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Chapter 14: Mendel and the Gene Idea

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  1. Chapter 14: Mendel and the Gene Idea

  2. Essential Knowledge • 3.a.3 – The chromosomal basis of inheritance provides an understanding of the pattern of passage (transmission) of genes from parent to offspring (14.1-14.4). • 4.c.2 – Environmental factors influence the expression of the genotype in an organism – (14.3). • 4.c.4 – The diversity of species within an ecosystem may influence the stability of the ecosystem (14.3).

  3. Past/Present Genetic Hypotheses 1. Blending Theory - traits were like paints and mixed evenly from both parents 2. Incubation Theory - only one parent controlled the traits of the children Ex: Spermists and Ovists 3. Particulate Model - parents pass on traits as discrete units that retain their identities in the offspring

  4. Gregor Mendel • Father of Modern Genetics • Mendel’s paper published in 1866, but was not recognized by science until the early 1900’s • Died prior to his “fame”

  5. Reasons for Mendel's Success • Used an experimental approach (scientific method) • Applied mathematics to the study of natural phenomena • Ratios and probability • Kept good records and observations • Large testsample/size

  6. Why Use Peas? • Short life span • Bisexual *Both sexes in one flower/plant *Stamens and carpels • Many traits known *Easy to see/observe traits • Cross- and self-pollinating *Easy to control reproduction • You can eat the failures 

  7. Cross-pollination • Cross between two different parents • Results in hybrid offspring • The offspring may be different than the parents.

  8. Self-pollination • Cross with only one flower • Stamens/carpels fertilize each other! • Naturally occurring event in pea plants • Results in pure-bred offspring where the offspring are identical to the parents • Is this asexual reproduction??? • NO…you still have gametes

  9. Mendel's Work • Used seven characters, each with two expressions or traits • Example: • Character - height • Traits - tall or short

  10. Monohybrid or Mendelian Crosses • Mono = one • Crosses that work with a single characterat a time • Example - Tall X short

  11. P Generation • The Parental generation or the first two individuals used in a cross • Example - Tall X short • Mendel used reciprocal crosses, where the parents alternated for the trait

  12. Offspring • F1 - first filial generation • Filial – Latin for “son” • F2 - second filial generation, • Bred by crossing two F1 plants together or allowing a F1 to self-pollinate

  13. Notice: TWO P1 plants shown (cross fertilz.) Notice: only ONE plant shown (self-fertilz.)

  14. Another Sample Cross P1 Tall X short (TT x tt) F1 all Tall (Tt) F2 3 tall to 1 short (1 TT: 2 Tt: 1 tt) Short Tall

  15. Results - Summary • Mendel observed SAME pattern in ALL 7 characters • F1 generation showed only one of the traits (regardless of sex) • The other trait reappeared in the F2 at ~25% • 3:1 ratio; 3 dominant – 1 recessive • Remember: the % are estimates (still have mutations that could change %)

  16. Mendel's Hypothesis • Genes can have alternate versions called alleles • Each offspring inherits two alleles, one from each parent • He made this conclusion without having knowledge of chromosomes/DNA makeup

  17. Homologous chromosomes ** Remember: Each diploid cell has a pair of homologous chromosomes -Therefore, any gene has 2 loci *one on maternal chromo *one on paternal chromo

  18. Mendel's Hypothesis • If the two alleles differ, the dominant allele is expressed • The recessive allele remains “hidden” (unseen) unless the dominant allele is absent • Now called Mendel’s Law of Dominance

  19. Mendel's Hypothesis • The two alleles for each trait separate during gamete formation (meiosis) • This now called Mendel's Law of Segregation

  20. Law of Segregation

  21. Genetics Vocabulary • Phenotype- the physical appearance of the organism • Genotype- the genetic makeup of the organism, usually shown in a code • T = tall • t = short

  22. Vocabulary • Homozygous- When the two alleles are the same (TT/tt) • Heterozygous- When the two alleles are different (Tt) • Notice (for single-gene traits: • Three choices for genotypes • Homo Dom (TT), Homo Rec (tt), Hetero (Tt)

  23. 6 Mendelian Crosses are Possible CrossGenotypePhenotype TT X tt all Tt all Dom Tt X Tt 1TT:2Tt:1tt 3 Dom: 1 Res TT X TT all TT all Dom tt X tt all tt all Res TT X Tt 1TT:1Tt all Dom Tt X tt 1Tt:1tt 1 Dom: 1 Res Notice the 3:1 ratio!!!

