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Mendelian Genetics

Gregor Johann Mendel, an Austrian monk, studied the inheritance of traits in pea plants and developed the laws of inheritance. Known as the "Father of Genetics," Mendel's work laid the foundation for the understanding of genetic inheritance.

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Mendelian Genetics

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  1. Mendelian Genetics

  2. Gregor Mendel(1822-1884) Responsible for the Laws governing Inheritance of Traits

  3. Gregor Johann Mendel • Austrian monk • Studied the inheritance of traits in pea plants • Developed the laws of inheritance • Mendel's work was not recognized until the turn of the 20th century

  4. Gregor Johann Mendel • Between 1856 and 1863, Mendel cultivated and tested some 28,000 pea plants • He found that the plants' offspring retained traits of the parents • Called the “Father of Genetics"

  5. Site of Gregor Mendel’s experimental garden in the Czech Republic

  6. Mendel stated that physical traits are inherited as “particles” Mendel did not know that the “particles” were actually Chromosomes & DNA Particulate Inheritance

  7. Genetic Terminology • Trait - any characteristic that can be passed from parent to offspring • Heredity - passing of traits from parent to offspring • Genetics - study of heredity

  8. Designer “Genes” • Alleles – one of the two forms of a gene (dominant & recessive) • Dominant - stronger of two genes expressed in the hybrid; represented byacapital letter (R) • Recessive - gene that shows up less often in a cross; represented by alowercase letter (r)

  9. Genetic Terminology • Monohybrid - cross involving a single trait e.g. flower color • Dihybrid – cross involving two traits e.g. flower color & plant height • True Breeding – all offspring will have the same characteristic generation after generation

  10. Genotypes • Homozygousgenotype - gene combination involving 2 dominant or 2 recessive genes (e.g. RR or rr); also calledpure  • Heterozygousgenotype - gene combination of one dominant & one recessive allele    (e.g. Rr); also calledhybrid

  11. More Terminology • Genotype - gene combination for a trait(e.g. RR, Rr, rr) • Phenotype - the physical feature resulting from a genotype(e.g. red, white)

  12. Genotype & Phenotype in Flowers Genotype of alleles:R= red flowerr= yellow flower All genes occur in pairs, so 2alleles affect a characteristic Possible combinations are: GenotypesRRRrrr PhenotypesRED RED YELLOW

  13. Genotypes • Parental Generation (P1) - the parental generation in a breeding experiment • First Filial generation (F1) – First generation created by parents • Second Filial generation (F2) - the second-generation offspring in a breeding experiment.

  14. Mendel’s Pea Plant Experiments

  15. Reproduction in Flowering Plants Pollen contains sperm Produced by the stamen Ovary contains eggs Found inside the flower • Pollen carries sperm to the eggs for fertilization • Self-fertilization can occur in the same flower • Cross-fertilization can occur between flowers

  16. Mendel’s Experimental Results

  17. Did the observed ratio match the theoretical ratio? The theoretical or expected ratio of plants producing round or wrinkled seeds is 3 round :1 wrinkled Mendel’s observed ratio was 2.96:1 The discrepancy is due to statistical error The larger the sample the more nearly the results approximate to the theoretical ratio

  18. 100% purple-flower peas F1 generation (hybrids) 100% self-pollinate 75% purple-flower peas 25% white-flower peas 3:1 F2 generation Looking closer at Mendel’s work true-breeding purple-flower peas true-breeding white-flower peas X P

  19. Test cross • Cross-breed the dominant phenotype — unknown genotype — with a homozygous recessive (pp) to determine the identity of the unknown allele x is itPP or Pp? pp

  20. x x Test cross PP pp Pp pp p p p p Pp Pp Pp Pp P P 50%:50%1:1 100% P Pp Pp p pp pp

  21. Mendel’s Laws

  22. P P P p p PP Pp pp p Mendel’s laws of heredity (#1) • Law of segregation • when gametes are produced during meiosis, homologous chromosomes separate from each other • each allele for a trait is packaged into a separate gamete

  23. Meiosis 1 Law of Segregation • What meiotic event creates the law of segregation?

  24. Monohybrid cross • Some of Mendel’s experiments followed the inheritance of single characters • flower color • seed color • monohybrid crosses • Other of Mendel’s experiments followed the inheritance of 2 different characters • seed color andseed shape • dihybrid crosses

  25. yellow, round peas F1 generation (hybrids) 100% self-pollinate 9/16 yellow round peas 3/16 green round peas 3/16 yellow wrinkled peas 1/16 green wrinkled peas 9:3:3:1 F2 generation Dihybrid cross true-breeding yellow, round peas true-breeding green, wrinkled peas P x YYRR yyrr Y = yellow R = round y = green r = wrinkled YyRr

  26. 9/16 yellow round 3/16 green round YR Yr 3/16 yellow wrinkled yR 1/16 green wrinkled yr Dihybrid cross YyRr x YyRr YR Yr yR yr YYRR YYRr YyRR YyRr YYRr YYrr YyRr Yyrr YyRR YyRr yyRR yyRr YyRr Yyrr yyRr yyrr

  27. Mendel’s laws of heredity (#2) • Law of independent assortment • each pair of alleles segregates into gametes independently • 4 classes of gametes are produced in equal amounts • YR, Yr, yR, yr • only true for genes on separate chromosomes YyRr Yr Yr yR yR YR YR yr yr

  28. Meiosis 1 Law of Independent Assortment • What meiotic event creates the law of independent assortment?

  29. The chromosomal basis of Mendel’s laws… Trace the genetic events through meiosis, gamete formation & fertilization to offspring

  30. Review: Mendel’s laws of heredity • Law of segregation • monohybrid cross • single trait • each allele segregates into separate gametes • established by Meiosis 1 • Law of independent assortment • dihybrid (or more) cross • 2 or more traits • each pair of alleles for genes on separate chromosomes segregates into gametes independently • established by Meiosis 1

  31. Genetics & Probability • Mendel’s laws: • segregation • independent assortment reflect same laws of probability that apply to tossing coins or rolling dice

  32. P P P p PP Pp Probability & genetics • Calculating probability of making a specific gamete is just like calculating the probability in flipping a coin • probability of tossing heads? 50% • probability making a P gamete… 50% 100%

  33. male / sperm P P PP P p P p p P p p Pp pp P female / eggs p Calculating probability sperm egg offspring Pp x Pp 1/2 x 1/2 = 1/4 1/2 x 1/2 = 1/4 PP Pp 1/2 x 1/2 = 1/4 1/2 Pp pp 1/2 x 1/2 = 1/4

  34. Rule of multiplication • Chance that 2 or more independent events will occur together • probability that 2 coins tossed at the same time will land heads up • probability of Pp x Pp  pp 1/2 x 1/2 = 1/4 1/2 x 1/2 = 1/4

  35. Calculating dihybrid probability • Rule of multiplication also applies to dihybrid crosses • heterozygous parents — YyRr • probability of producing yyrr? • probability of producing y gamete = 1/2 • probability of producing r gamete = 1/2 • probability of producing yr gamete = 1/2 x 1/2 = 1/4 • probability of producing a yyrr offspring =1/4 x 1/4 = 1/16

  36. NOW TO PRACTICE!

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