple flowers Pp ? purple flowers pp ? white flowers

29. 29 LAW OF SEGREGATION The two alleles for each character segregate during gamete production Gametes are formed by meiosis Meiosis separates homologous chromosomes Meiosis separates pairs of alleles Each gamete receives only one allele of each gene

30. 30 MENDEL�S EXPERIMENTS Parental generation True-breeding Possess identical alleles �Homozygous� for the relevant gene e.g., Genotype �AA� or �aa� F1 generation Hybrids Possess non-identical alleles �Heterozygous� for the relevant gene e.g., �Aa�

31. 31 MENDEL�S EXPERIMENTS F2 generation Both phenotypes present What genotypes are present?

32. 32 PUNNETT SQUARES A Punnett square can be used to determine the genotypes of potential offspring from a given mating Genotypes of female gametes listed on one axis Genotypes of male gametes listed across other axis Offspring inside boxes Products of fertilizations

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34. 34 MENDEL�S EXPERIMENTS The F2 generation in a monohybrid cross displays specific ratios 3:1 phenotypic ratio 1:2:1 genotypic ratio

35. 35 MENDEL�S EXPERIMENTS

36. 36 MENDEL�S EXPERIMENTS Individuals with the dominant phenotype may be either homozygotes or heterozygotes 1/3 of the purple-flowered plants are expected to be homozygous dominant 2/3 of the purple-flowered plants are expected to be heterozygous

37. 37 TEST CROSS A test cross can be used to determine the genotype of an individual with the dominant phenotype This individual is crossed to a recessive homozygote Phenotypes of offspring will uncover the genotype of the parent

38. 38 MULTIPLE CHARACTERISTICS Mendel also analyzed crosses involving two different traits Simultaneously investigated the pattern of inheritance for two different traits e.g., Seed color and seed texture Two-factor crosses Dihybrid crosses

39. 39 MULTIPLE CHARACTERISTICS One of Mendel�s two factor crosses Seed color Yellow seeds are dominant to green seeds Seed texture Smooth seeds are dominant to wrinkled seeds

40. 40 DIHYBRID CROSS YYRR (yellow, smooth) x yyrr (green, wrinkled) The F1 generation was all YyRr (yellow, smooth) �Dihybrids� What would the F2 generation look like?

41. 41 DIHYBRID CROSS Will the segregation of one allele pair influence the segregation of a second allele pair? Perhaps the genes for these two traits are physically linked and inherited as a single unit Perhaps the genes for these two traits can assort themselves independently during meiosis

42. 42 DIHYBRID CROSS YYRR x yyrr ? YyRr F1 generation YyRr x YyRr ? more complex F2 generation The F2 generation consisted of individuals with four different phenotypes Yellow, round Yellow, smooth Green, round Green, smooth

43. 43 DIHYBRID CROSS What is the expected frequency of each of the following phenotypes in this dihybrid cross? Y_R_ (yellow, round) Y_rr (yellow, wrinkled) yyR_ (green, round) yyrr (green, wrinkled)

44. 44 DIHYBRID CROSS The ratio Mendel observed was 315:108:101:32 Similar results were obtained for different combinations of traits How close is this ratio to the expected ratio? (First, reduce the ratio to ?:?:?:1 by dividing all numbers by the smallest) 9.8:3.4:3.2:1 is obtained Within experimental error of expected 9:3:3:1

45. 45 DIHYBRID CROSS The ratio Mendel observed was 315:108:101:32 Similar results were obtained for different combinations of traits How close is this ratio to the expected ratio? (Reduce the ratio to ?:?:?:1 by dividing all numbers by the smallest) 9.8:3.4:3.2:1 is obtained Within experimental error of expected 9:3:3:1

46. 46 DIHYBRID CROSS Mendel obtained similar results with other pairs of traits e.g., TtYy x TtYy dihybrid cross 9:3:3:1 ratio in F2 generation

47. 47 DIHYBRID CROSS For a moment, ignore one characteristic and focus only on the other What is the yellow:green ratio? What is the smooth:wrinkled ratio?

48. 48 DIHYBRID CROSS Viewing only one characteristic while ignoring the other generates a ratio we have seen before The more complex ratio (9:3:3:1) seen in a dihybrid cross results from the superimposition of two simpler ratios (3:1)

49. 49 INDEPENDENT ASSORTMENT Mendel�s Law of Independent Assortment Two pairs of alleles segregate independent of each other during gamete formation The segregation of alleles for seed color has no effect on the segregation of alleles for seed shape etc.

