Biology 212 General Genetics Spring 2007Lecture 4: "Mendelian Genetics I"Reading: Chap. 2 pp. 34-45Lecture Outline:1. Mendel's experiments2. Molecular basis for round vs. wrinkled3. Principle of segregationLecture: Mendelian Genetics=Classical Genetics=Transmission Genetics=Study of how genes are transmitted from generation to generation.1. Mendel's experimentsGregor Mendel 1856-1863- Monk, teacher of physics and natural history.- Experiments on garden peas.Traits in peas: 7 in all, including- Round vs. wrinkled seeds- Green vs. yellow seeds- Purple vs. white flowers- Tall vs. short stemsMendel's crosses- Chose very distinct traits- Selected stocks that were true-breeding for these traits- Carried out controlled breeding experiments- Crossed pollen of one plant with the ova of another- Can self-cross plants: allow to self pollinateMendel found- Each parent contributes distinct "factors" (=genes) to offspring- Factors remain unchanged as they are passed onExample of a monohybrid cross (=cross between organisms that differ in one trait)P1 generation True-breeding wrinkled x true-breeding roundF1 generation All round1Therefore round is dominant trait, wrinkled is recessive trait. See Fig. 2.52. Molecular Basis for Mendel's traits: round vs. wrinkledround seed gene wrinkled seed gene: has insertion mutationSB enzyme I defective SB enzyme IRound have enzyme to make Wrinkled have defectivestarch; shrink evenly. enzyme for making starch; shrink unevenly.Nomenclature:W=round w=wrinkledCapital letter for Lower case letter for recessive trait dominant traitRound is wild type= Wrinkled is mutant,commonly found type rarely foundWays of distinguishing variants (=alleles of traits):- Observe physical differenceo Example shape, color- Measure molecular differenceo Example: electrophoresis of DNASmall version of gene=roundLarger version of gene=wrinkledElectrophoresis diagrams of - Wrinkled- Round- F1 hybridSee Figure 2.6- Visible traits are frequently dominant or recessive. - Often have codominant molecular forms. Neither form is dominant at the molecular level.23. Principle of segregationBefore Mendel- Blending inheritance- Traits of parents become blended in the hybrid- Mendel believe traits behaved as discrete units; don't blend - To observe Mendelian inheritance must observe a couple generations of crossesP1 round X wrinkledF1 round X F1 round (self-cross)F2 5474 round F2 ratio 2.96:11850 wrinkledResults: In F2 generation, the recessive wrinkled trait reappears. The ratio of dominant to recessive is about 3:1.Mendel predicteda. Genes come in pairs (=alleles)b. Paired genes separate (segregate) in gamete formationc. gametes unite at random in fertilizationFig. 2.6 Illustration of Mendel's concepts in monohybrid cross of round and wrinkled peas.WW x wwRound wrinkledGametes W w One copy of each gene in gameteFertilization WwIntercrossing of hybrid: Ww x Ww"Punnett square"W wW WW Www Ww ww3/4 round (WW and Ww) to 1/4 wrinkled (ww)3Principle of segregation: In the formation of gametes, the paired genes separate andare equally likely to be distributed to the mature gametes.Summary of Mendel's observations for monohybrid crosses- One trait is dominant, one is recessive.- In the F1 generation, the recessive trait does not appear in a cross of a true-breeding dominant individual and a true breeding recessive individual.- The recessive trait will reappear in 1/4 of the progeny in an F1 x F1 monohybrid cross- the dominant trait will occur in 3/4 of the individualsOther terms used in classical genetics:- Gene: the hereditary determinant of a trait- Allele: different forms of a particular gene- Genotype: genetic make-up of an individual- Heterozygote: individual with two different alleles for a gene- Homozygote: individual with two of the same types of alleles for a gene- Phenotype: the observable traits of an organism- Recessive trait: a trait that is only expressed when genotype is heterozygous- Dominant trait: a trait that is expressed when the genotype is either homozygous or