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MENDELIAN GENETICS

MENDELIAN GENETICS. ANNOUNCEMENTS. First set of genetics problems will be posted on course web site today!. OBJECTIVES. Understand Mendel’s principles/laws governing genetics Understand meaning of relevant vocabulary

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MENDELIAN GENETICS

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  1. MENDELIAN GENETICS

  2. ANNOUNCEMENTS • First set of genetics problems will be posted on course web site today!

  3. OBJECTIVES • Understand Mendel’s principles/laws governing genetics • Understand meaning of relevant vocabulary • Be able to predict results of mono & dihybrid crosses using a Punnett square • Know exceptions to rules

  4. INTRODUCTION TO GENETICS TERMINOLOGY • Diploid: • Cell (or organism) with pairs of homologous chromosomes • Haploid: • Cell/org. with only one chromosome of each kind

  5. Gene (Character): A feature that is heritable Locus: Specific area on chromosome where a gene is found Allele(Trait): Diff. forms of the SAME gene Genotype: The genetic makeup of an organism (combination of genes in its nucleus) Phenotype: The physical appearance of an organism GENETICS TERMINOLOGY Brown Eyes Blue Eyes Black Hair Red Hair

  6. GENETICS • Genetics: • The scientific study of heredity • Heredity: • The transmission of traits from parents to offspring

  7. MENDEL • Gregor Mendel • Interested in basic patterns governing the transmission of traits from parent to offspring

  8. MENDEL • Mendel’s work precedes Darwin and an understanding of meiosis/genes/chromosomes

  9. HISTORICAL HYPOTHESES REGARDING INHERITANCE Blending Inheritance Hypothesis: • Offspring represent blend of characteristics from two parents Inheritance of Acquired Characteristics Hypothesis: • Traits are modified, through use, and passed on to offspring in modified form

  10. MENDEL • Mendel performed crosses between pure bred garden peas • Began studying inheritance of ONE trait • Monohybrid cross • Total of 7 traits were studied • Later, multiple traits studied together

  11. MENDEL’S CROSSES • Mendel's plants had twoobservable“forms” (alleles) of each “character” (gene) • Green or Yellow seeds • Round or Wrinkled seeds

  12. Forms of Trait (Phenotype) Trait or Seed shape Round Wrinkled Round Wrinkled or Seed color Green Yellow Yellow Green Pod shape or Constructed Inflated Smooth Constricted or Pod color Green Yellow Yellow Green

  13. Trait Forms of Trait (Phenotype) or Flower color Purple White White Purple or Flower and pod position Axial (on stem) Terminal (at tip) Axial (on stem) Terminal (at tip) or Stem length Dwarf Tall Tall Dwarf

  14. MENDEL • Looked at traits exhibited by progeny of crosses • He studied phenotype and INFERRED GENOTYPE! • Found observableevidence of how parents transmit traits to offspring

  15. MENDEL’S PEA PLANTS • Pisum sativum • Can self fertilize • Male & female parts on same flower

  16. Male parts = anthers • Pollen grain contain sperm • Female parts = pistil • Ovule = eggs SELF FERTILIZATION Stigma (receives pollen) Anthers (produce pollen grains, which contain male gametes) Ovules (produce female gametes)

  17. MENDEL’S PEA PLANTS Controlled Mating: • Cut off reproductive organ to stop self fertilization • Cross fertilize plants in a controlled fashion • Plant that contributes pollen is considered “male” • Plant that receives the pollen is considered “female”

  18. CROSS-POLLINATION 1. Remove anthers from one plant. 2. Collect pollen from a different plant. 3. Transfer pollen to stigma of the 1st plant (plant without anthers)

  19. FIRST HALF OF RECIPROCAL CROSS …to stigma of wrinkled-seeded parent Pollen from round- seeded parent…

  20. SECOND HALF OF RECIPROCAL CROSS Polen from wrinkled- seeded parent To stigma of round-seeded parent

  21. MENDEL’S CROSSES Round Parent x WrinkledParent • All Round Offspring (F1) (Wrinkled trait disappeared!) **Reciprocal cross (control for sex) produced same results**

