Presentation on theme: "Recall: several hypotheses about inheritance"— Presentation transcript:
1 Recall: several hypotheses about inheritance What made Mendel successful?3 key points to any successful biological experiment:Choosing the appropriate organism to studyDesigning and performing the experiment correctlyAnalyzing the data properly
2 Pisum sativum – The common pea plant What organism did he study???Pisum sativum – The common pea plantWhy the pea plant?Commercially available across EuropeEasy to grow and matured quicklySexual organs entirely enclosed in flower (self-pollinate)Control which plants reproducedDifferent traits could be observed easily
3 To setup his experiment, Mendel obtained purebreed plants Purebreed – decended from ancestors of a distinct type or breedIe. All tall vs. All short plantsThus, Mendel produced plants that were true breeding – only producing offspring that grew either tall or shortMendel repeated this several times for 7 different traits:- Stem length, flower position, seed shape, seed colour, pod shape, pod colour, flower colour
4 Mendel’s First Experiment: Monohybrid Crosses P generation (parent generation)Crossed a true-breeding TALL pea plant with a true-breeding SHORT pea plant
5 The offspring were the 1st filial generation (F1 generation) This generation consisted of hybrid plants since they were the result of a cross between 2 purebred plantsMonohybrid cross when only 1 trait is involvedDiscussion:What trait did the F1 generation demonstrate?Tall or short or medium?...or something we’ve never even heard about?
7 Mendel concluded that the “TALL” trait is dominant and the “SHORT” trait is recessive Dominant trait – characteristic that is always expressed in an individualRecessive trait – characteristic that is latent and therefore not usually expressed in an individual***May be expressed if it is the only trait present
8 He did this several times using true breeding plants for 7 traits that he chose to study Stem lengthFlower positionSeed shapeSeed colourPod shapePod colourFlower colour
9 When individuals with contrasting traits are crossed, the offspring Every time: one trait was alwaysdominant over the otherTherefore...Mendel formulatedthe Principle of Dominance:When individuals with contrastingtraits are crossed, the offspringwill express only the dominanttrait.
10 Law of Segregation Next step: Breeding the F1 generation Offspring was the 2nd filial generation (F2 generation)Discussion:What trait did the offspring of the F2 generation express?
11 Answer: The same result occurred with all 7 traits 3:1 ratio = 3 out of 4 plants in the F2 generation were tall and 1 was shortThe same resultoccurred with all 7traits75% of the time– expressed dominant25% of the time– expressed recessive3:1 ratio =Mendelian ratio
13 Mendel’s Law of Segregation Thus, Mendel drew the following conclusions:In the F1 generation, each parent starts with 2 heredity “factors” – one dominant, one recessiveThe factors segregate and only one factor from each parent is given to the F2 generation offspring; therefore, the offspring has 1 factor from each parentIf the dominant factor is present it will be expressed (even if the recessive factor is also present)The recessive factor will only be expressed if only recessive factors are present
14 Modern day’s terminology: “factors” = genes Genes have: Dominant allelesRecessive alleles*Note: When a dominant and recessive allele are together, the dominant allele will be expressed but the recessive allele is still there and still can be passed onto the next generation
15 Homozygous vs. Heterozygous HOMOZYGOUS: the alleles are the sameHETEROZYGOUS: alleles are differentEx. Trait: Tall and shortT = tall t = shortTT = homozygous dominant talltt = homozygous recessive shortTt / tT = heterozygous dominant tall(Demonstrate generations on board)
16 Punnett SquaresTTUsed to calculate probability of inheriting a particular trait.Allows you to determine the:Genotype: the genetic make-up behind a traitPhenotype: The actual physical appearance of the traittt
17 Example: Applying Mendel’s first law: a monohybrid cross Problem: A plant grown purebred homozygous tall seed crossed with a homozygous short seedShow the possible gametes from each parent (possible offspring)Show the possible gamete combinations at fertilizationWhat are the possible genotypes of F1What are the possible phenotypes of F1
18 Example 2:Problem: A plant grown from heterozygous round seeds is crossed with a plant grown from wrinkled seeds (Let R = dominant round, r = recessive wrinkled)Show the possible gametes produced from each parentShow the possible gamete combinationsWhat are the possible genotypes?What are the possible phenotypes?
19 Example 3:In sheep, the allele for white wool (W) is dominant over the allele for black wool (w). If two white sheep mated,What are the possible genotypes of the two parent?What is the probability that an offspring of the two white sheep will be black? (Create Punnet squares for each possible parent combinations)
20 Pedigrees How can we apply this to us? Pedigrees are diagrams that illustrate the genetic relationships among a group of related individualsIt organizes medical, historical, and family records extending across several generationsPedigrees been able to show simple dominant traits (only 2 possibilities – dominant, recessive)Ie. Widow’s peak, tongue-rolling, freckles, long eyelashes, unattached ear lobes, etc
21 MaleFemaleAffected Male (homozygous)Affected Female (homozygous)Known Heterozygous FemaleKnown Heterozygous MaleMatingParentsSiblings