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Mendelelian Genetics

Mendelelian Genetics. Gregor Mendel “father of Genetics”. Responsible for the Laws governing Inheritance of Traits. Gregor Johann Mendel. Austrian monk Studied the inheritance of traits in pea plants He found that the plants' offspring retained traits passed from parental generation.

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Mendelelian Genetics

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  1. Mendelelian Genetics

  2. Gregor Mendel“father of Genetics” Responsible for the Laws governing Inheritance of Traits

  3. Gregor Johann Mendel • Austrian monk • Studied the inheritance of traits in pea plants • He found that the plants' offspring retained traits passed from parental generation

  4. 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

  5. Types of Genetic Crosses • Monohybrid cross - cross involving a single traite.g. flower color • Dihybrid cross - cross involving two traits e.g. flower color & plant height

  6. Punnett Square Used to help solve genetics problems

  7. Genetic Terminology • Alleles - 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)

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

  9. Genotype & Phenotype in Peas

  10. Genotype & Phenotype in Peas Genotype of alleles:R= Round seedsr = Wrinkled seeds All genes occur in pairs, so 2alleles affect a characteristic Possible combinations are: GenotypesRRRrrr PhenotypesROUND ROUND WRINKLED

  11. 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

  12. Law of Dominance In a cross of parents that are pure for contrasting traits, only one form of the trait will appear in the next generation. All the offspring will be heterozygous and express only the dominant trait. RR x rr yields all Rr (round seeds)

  13. Law of Dominance

  14. Mendel’s Pea Plant Experiments

  15. Why peasplants? • Can be grown in a small area • Produce lots of offspring • Produce pure plants when allowed to self-pollinate several generations • Can be artificially cross-pollinated

  16. 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

  17. Some Pea Plant Traits • Seed shape--- Round (R) or Wrinkled (r) • Seed Color---- Yellow (Y) or  Green (y) • Pod Shape--- Smooth (S) or wrinkled (s) • Plant Height--- Tall (T) or Short (t) • Flower color --- Purple (P) or white (p)

  18. Generation “Gap” • Parental P1 Generation= the parental generation in a breeding experiment. • F1 generation= the first-generation offspring in a breeding experiment. • F2 generation= the second-generation offspring in a breeding experiment.

  19. Monohybrid Crosses

  20. Monohybrid Cross • Trait: Plant Height • Alleles: T– Tall t – short • Cross: Tall plants xshort plants • TTxtt Genotype:Tt Phenotype:tall GenotypicRatio:All alike PhenotypicRatio: All alike T T Tt Tt t t Tt Tt

  21. Monohybrid Cross • Trait: Plant Height • Alleles: T– Tall t – short • Cross: Tall plants xtall plants • TtxTt T t Genotypic Ratio:1 TT:2 Tt:1 tt Phenotypic Ratio: 3 tall:1 short TT Tt T t Tt tt

  22. Practice Your Crosses • Refer to page 179 • Provide the genotypic AND phenotypic ratios • 1. Cross ahomozygous greenseed with a heterozygous yellow seed • 2. Cross aheterozygous round plant with a heterozygous round plant • 3. Cross ahomozygous purple seed with a homozygous white seed

  23. Review of Monohybrid Crosses • Phenotype is based on Genotype • Each traitis based ontwo genes, one from the mother and the other from the father • True-breeding individuals are homozygous, which means both allelesare the same.

  24. Mendel’s Laws

  25. 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 the offspring.

  26. Applying the Law of Segregation

  27. Law of Independent Assortment • Alleles for differenttraits are distributed to sex cells (& offspring) independently of one another.

  28. Summary of Mendel’s laws

  29. Exceptions To Mendel’s Original Principles • Incomplete dominance • Co-dominance • Multiple alleles • Polygenic traits • Environmental effects on gene expression • Linkage • Sex linkage

  30. Incomplete DominanceandCodominance

  31. r r R R Incomplete Dominance • Neither allele is dominant • Heterozygotes have an intermediate phenotype • Example:Japanese “Four o’clock” flower • red (RR) x white (rr) RR = red flower rr = white flower

  32. r R Rr Rr All Rr = pink (heterozygous pink) R Rr Rr Incomplete Dominance Cross: red (RR) x white (rr) r

  33. Incomplete Dominance Checking your understanding… Show a cross between 2 pink 4 o’clock flowers. Give the genotypic and phenotypic ratios.

  34. Codominance • Neither allele is dominant and both alleles are expressed in heterozygous individuals. • Example: blood type

  35. IA i IAIB IBi IB 50% = IAIB 50% = IBi IB IAIB IBi Codominance Problem • Example: Homozygous male Type B (IBIB) • x Heterozygous female Type A (IAi)

  36. IA IB IAi IBi i 50% = IAi 50% = IBi i IAi IBi Another Codominance Problem • Checking your understanding… • Example:male Type O (ii) x female type AB (IAIB)

  37. Codominance • Critical Thinking Question: If a boy has a blood type O and his sister has blood type AB, what are the genotypes and phenotypes of their parents? • boy - type O (ii) X girl - type AB (IAIB)

  38. IA i IAIB IB i ii Codominance • Answer: Parents: genotypes = IAi and IBi phenotypes = A and B

  39. Sex-linked Traits • Traits (genes) located on the sex chromosomes • Sex chromosomes are X and Y • XXgenotype for females • XYgenotype for males • Many sex-linked traits carried on X chromosome • Examples: color blindness, hemophilia

  40. Xr Xr XR Y Sex-linked Trait Problem • Example: Eye color in fruit flies • (red-eyed male) x (white-eyed female)XRY x XrXr • Remember: the Y chromosome in males does not carry traits. • RR = red eyed • Rr = red eyed • rr = white eyed • XY = male • XX = female

  41. Xr Xr XR Xr XR Xr XR Y Xr Y Xr Y Sex-linked Trait Solution: 50% red eyedfemale 50% white eyed male

  42. Female Carriers

  43. Genetic Practice Problems

  44. t t T T Practice • tall (TT) x dwarf (tt) pea plants

  45. r r R Rr Rr R Rr Rr All Rr= ROUND (heterozygous round) Solution: Round (RR) vs. wrinkle (rr) pea plants

  46. T t T t practice • tall (Tt) vs. tall (Tt) pea plants

  47. T t Tt TT T 1/4 (25%) = TT 1/2 (50%) = Tt 1/4 (25%) = tt Tt tt t 1:2:1 genotype 3:1 phenotype Solution: tall (Tt) x tall (Tt) pea plants

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