  24. Test Cross • Cross of a suspected heterozygote with a homozygous recessive • Goal: to determine genotype of unknown • Ex: T? X tt *If TT - all Dominant *If Tt - 1 Dominant: 1 Recessive

  25. Dihybrid Cross • Cross with two genetic traits • Di = two • Need 4 letters(two for each trait) to code for the cross • Ex: TtRr (Mono = Tt OR Rr) • Each Gamete - Must get 1 letter for each trait • Ex. TR, Tr, etc. (when combine = 4 letters)

  26. Number of Kinds of Gametes • Critical to calculating the results of higher level crosses • Look for the number of heterozygous traits

  27. Equation • The formula 2n can be used, where “n” = the number of heterozygous traits. • Ex: TtRr, n=2 (2 heterozygous traits) • 22 or 4 different kinds of gametes are possible (TR, tR, Tr, tr) • Ex:TtRR, n = ? • 21 or 2 different gametes are possible

  28. Dihybrid Cross TtRr X TtRr • Each parent can produce 4 types of gametes. (n=2; 22=4) • TR, Tr, tR, tr • Cross is a 4 X 4 = 16 possible offspring

  29. Results • 9 Tall, Red flowered • 3 Tall, white flowered • 3 short, Red flowered • 1 short, white flowered Or: 9:3:3:1 ratio

  30. Law of Independent Assortment • The inheritance of 1st genetic trait is NOT dependent on the inheritance of the 2nd trait • Ex: Inheritance of height is independent of the inheritance of flower color • This relates to dihybrid crosses – one character’s inheritance is NOT connected to the inheritance of another!

  31. Comment • Ratio of Tall to short is 3:1 • Ratio of Red to white is 3:1 • The cross is really a product of the ratio of each trait multiplied together. • (3:1) X (3:1) = 9:3:3:1 • *Use FOIL method to attain ratio

  32. Probability • Genetics is a specific application of the rules of probability • Probability - the chance that an event will occur out of the total number of possible events

  33. Genetic Ratios • The monohybrid “ratios” are actually the “probabilities” of the results of random fertilization Ex: 3:1 75% chance of the dominant 25% chance of the recessive

  34. Rule of Multiplication • The probability that two alleles will come together at fertilization, is equal to the product of their separate probabilities • Steps to determining probability: • 1) Determine ratios for each character/trait • How? Do “little” Punnett squares for EACH trait • 2) Multiply ratios together

  35. Example: TtRr X TtRr • The probability of getting a tall offspring is ¾. • The probability of getting a red offspring is ¾. (use same Punnett square as above – only with R/r) • The probability of getting a tall red offspring is ¾ x ¾ = 9/16

  36. Product Rule • Use the Product Rule to calculate the results of complex crosses rather than work out the Punnett Squares • Ex: TtrrGG X TtRrgg

  37. Solution TtrrGG X TtRrgg “T’s” = Tt X Tt = 3:1 “R’s” = rr X Rr = 1:1 “G’s” = GG x gg = 1:0 Product is: (3:1) X (1:1) X (1:0 ) = 3:3:1:1

  38. Variations on Mendel 1. Incomplete Dominance 2. Codominance 3. Multiple Alleles 4. Epistasis 5. Polygenic Inheritance

  39. Incomplete Dominance • When the F1 hybrids show a phenotype somewhere between the phenotypes of the two parents • Ex. Red X White snapdragons F1 = all pink F2 = 1 red: 2 pink: 1 white • NOT BLENDING!!!!!

  40. Not enough red pigment made

  41. Result from Inc Dominance • No hidden recessive • 3 phenotypes and 3 genotypes (Hint! – often a “dose” effect) • Red = CR CR • Pink = CRCW • White = CWCW

  42. Another example

  43. Codominance • Both alleles are expressed equally in the phenotype • NOT an intermediate (like incomplete dominance • Ex. MN blood group • MM, MN, NN • Ex: Rooster/chicken feathers • Ex: flower petal color

  44. Result from Codominance • No hidden recessive • 3 phenotypes and 3 genotypes (but not a “dose” effect)

  45. Multiple Alleles • When there are more than 2 alleles for a trait • *Remember: only 2 alleles exist for Mendel’s pea plants • Ex. ABO blood group • IA - A type antigen • IB - B type antigen • i - no antigen