  22. MENDEL’S CROSSES Round x Round (F1) • Next, Mendel crossed F1 (bro-sis mating): Get both types of offsrping (F2) Wrinkled trait re-emerges in F2

  23. MENDEL’S CROSSES • Mendel Determines (Phenotypic) Ratio: F2: 5474 Round : 1850 Wrinkled • Nearly 3 : 1 • Similar pattern observed in other 6 traits

  24. MENDEL’S CONCLUSIONS 1. Genes do not blend together. The hereditary determinants, or genes, maintain their integrity from generation to generation. They do not blend together, and they do not acquire characteristics in response to actions by an individual 2. Peas have two versions, or alleles, of each gene.This is also true for many other organisms. Each gamete contains one allele of each gene. Pairs of alleles segregate during the formation of gametes 3. 4. Males and females contribute equally to the genotype of their offspring. When gametes fuse, offspring acquire a total of two alleles—one from each parent. Some alleles are dominant to others. When a dominant and recessive allele for the same gene are found in the same individual, that individual exhibits the dominant phenotype. 5.

  25. MENDEL’S SYMBOLS • Mendel assigned symbols to hereditary characters • Letters represent versions of a gene (alleles): • Ex: If Letter “r” represents a gene then: • Rsymbolizes dominant allele • r symbolizes recessive allele

  26. PUNNETT SQUARE • R.C. Punnett • Leading geneticist in early 1900s • Invented technique called the “Punnett Square” • Place gamete possibilities for 2 parents along axes • Internal boxes represent union of gametes • Used to predict genotype and phenotype of potential zygote

  27. MENDEL’S CROSS OF PURE LINE PEA PLANTS PARENT: R R R is round r is wrinkled PARENT: r r ** hint: gametes along top, “whole people” on inside

  28. MENDEL’S CROSS OF PURE LINE PEA PLANTS R = Dominant allele for seed shape (round) r = Recessive allele for seed shape (wrinkled) Round-seeded Father Wrinkled-seeded Mother Parental generation (homozygous) Meiosis Gametes Fertilization F1 generation Rr Rr Rr Rr All have Rr genotype (heterozygous) and round seed phenotype

  29. MENDEL’S CROSS OF F1 PLANTS Crossing the F1s: Rr x Rr R = round r = wrinkled

  30. PROBABILITY THEORY In Diploid Organisms, • Each allele has 50% chance of being found in a particular gamete • To calculate likelihood of two alleles combining (i.e. fertilization): • Multiply probabilities together

  31. MENDEL’S CROSS OF F1 PLANTS = Round- phenotype = Wrinkled phenotype Mother R = Dominant allele (round) r = Recessive allele (wrinkled) Rr Female gametes r R R R Father RR Rr Male gametes Rr r Rr rr Resulting genotypes: 1/4 RR : 1/2 Rr : 1/4 rr Resulting phenotypes: 3/4 : 1/4

  32. Three possible genotypes for each gene in a diploid cell: Homozygous: Cell (diploid) with two “doses” of same allele Homozygous Dominant: Both alleles of a gene are of the “Dominant” variety (RR) Homozygous Recessive: Both alleles of a gene are of the “Recessive” variety (rr) Heterozygous: Cell (diploid) with two differing alleles for a gene One dominant and one recessive version of gene POSSIBLE GENOTYPES Mendel DID NOT use these terms, only inferred them!

  33. MENDEL’S CROSSES • Dihybrid Crosses: • Mendel used pea plants to follow inheritance of TWOtraits • Another pattern began to emerge…

  34. MENDEL’S SYMBOLS • Seed Shape: • R is dominant allele (Round) • r is recessive allele (Wrinkled) • Seed Color: • Y is dominant allele (Yellow) • y is recessive allele (Green)

  35. IN CLASS EXERCISE • Make a Punnett Square to predict offspring of a homozygous dominant individual crossed with a homozygous recessive individual! • What are the genotypes of the parents? • What are the genotypes of the gametes? • What about the offspring?

