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Mendelian Genetics

Mendelian Genetics. Gregor Mendel. 1822-1884 Responsible for the laws governing inheritance of traits. Gregor Mendel. Austrian Monk Studied the inheritance of traits in pea plants Developed the Laws of Inheritance Mendel’s work was not recognized until the early 20 th century.

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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 Mendel • Austrian Monk • Studied the inheritance of traits in pea plants • Developed the Laws of Inheritance • Mendel’s work was not recognized until the early 20th century

  4. Gregor 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 • He is known as the “Father of Genetics”

  5. Gregor’s Garden • The site of Mendel’s garden in the Czech Republic

  6. Particulate Inheritance • Mendel stated that physical traits are inherited as “particles” • Mendel did not know that the “particles” were DNA and chromosomes.

  7. Punnett Square • Used to help solve genetic problems. • Uses probability.

  8. Punnett Square

  9. Genotype and Phenotype • Genotype – gene combination for a trait • Ex. RR, Rr, rr • Phenotype – the physical feature resulting from the genotype • Ex. Red, pink, white

  10. Genotype and Phenotype • Genotype of alleles: • R = red flower • r = yellow flower • All genes occur in pairs, so TWO alleles affect a characteristic • Possible combinations are: • Genotypes: RR Rrrr • Phenotypes: Red Red Yellow

  11. Genotypes • Homozygous • Gene combination involving two dominant or two recessive genes (ex. RR or rr) • Also called pure • Heterozygous • Gene combination of one dominant and one recessive allele (ex. Rr) • Also called hybrid

  12. Genes AND Environment determine characteristics

  13. Mendel’s Pea Plant Experiments

  14. Why Peas?(Pisum sativum) • Can be grown in a small area. • Can produce a lot of offspring. • Will produce pure plants when allowed to self-pollinate over several generations. • Can also be artificially cross-pollinated.

  15. Reproduction In Flowering Plants • Pollen contains sperm • Produced by the stamen • Carries sperm to the eggs for pollination • Ovary produces eggs • Found inside the flower • Self-pollination • Can occur within the same flower • Cross-pollination • Can occur between unrelated flowers

  16. Mendel’s Experimental Methods • Mendel hand-pollinated flowers using a paintbrush • He could snip the stamens to prevent self-pollination. • He traced traits through several generations.

  17. How Mendel Began • Mendel produced pure strains of pea plants by allowing the plants to self-pollinate over several generations.

  18. Eight Pea Plant Traits • Seed shape – round (R) or wrinkled (r) • Seed color – yellow (Y) or green (y) • Pod shape – smooth (S) or wrinkled (s) • Pod color – green (G) or yellow (g) • Seed coat color – gray (G) or white (g) • Flower position – axial (A) or terminal (a) • Flower color – purple (P) or white (p) • Plant height – tall (T) or short (t)

  19. Eight Pea Plant Traits

  20. Eight Pea Plant Traits

  21. Mendel’s Experimental Results

  22. 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. This discrepancy is due to statistical error. • The larger the sample, the more nearly results reflect the theoretical ratios

  23. Mendel’s Generations • Parental Generation (P1) • The original (parental) generation in a breeding experiment. • First Filial Generation (F1) • The first generation of offspring in a breeding experiment. • From breeding individuals in the P1 generation. • Second Filial Generation (F2) • The second generation of offspring in a breeding experiment. • From breeding individuals in the F1 generation.

  24. Following the Generations Cross two pure plants. TT x tt Cross two hybrids Get 3 tall : 1 short TT, Tt, tt Results in all Hybrids Tt

  25. Monohybrid Crosses

  26. P1 Monohybrid Cross • Trait : Seed Shape • Alleles : R – round, r – wrinkled • Cross : round seeds X wrinkled seeds RR x rr r r Genotype: Rr Phenotype: Round Genotypic Ratio: All alike Phenotypic Ratio: All alike R Rr Rr R Rr Rr

  27. P1 Monohybrid Cross Review • Homozygous dominant x homozygous recessive • Offspring all heterozygous (hybrids) • Offspring called F1 generation • Genotypic and phenotypic ratio is all alike.

