History of Horticulture: Lecture 331Lecture 33 Lecture 33 Agricultural Scientific Revolution: Genetic and BiologicalAncient Empirical Knowledge of InheritanceLike begets LikeBeginning of genetic wisdomWhen she has calved, then set the dam aside;And for the tender progeny provide;Distinguish all betimes with branding fire,To note the tribe, the lineage, and the sire;Whom to reserve for husband of the herd;Or who shall be to sacrifice preferred;Of whom thou shalt to turn thy glebe (soil) allow,To smooth the furrows, and sustain the plough;The rest, for whom no lot is yet decided,May run in pastures, and at pleasure fed.Virgil’s GeorgicsMy hounds are bred out of the Spartan kind,So flew’d, so sanded, and their heads are hungWith ears that sweep away the morning dew;Crook-knee’d, and dewlapp’d like Thessalian bulls;Slow in pursuit, but match’d in mouth like bells,Each under eachMidsummer’s Nights Dream 4.1.119–124ShakespeareHistory of Horticulture: Lecture 332Professor Botanic Gardens at Tübingen, 1688.Through study of dioeciousand monoecious plants explains function of pollen and egg; considered apices with pollen as male, first modern understanding of plant sexuality.Rudolph Camerarius(1665–1721)First systematic experiments on plant hybridization using tobacco(Nicotiana paniculata × N. rustica)Demonstrated that hybrids resembleboth parentsExperimentally verified the geneticcontribution of pollenFirst observed hybrid vigor (heterosis)18th& 19thCentury ExperimentsHybridizingJoseph Gottlieb Koelreuter(1733–1806)Described dominance and segregation in the garden pea but failed to make the brilliant leap of MendelInitiated fruit breeding Thomas Andrew Knight (1759–1838)History of Horticulture: Lecture 333On the Origin of the Species (1859)Discuses variability and evolution but does not have a rational genetic explanationCharles Darwin (1809–1882)Priest in Brünn, Austro-Hungarian Empire now Brno, Czech republicCrosses peas, intercrosses progeny, classifies and counts segregation of traitsPaper formulates the “Laws of Genetics” concerning transmission of genetic informationGregor Mendel (1822–1884)Father of GeneticsDelivers 2 lectures at the Brünn Society for the Study of Natural History (1865) Famous paper, Experiments on Plant Hybrids, published in 1866; widely distributed, but basically unread or not fully understoodThe most famous paper in biology (up to Watson-Crick’s paper on the structure of DNA) was written by a horticulturistHistory of Horticulture: Lecture 334Selected seven lines of peas with different traits tall and dwarf plant, round and wrinkled seeds, yellow or green seed, green or dark pods, smooth or constricted pods, axillary or terminal flowers)In these lines, traits were constant when self pollinated (bred true)Tall × tall (selfed or intercrossed) gave tall progenyDwarf plants (selfed or intercrossed) gave dwarf progenyClassical Genetics (Transmission Genetics)Crossing peas with certain contrasting traits (tall × dwarf) produced progeny with one of the two traits suggesting that one traits dominates the other Thus, when tall is crossed with dwarf (the source of the pollen is immaterial) the progeny was tall Thus, the factor responsible for tallness was dominant over dwarfDwarf is said to be recessive to tallIn the next generation, when the tall hybrids were selfed or intercrossed, the resulting offspring segregated for both traits (either tall plants or dwarf plants) The ratio of the two classes was predictable:3 plants with the dominant trait (tallness) and one plant of the recessive trait (dwarf)The recessive plants bred true when selfedBut the plants with the dominant trait (tall) show two types of segregationOne third of them bred true for tallness and two thirds produced a 3:1 ratio of tall to dwarfHistory of Horticulture: Lecture 335Furthermore, when the tall hybrid plant was crossed to the tall parent, all the offspring were tallWhen the tall hybrid was crossed to the dwarf parent the offspring segregated in a ratio of 1 tall:1 dwarfSegregation of plants with different traits (say tallness and seed color) segregated independentlyThese results could be explained by assuming each trait was controlled by a controlling element or factor (now known as a gene)These results are explainable assuming true-breeding tall plants contained the gene T in a doubled form (TT) and that true-breeding dwarf plants contained the gene t in a doubled form (tt).The gametes contain only 1 copy of each gene pair.Tall (TT) × dwarf (tt) produced tall hybrids (Tt) because T dominated t.Intercrossing the hybrids (Tt) produced progeny in a ratio of 3 tall to 1 dwarfTt × Tt→ 1 TT : 2 Tt : 1 tt (3 tall : 1 dwarf)In the backcross of hybrids to parentsTt × TT→ 1 TT : 1 Tt (all tall)Tt × tt→ 1 Tt : 1 tt (1 tall : 1 dwarf)History of Horticulture: Lecture 336The “gene” conditioning each trait can have different forms now called “alleles”The mature plants had two alleles; the gametes had only one of each allelesThus, tall plants that bred true were TT;all gametes were TDwarf plants that bred true were tt;all gametes were tThe hybrid was Tt; it was tall indicating that Tdominates t in expression; gametes had T or tWhen hybrids were selfed or crossed (Tt × Tt) the T or t gametes combined at random, the offspring were either TT, tT, Tt, or ttor 1TT : 2Tt : 1tt. This is the F2generationBecause Tt is tall the “phenotypic” ratio was3 tall to 1 dwarfOf the tall progeny, 1/3 (TT) were true breeding for tallness while 2/3 (Tt) segregated in a ratio of 3 tall : 1 dwarfBackcross ratios (F1hybrid × parents)Tt × TT gave all tall progeny; Tt × tt produced 3 tall : 1 dwarfMendelian segregation illustrating incomplete dominance in the four o’clock (Mirabilis jalapa) Note that the phenotypic ratio of 1 red:2 pink:1 white is also 3 colored:1 noncoloredHistory of Horticulture: Lecture 337The independent assortment of two genes in peasWhat Mendel proved was that the traits are produced by factors (genes) that pass from one generation to the otherFurthermore, the factors that control inheritance were not changed by their transmission Factors segregated in predictable patternsIt was inferred that variability was due to the segregation and interaction of different genesNo one paid much attention to Mendel’s paper; although cited, it had no immediate impactHowever, in 1900 three investigators (Hugo DeVries, Carl Correns, and Erich von Tschermak) who
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