  36. MENDEL’S CROSSES WITH PEAS THAT DIFFER IN TWO TRAITS R = Dominant allele for seed shape (round) r = Recessive allele for seed shape (wrinkled) Y = Dominant allele for seed color (yellow) y = Recessive allele for seed color (green) Mother Father Parental generation Meiosis Gametes Fertilization F1 generation

  37. IN CLASS EXERCISE • Now, cross the F1s • What is the phenotypic ratio of the offspring? (What Mendel Observed) • What about the genotypic ratio?

  38. PUNNETT SQUARE FOR TWO TRAITS R = Dominant allele for seed shape (round) r = Recessive allele for seed shape (wrinkled) Y = Dominant allele for seed color (yellow) y = Recessive allele for seed color (green) Parental generation RRYY rryy RrYy F1 generation ALL Female gametes RrYy 1/4 RY 1/4 Ry 1/4 rY 1/4 ry 1/4 RY RRYY RRYy RrYY RrYy 1/4 Ry RrYy RRYy RRyy Rryy RrYy Male gametes 1/4 rY RrYY RrYy rrYY rrYy 1/4 ry RrYy Rryy rrYy rryy Resulting genotypes: 9/16R-Y- : 3/16R-yy : 3/16rrY- : 1/16rryy 9/16 3/16 3/16 1/16 Resulting phenotypes:

  39. F2 OFFSPRING FROM DIHYBRID CROSS Yellow & Green Wrinkled Round F2 generation phenotype Mendel’s Numbers 315 101 108 32 = 556 Fraction of progeny 9/16 3/16 3/16 1/16 = 1 Note: New mixtures of traits are observed in F2!!

  40. How does the 9 : 3 : 3 : 1 ratio for two traits relate to the 3 : 1 ratio for one trait? Round seeds : Wrinkled seeds 315 + 108 : 101 + 32 423 : 133 3 : 1 Yellow seeds : Green seeds 315 + 101 : 108 + 32 416 : 140 3 : 1

  41. MENDEL’S PRINCIPLE OF SEGREGATION • Principle of Segregation: • Holds true for one or two traits: • For a single trait, 3:1 ratio (dominant : recessive) • For two traits, 9:3:3:1 ratio • Each trait, considered individually, showed 3:1 ratio

  42. PRINCIPLE OF SEGGREGATION

  43. MENDEL’S PRINCIPLE OF INDEPENDENT ASSORTMENT • Law of Independent Assortment: • Genes residing on differentchromosomes separate without regard for one another

  44. Hypothetical Example of Independent Assortment During meiosis I, tetrads can line up two different waysbefore homologs separate. OR Brown eyesBlack hair Blue eyesRed hair Brown eyesRed hair Blue eyesBlack hair

  45. THE TEST CROSS • To determine genotype of an individual, cross with a homozygous recessive individual

  46. THE TEST CROSS Homozygous recessive parent YOUR HOMEWORK! ? rryy ry All 1/4 1/4 1/4 1/4

  47. MENDEL Why Mendel Was Successful: • Unbiased • Selected good model organism • Used pure breeds as parents • Large sample size • Quantitative analysis • Controlled experiments • Studied obvious traits

  48. EXTENSION OF MENDEL’S RULES • Phenotypic variation is not always due to simple dominance/recessive-ness of genes

  49. EXTENSION OF MENDEL’S RULES Some Traits Deviate from Mendelian Patterns: • Incomplete Dominance: • No allele is completely dominant over another • Co Dominance: • Two alleles are dominant & expressed • Environmental Effects: • Environmental conditions help determine trait • Quantitative Traits: • Trait due to interaction between more than one gene

  50. INCOMPLETE DOMINANCE • No Allele Is Completely Dominant • Neither red nor white allele is completely dominant

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