  28. F1 Monohybrid Cross • Trait : Seed Shape • Alleles : R – round, r – wrinkled • Cross : round seeds X round seeds Rr x Rr R r Genotype: RR, Rr, rr Phenotype: Round & Wrinkled Genotypic Ratio: 1 : 2 : 1 Phenotypic Ratio: 3 : 1 R RR Rr r Rr rr

  29. F1 Monohybrid Cross • Heterozygous X Heterozygous • Offspring • 25% homozygous dominant --RR • 50% heterozygous -- Rr • 25% homozygous recessive – rr • Offspring called F2 generation • Genotypic ratio is 1 : 2 : 1 • Phenotypic ratio is 3 : 1

  30. What do the peas look like?

  31. Test Cross • Mendel then crossed a pure and a hybrid from his F2 generation. • This is known as an F2 or test cross • There are two possible test crosses • Homozygous dominant x hybrid • Homozygous recessive x hybrid

  32. F2 Monohybrid Cross (1st) • Trait : Seed Shape • Alleles : R – round, r – wrinkled • Cross : round seeds X round seeds RR x Rr R r Genotype: RR, Rr Phenotype: Round Genotypic Ratio: 1 : 1 Phenotypic Ratio: All alike R RR Rr R RR Rr

  33. F2 Monohybrid Cross (2nd) • Trait : Seed Shape • Alleles : R – round, r – wrinkled • Cross : wrinkled seeds X round seeds rr x Rr R r Genotype: Rr, rr Phenotype: Round & Wrinkled Genotypic Ratio: 1 : 1 Phenotypic Ratio: 1 : 1 r Rr rr r Rr rr

  34. F2 Monohybrid Cross Review • Homozygous x heterozygous (hybrid) • Offspring • 50% homozygous – RR or rr • 50% heterozygous – Rr • Phenotypic Ratio – 1 : 1 • Genotypic Ratio – 1 : 1 • Called a test cross because the offspring have the SAME genotype as parents.

  35. Mendel’s Laws

  36. Results of Monohybrid Crosses • Inheritable factors or genes are responsible for all heritable characteristics. • Phenotype is based on genotype. • Each trait is based on two alleles, one from ma, the other from pa. • True breeding individuals are homozygous (both alleles are the same).

  37. Law of Dominance • In a cross of parents that are PURE for CONSTRASTING TRAITS, only ONE FORM of the trait WILL APPEAR in the next generation. • All offspring will be HETEROZYGOUS and will EXPRESS only the DOMINANT TRAIT. • RR x rr yields all Rr

  38. Law of Segregation • During the formation of gametes (eggs or sperm), the two alleles responsible for a trait separate from each other. • Alleles for a trait are then recombined at fertilization, producing the genotype for the traits of offspring.

  39. Law of Segregation

  40. Law of Independent Assortment • Alleles for different traits are distributed to sex cells & offspring independently from one another. • Each pair of alleles segregates independently during gamete formation. • Formula = 2n (n = the # of heterozygotes) • This law can be illustrated using dihybrid crosses.

  41. How many gametes will be produced for the following allele arrangements? • 2n = (n = # of heterozygotes) • RrYy • AaBbCCDd • MmNnOoPPQQRrssTtUu

  42. Answers: • RrYy • 2n = 22 = 4 gamete combinations • RY Ry rY ry • AaBbCCDd • 2n = 23 = 8 gamete combinations • ABCD ABCd AbCD AbCd aBCD aBCd abCD aBcD • MmNnOoPPQQRrssTtUu • 2n = 26 = 64 gamete combinations

  43. Dihybrid Crosses • A breeding experiment that tracks the inheritance of TWO different TRAITS. • Ex. Tail size and Fur Color

  44. Dihybrid Crosses • Traits = seed shape & seed color • Alleles • R (round) • r (wrinkled) • Y (yellow) • y (green) RrYy x RrYy RY Ry rY ry RY Ry rY ry All possible gamete combinations

  45. Dihybrid Cross RY Ry rY ry RY Ry rY ry

  46. Dihybrid Cross RY Ry rY ry RY RRYY RRYy RrYY RrYy Round /Yellow: 9 Round/Green: 3 Wrinkled/Yellow: 3 Wrinkled/Green: 1 Phenotypic Ratio— 9 : 3 : 3 : 1 Ry RRYy RRyy RrYy Rryy rY RrYY RrYy rrYY rrYy ry RrYy Rryy rrYy rryy

  47. Dihybrid Cross • Round/Yellow: 9 • Round/Green: 3 • Wrinkled/Yellow: 3 • Wrinkled/Green: 1 9 : 3 : 3 : 1

  48. bC b___ bc Test Cross • A mating between an individual with unknown genotype and a homozygous recessive individual. • Ex. bbC_ x bbcc • BB = brown eyes • Bb = brown eyes • bb = blue eyes • CC = curly hair • Cc = curly hair • cc = straight hair

  49. bC b___ C bC b___ c bc bbCc bbCc or bc bbCc bbcc Test Cross • Possible Results

  50. LAW PARENT CROSS OFFSPRING DOMINANCE TT x tttall x short 100% Tttall SEGREGATION Tt x Tttall x tall 75% tall 25% short INDEPENDENT ASSORTMENT RrGg x RrGground & green x round & green 9/16 round seeds & green pods 3/16 round seeds & yellow pods 3/16 wrinkled seeds & green pods 1/16 wrinkled seeds & yellow pods Summary of Mendel’s Laws

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