Exploring Genetics: Mendel’s Legacy and Experiments

1. Fundamentals of Genetics • Mendel’s Legacy • Mendel’s Experiments • Mendel’s Results & Conclusions • Genetic Crosses • Probability • Monohybrid Crosses • Dihybrid Crosses

2. Historical Background of Genetics • To View Video: • Move mouse cursor over slide title-link • When hand appears, click once • ASF Video plays about 2-1/2 min

3. Mendel’s Legacy • Gregor Mendel • Mendel’s Garden Peas • Mendel’s Methods • Mendel’s Experiments • Mendel’s Results & Conclusions • Dominance & Recessiveness • Law of Segregation • Law of Independent Assortment • Chromosomes & Genes

4. LearningObjectives • TSW … • Describe the steps involved in Mendel’s experiments on garden peas • Distinguish btw/ dominant & recessive traits • State 2 laws of heredity that were developed from Mendel’s work • Describe how Mendel’s results can be explained by scientific knowledge of genes & chromosomes

5. Gregor Mendel mathematician natural philosopher priest & abbot

6. The Garden PeaPisum sativum • Annual plant • Heritable features easily observed • Monoecious: flowers have both male and female organs • plants can self-pollinate or cross-pollinate • male stamens can easily be removed to prevent self-pollination • pollen easily transferred by dusting to cross-pollinate

7. Genetic Terminology • Heredity– transmission of characters from parents to offspring • Character – heritable feature • flower color • pea pod color • stem length • Trait – heritablevariant of a character • purple flower -v- white flower • green pea pod -v- yellow pea pod • tall stems -v- dwarf (short) stem

8. Pea Plant Traits • Features investigated by Mendel • flower color: purple or white • flower position: axial or terminal • seed color: yellow or green • seed shape: smooth or wrinkled • pod shape: inflated or constricted • pod color: green or yellow • stem length: tall or short

9. Mendel’s Methods:Flower Structure • Pollination – pollen grains from male flower part are transferred to female flower part • Anther (of stamen): male flower part • Stigma (of carpal): female flower part

10. Mendel’s Methods:Pollination Techniques • Self-pollination – pollen transferredfrom anthers of a flower to a stigma of the same flower or a different flower on the same plant • Cross-pollination – pollen transferredfrom anthers of a flower on one plant to a stigma of a flower on a different plant

11. Mendel’s Experiments:Parental Generation • P1 generation= strain of plants pure for a trait (true-breeding) • “Parental” plants • Developed by self-pollinating for generations to produce offspring that always have the same trait as the parents Ex: purple-flowered parentsproduce only purple-flowered offspring

12. Mendel’s Experiments:Filial Generations • F1 generation= offspring (hybrids) of the P1 generation • Produced by cross-pollinating 2 pure (P1) strains Ex: purple-flowered strain cross-pollinated w/white flowered strain • F2 generation= offspring of the F1 generation • Produced by self-pollinating F1s

13. Mendel’s Results

14. Results of P1 & F1 Crosses • 7 characters, each showing 2 possible traits, were tested • 1,000s of crosses were made in each case • In every P1 cross – 1 trait disappearedin the F1 • In every F1 cross – the trait reappeared in the F2 generation

15. Mendel’s Experiments & Conclusions • To View Video: • Move mouse cursor over slide title-link • When hand appears, click once • ASF Video plays about 4-1/2 min

16. Mendel’s Conclusions • Based on mathematics, each character (ex., flower color) was controlled by factors for 2 contrasting traits (exs., purple flower or white flower) • One trait-factor dominated the other in expression of the character when both were present in the same plant

17. Mendel’s Conclusions:Dominance & Recessiveness • Dominant factor (trait) – masks or “dominates” the other trait in the F1 • Recessive factor (trait) – disappears (or “recedes”) in the F1 generation but reappears in the F2 • Mendel discovered this after carrying out monohybrid crosses for specific characters.