50. 50 PUNNETT SQUARES A Punnett square can be used to predict the outcomes of various crosses One allele pair: 2 x 2 = 4 boxes Two allele pairs: 4 x 4 = 16 boxes Three allele pairs: 8 x 8 = 64 boxes etc. Larger Punnett squares become unmanageable Do you really want to draw 64 boxes? You do?? Are you insane???

51. 51 NO PUNNETT SQUARES Predicted outcomes can be determined mathematically without drawing unnecessarily large Punnett squares The segregation of one allele pair is independent of other allele pairs e.g., YyRr x YyRr � of the offspring are expected to be yellow � of the offspring are expected to be smooth (�)*(�) = 9/16 are expected to be yellow and smooth What fraction are expected to be yellow and wrinkled?

52. 52 LOSS-OF-FUNCTION ALLELES Most genes encode proteins Mendel studied seven protein-encoding genes The recessive alleles of these genes were defective Rendered inactive by mutation Did not encode a functional protein �Loss-of-function alleles� Provide critical clues concerning the function of the encoded protein

53. 53 LOSS-OF-FUNCTION ALLELES Flower color gene in Mendel�s peas Encodes an enzyme required for the synthesis of purple pigment White allele does not encode functional enzyme Homozygous recessive individuals are white because they cannot make purple pigment

54. 54 PEDIGREE ANALYSIS Peas are very convenient model organisms for genetic analysis Humans are much less convenient �Could you please reproduce with that person over there so I can see what your litters of offspring look like?� Are there any ethical issues with this? etc.

55. 55 PEDIGREE ANALYSIS Inheritance of traits in humans can be followed using pedigree analysis Often less definitive than Mendel�s crosses Commonly used to determine the inheritance patterns of human genetic diseases e.g., Dominant or recessive Frequently able to provide important clues concerning inheritance patterns of various traits e.g., Cancer

56. 56 PEDIGREE ANALYSIS Reading human pedigrees Shape denotes gender Circle = female Square = male Lines denote relationships Parents connected by horizontal line Vertical line from parents denotes offspring Oldest on left, youngest on right

57. 57 PEDIGREE ANALYSIS Reading human pedigrees Shading denotes condition Filled in = disorder Half filled = known heterozygote Heterozygotes cannot always be identified

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59. 59 PROBABILITY Laws of inheritance can b used to predict the outcome of genetic crosses Useful in many ways Agriculture Inherited diseases

60. 60 PROBABILITY Mendel�s laws of segregation and independent assortment reflect the rules of probability So do the results of flipping coins, rolling dice, etc. What is the chance of flipping two coins and getting heads on both? �one of each?

61. 61 PROBABILITY A probability calculations allows us to predict the likelihood that an event will occur in the future Probability = number of times an event occurs total number of events Deviation between expected and observed is due to random sampling error Difference between predicted and expected percentages can be large for small sample sizes Random sampling error should be much smaller for large samples

62. 62 SUM RULE Sum rule The probability that one of two or more mutually exclusive events will occur is equal to the sum of the individual probabilities of the events

63. 63 SUM RULE In the cross AaBb x AaBb, what is the probability that an offspring will have either the dominant phenotype for both traits or the recessive phenotype for both traits? Calculate the individual probabilities of each phenotype Dominant for both = 9/16 Recessive for both = 1/16 Add together the individual probabilities 9/16 + 1/16 = 10/16 =5/8

64. 64 PRODUCT RULE Product rule The probability that two or more independent events will occur is equal to the product of their individual probabilities

65. 65 PRODUCT RULE In the cross Aa x Aa, what is the probability that none of the first three offspring will be albino? Calculate the individual probability of this phenotype Accomplished using a Punnett square Probability of unaffected = 3/4 Multiply the individual probabilities � * � * � = 27/64

66. 66 PRODUCT RULE Product rule Can also be used to predict the outcome of a cross involving two or more genes

67. 67 PRODUCT RULE In the cross AaBbCC x AabbCc, what is the probability that the first offspring will have the dominant phenotype for all three traits? Calculate the individual probabilities of this phenotype Accomplished using a Punnett square PA= � PB = � PC = 1 Multiply the individual probabilities � * � * 1 = 3/8