PowerPoint PresentationSection 12. Mendelian GeneticsGregor MendelMendel Was Not the First to Try: Why Did He Succeed in Deducing Laws of Heredity Where His Predecesssors Failed?Slide 5What Mendel DidSlide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Dihybrid CrossesSlide 15Slide 16Slide 17Slide 18Slide 19Analysis as a two-factor cross requires two steps to predict F2: 1. Use Punnett square to get all possible combinations of alleles in gametes:Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Mendelian Genetics in TetradsSlide 30Slide 31This presentation was originally prepared by C. William Birky, Jr.Department of Ecology and Evolutionary BiologyThe University of ArizonaIt may be used with or without modification for educational purposes but not commercially or for profit.The author does not guarantee accuracy and will not update the lectures, which were written when the course was given during the Spring 2007 semester.Section 12. Mendelian GeneticsGregor MendelBorn 1822 in Heizenberg, Austria, son of a farmer.Very bright as a student, sent to gymnasium, but father was crippled and family couldn’t afford to keep him in school, so he joined monastery to get an education and to be teacher. 1843 joined Augustinian monastery at Brünn in Moravia.1847 ordained into priesthood.1849 assigned to teach in secondary school, took teaching examination, failed due to lack of knowledge.1851 was sent to U. Vienna where got brief but extremely sound scientific education.1856 failed teaching exam again; test anxiety?Began experiments with peas in 1850's. 1865 read paper on his results to Brunn Natural History Society.1866 paper published in Proceedings of Brunn Natural History Society. (A date to remember!)Besides studies on heredity, did other kinds of natural history.1868 scientific career ended when became abbot of the monastery.1884 died.Mendel Was Not the First to Try: Why Did He Succeed in Deducing Laws of Heredity Where His Predecesssors Failed?He was really smart!Better scientific background than those before him:•cell theory; probably knew adult plant comes from egg by succession of cell divisions•fertilization: pollen grain + egg -> zygote; knew, from his own experiments, that one pollen grain fertilized one egg•took math including early probability theory; ready to see and understand random variation•took physics from Doppler, saw power of quantitative data and mathematical lawsWhat Mendel did not know:Genes on chromosomes in nucleusMitosisMeiosisSimplified problemFocused on discontinuous variation (either/or traits), usually controlled by one or two genes, instead of continuous variation controlled by many genes and the environment.Good choice of experimental organismWorked with plants, as did nearly all geneticists. Selected peas because:•many different phenotypes; got ≥ 27 varieties that differed in various phenotypic traits•could do controlled crosses or selfing = self-fertilization, mating plant with itselfCopyrighted figure removed.What Mendel Did1. From commercial seed dealers, selected many pea strains differing in discrete characters. Chose some differing in 7 traits.2. Subjected these to several generations of selfing. Bred true; e.g. plant green seeds, grow plants, self plants --> seeds all green.We know, and Mendel deduced, that selfing (or any other form of inbreeding) produces pure lines, homozygous plants that produce only homozygous offspring.3. Did crosses between strains differing in one or more traits. Monohybrid cross: parents differ in only one trait. Most of Mendel’s crosses were dihybrid or trihybrid. Any cross can be analyzed as monohybrid crosse by following only one trait.Copyrighted figure removed.P0round wrinkled pure lines homozygous diploidSelf gametes P1 round wrinkled Cross gametesF1 round heterozygous diploidSelf or cross gametes inter seF2 423 round 0.76 ≈ 3/4 133 wrinkled 0.24 ≈ 1/4 556 1.00P0round wrinkled pure lines R r homozygous diploidSelf R r gametes P1 round wrinkled R r Cross R r gametesF1 round R heterozygous diploidSelf or Cross gametes Inter seF2 423 round R133 wrinkled rTwo phenotypes produced by two different hereditary factors.P0round wrinkled pure lines R r homozygous diploidSelf gametes P1 round wrinkled R r Cross gametesF1 round Rr heterozygous diploidSelf or Cross R r gametes Inter seF2 423 round R133 wrinkled rTwo phenotypes produced by two different hereditary factors.F1 produces F2 with both phenotypes so must have and transmit both hereditary factors..P0round wrinkled pure lines RR rr homozygous diploidSelf R R r r gametes P1 round wrinkled RR rrCross R r gametesF1 round Rr heterozygous diploidSelf or Cross R r gametes Inter seF2 423 round RR and Rr133 wrinkled rrTwo phenotypes produced by two different hereditary factors.F1 must produce F2 with both phenotypes so must have and transmit both hereditary factors.If F2 has two factors, all plants have to have two. Inbred parents only produce one kind of gamete, so have only one kind of hereditary factor.P0round wrinkled pure lines RR rr homozygous diploidSelf R R r r gametes P1 round wrinkled RR rrCross R r gametesF1 round Rr heterozygous diploidSelf or Cross R r gametes Inter seF2 423 round 0.76 ≈ 3/4 = 1/4 RR and 1/2 Rr133 wrinkled 0.24 ≈ 1/4 all rr556 1.00Now Mendel can explain the ratio of phenotypes in the F2. If the two kinds of F1 gametes are paired randomly in all possible combinations, 1/4 will be RR, 1/2 Rr, and 1/2 rr. Rr will be round, as in the F1. R is dominant, so Rr is round.How explain F2? Mendel came up with a model:•Mendel’s first law or law of segregation: Alleles segregate during formation of the gametes, 1/2 of the gametes get one allele and 1/2 the other.F1 gametes are 1/2 R and 1/2 r.•Fertilization is random with respect to genotype.Make Punnett square to see different combinations of egg and pollen.Genotypic ratio 1/4 RR : 1/2 Rr : 1/4 rrPhenotypic ration 3/4 round : 1/4 wrinkled PollenEggs1/2 R 1/2 r1/4 RR1/2 R1/2 r1/4 Rr1/4 rr1/4 rRMendel didn’t know about meiosis or even about chromosomes so he couldn’t interpret his data in those terms.Walter Sutton (1902), Theodore Boveri (1903): Chromosome theory of heredity: •Genes are on chromosomes. •Different chromosomes have different sets of