18. Mendel’s Conclusions:Segregation • Law of Segregation– A pair of factors is segregated (or separated) during the formation of gametes • Mendel discovered this after carrying out P1 & F1 crosses for specific characters. • The two paired factorsthat control the expression of a trait separate during the formation of reproductive cells

19. Mendel’s Conclusions: Independent Assortment • Law of Independent Assortment– Factors for different characters are distributed to gametes independently • Mendel discovered this after carrying out P1 & F1 crosses for 2 specific characters. • Plants showing the dominant trait for one character could also show the recessive trait for a different character

20. Law of Independent Assortment • When considering two or more different characters(genes), the traits(alleles) for each segregate into different gametes independently of each other • the different characters reside on different pairs of homologous chromosomes Flower color Pod color

21. Law of Independent Assortment • Mendel discovered this after carrying out repeated dihybrid crosses. • he crossed a true-breeding plant w/ yellow, round seeds (dominants) and a true-breeding plant w/ green, wrinkled seeds (recessives) • all the F1s had the dominant traits of yellow, round seeds

22. Law of Independent Assortment • he then crossed the yellow, round F1s • in the F2 generation, plants appeared with: yellow, round seeds (56%); yellow, wrinkled (19%); green, round (19%); green, wrinkled (6%)

23. Factors & Chromosomes • To View Video: • Move mouse cursor over slide title-link • When hand appears, click once • ASF Video plays about 2-1/4 min

24. Chromosomes & Genes • Chromosomes are DNA molecules • DNA molecules are divided into many distinct segments called genes • Genes control specific hereditary traits • Chromosomes occur in pairs • Thereforegenes also occur in pairs

25. Chromosomes & Genes • Alleles • Alternateversions of the same gene • 1 allele carried on eachhomologouschromosome • Occur at the samelocus • the DNA of each allele has a slightly different nucleotide sequence • results in slightly different variations of the same character

26. Alleles • Example: • The gene for the character “flower color” on one homologue can have an allele (factor) forpurple flowers • The gene for the character “flower color” on the other homologue can have an allele (factor) for white flowers

27. Genetic Symbology pp PP Pp Dominant alleles are symbolized w/ a capital letter Recessive alleles are symbolized w/ a the lower case letter used for the dominant allele

28. Chromosomes & Inheritance • To View Video: • Move mouse cursor over slide title-link • When hand appears, click once • ASF Video plays about 1 min

29. Video Quiz:Genetics & Meiosis • To View Video: • Move mouse cursor over slide title-link • When hand appears, click once • ASF Video plays about 2 min • 5 Questions

30. Genetic Crosses • Genotype & Phenotype • Probability • Predicting Results of Monohybrid Crosses • 6 examples • Predicting Results of Dihybrid Crosses • 2 examples

31. LearningObjectives • TSW … • Explain how probability is used to predict the results of genetic crosses • Use a Punnet square to predict the results of monohybrid & dihybrid crosses • Explain how a testcross is used to show the genotype of an individual w/ the dominant phenotype • Differentiate a monohybrid cross from a dihybrid cross

32. Genetic Vocabulary • Phenotype • outward appearance of an organism Ex: purple flowers • Genotype • genetic make-up of an organism Ex: PP, Pp(– purple flowers) pp(– white flower)

33. Genetic Vocabulary • Homozygous • an organism having a pair of identical alleles for a character Ex: PP= homozygous dominant Ex: pp = homozygous recessive • Heterozygous • an organism having 2 different alleles for a character Ex: Pp

34. Probability • Probability – the likelihood that a specific event will occur • Expressed as: • decimal • percentage • fraction Probability = # times an event is expected to occur # opportunities for an event to occur

35. Probability Q. What is the probability that the dominant trait for purple flowers will appear in an F2 generation? Probability = # times an event occurs # opportunities for an event to occur Mendelian Experiment: Probability = 6,022 purple plants = 0.75 8,023 total plants produced

36. Genetic PredictionsBased on laws of probability • Multiplication Rule • The probability that 2 or more independent events will occur together in a specific way is determined by multiplying the probability of 1 event by the probability of the other event Ex: the probability of a coin flip ending up heads is 50% or ½. The probability that a second coin flip will end up heads is also 50% or ½. Therefore, the probability that 2 coins flipped together will both end up heads is ½ X ½ = ¼ or 25%

37. Genetic PredictionsBased on laws of probability • Addition Rule • The probability that any 1 of 2 or more independent events will occur is determined by adding their individual probabilities Ex: the probability that flipped coin A will come up heads while flipped coin B will come up tails is 25% or ¼. The probability that flipped coin A will come up tails while flipped coin B will come up heads is also 25% or ¼. Therefore, the probability that both events will occur is ¼ + ¼ = ½ or 50%

38. Predicting Results of Monohybrid Crosses • Monohybrid cross – a cross btw/ individuals that involves 1 pair of contrasting traits • Punnett square – a diagram used as an aid in predicting the probability that certain traits will be inherited by offspring Monohybrid crosses investigate inheritance patterns for a single character

39. Mendel’s Crosses & Punnett Squares • To View Video: • Move mouse cursor over slide title-link • When hand appears, click once • ASF Video plays about 6 min

40. Monohybrid Cross:Procedure • Step 1: determine the genotypes for each mate in the cross. The genotype shows the allelic combination. Ex: Tt X Tt • Step 2: determine all the possible kinds of gametes that each mate can produce. Each gamete will have 1 allele from the genotype. Ex: T & t for one parent; T & tfor the other parent

41. Monohybrid Cross:Procedure • Step 3: construct a Punnett square. Place the gametes for one parent across the top & the gametes for the other parent along the left side. Ex: 4 boxes for a monohybrid cross. T & tacross top; T & t along side • Step 4: combine the possible gametes of each parent in the 4 boxes Ex: TT in top left box; Ttin top right & bottom left boxes; tt in bottom right box

42. Example 1:Homozygous X Homozygous • A plant that is true-breeding for purple flower color (dominant) is crossed with a plant that is true breeding for white flower color (recessive). • State the probability that any offspring will have the following flower color: • Purple • White

43. ♀ ♂ P PXpp Purple: 100% purplewhite P P P p P p p purple purple P p P p p purple purple

44. Example 2:Homozygous X Heterozygous • A plant that is true-breeding for purple flower color is crossed with a purple flowered plant that carries the allele for white flower color. • State the probability that any offspring will have the following flower color: • Purple • White

45. ♀ ♂ P PXPp Purple: 100% purplepurple P P P P P P P purple purple P p P p p purple purple

46. Example 3:Heterozygous X Heterozygous • Two purple flowered plants that carry the allele for white flower color are crossed. • State the probability that any offspring will have the following flower color: • Purple • White

47. ♀ ♂ Purple: 75% White: 25% P pXPp purplepurple p P P P P p P Phenotypic ratio 3:1 Genotypic ratio 1:2:1 purple purple P p p p p purple white

48. GeneticRatios 1 PP : 2 Pp : 1 pp • Phenotypic ratio • ratio of offspring w/ dominant appearance to offspring w/ recessive appearance Ex: for offspring of 2 heterozygotes = 3:1 for monohybrid crosses • purple-flowered to white flowered individuals after a cross of Pp X Pp • Genotypic ratio • ratio of homozygous dominants to heterozygotes to homozygous recessives Ex: for offspring of 2 heterozygotes = 1:2:1 for monohybrid crosses

49. Testcross If an organism has the recessivephenotype, we know its genotype because there is onlyoneway that the recessive form of the character can occur. Ex: White-flowered plants always have the pp genotype Question: If an organism has the dominant phenotype, how can we determine which of the 2 possible genotypes it has? Ex: Purple-flowered plants can have either the PP or the Ppgenotypes ?

50. Testcross • If you cross a homozygous recessive, pp, with an organism of the dominant phenotype, P?, there are only two possible results. • If 100% of the offspring have the dominant phenotype, the dominant parent MUST BE a homozygous dominant,PP • If any of the offspring have the recessive phenotype, the dominant parent MUST BE a heterozygote,Pp