MCB140 9 3 08 1 Gregor Mendel Versuche ber Pflanzenhybriden The Second Law MCB140 9 3 08 2 Black box MCB140 9 3 08 3 The offspring of hybrids in which several differing traits are associated In the experiments above described plants were used which differed only on one essential character The next task consisted in ascertaining whether the law of development discovered in these applied to each pair of differentiating characters when several diverse characters are united in the hybrid by crossing dihybrid cross MCB140 9 3 08 4 Two experiments were made with a considerable number of plants In the first experiment the parental plants differed in the form of the seed and in the color of the albumen in the second in the form of the seed in the color of the albumen and in the color of the seed coats Experiments with seed characters give the result in the simplest and most certain way In order to facilitate study of the data in these experiments the different characters of the seed plant will be indicated by A B C those of the pollen plant by a b c and the hybrid forms of the characters by Aa Bb and Cc MCB140 9 3 08 5 MCB140 9 3 08 6 MCB140 9 3 08 7 2 11 MCB140 9 3 08 8 Ta daaa The second law There is therefore no doubt that for all of the traits involved in the experiments this statement is valid the offspring of the hybrids in which several essentially different characters are combined exhibit the terms of a series of combinations in which the developmental series for each pair of differentiating traits are combined It is demonstrated at the same time that the relation of each pair of different traits in hybrid union is independent of the other differences in the two parental plants Es unterliegt daher keinem Zweifel dass f r s mmtliche in die Versuche aufgenommenen Merkmale der Satz Giltigkeit habe die Nachkommen der Hybriden in welchen mehrere wesentlich verschiedene Merkmale vereinigt sind stellen die Glieder einer Combinationsreihe vor in welchen die Entwicklungsreihen f r je zwei differirende Merkmale verbunden sind Damit ist zugleich erwiesen dass das Verhalten je zweier differirender Merkmale in Hybrider Verbindung unabh ngig ist von den anderweitigen Unterschieden an den beiden Stammpflanzen MCB140 9 3 08 9 Mendel s laws 1 2 For any given autosomal locus a diploid organism makes an equal number of gametes carrying one allele and the other allele the two alleles segregate equally to the gametes law of equal segregation For example an organism heterozygous for a given locus Aa will make 50 A and 50 a gametes Mendel s first law holds for each locus if we study two separate unlinked autosomal loci The alleles at each locus sort themselves to the gametes according to Mendel s first law independently of the way alleles for the other locus segregate law of independent assortment For example an organism doubleheterozygous AaBb will make AB aB Ab and ab gametes in equal proportion because the A locus and the B locus each obey Mendel s first law without regard for what alleles for the other locus are doing MCB140 9 3 08 10 Gasp Even the validity of the law formulated for Pisum requires still to be confirmed and a repetition of the more important experiments is therefore desirable In the meantime we may assume that no basic difference could exist in important matters since unity in the developmental plan of organic life is beyond question Indessen d rfte man vermuthen dass in wichtigen Puncten eine principielle Verschiedenheit nicht vorkommen k nne da die Einheit im Entwicklungsplane des organischen Lebens ausser Frage steht MCB140 9 3 08 11 Gregor Mendel to Carl N geli Dec 31 1866 Highly esteemed Sir The acknowledged preeminence your Honor enjoys in the detection and classification of wild growing plant hybrids makes it my agreeable duty to submit for your kind consideration the description of some experiments in artificial fertilization MCB140 9 3 08 12 Gregor Mendel to Carl N geli Dec 31 1866 I am not surprised to hear your honor speak of my experiments with mistrustful caution I knew that the results I obtained were not easily compatible with our contemporary scientific knowledge and that under the circumstances publication of one such isolated experiment was doubly dangerous dangerous for the experimenter and for the cause he represented Thus I made every effort to verity with other plants the results obtained with Pisum MCB140 9 3 08 13 Apomixis In botany apomixis is asexual reproduction without fertilization and modified meiosis The modified meiosis yields seeds that are genetically identical to the one of the parental plants In zoology the cognate phenomenon is known as parthenogenesis MCB140 9 3 08 14 MCB140 9 3 08 15 Charles Darwin 1859 The Origin of Species by Means of Natural Selection 1 2 3 4 Living organisms multiply resources are limited Organisms vary Some variation affects survival and reproduction Like begets like Populations of organisms will evolve those organisms with characteristics most favourable for survival and reproduction will not only have more offspring but will pass their characteristics onto those offspring the characteristics seen in the population will change heritable change in animals selection by environment adaptation to environment MCB140 9 3 08 16 The problem with step 2 What was missing from Darwin s theory was a source for the variations on which natural selection acted Darwin addressed the problem of heredity He believed that smalled units which he called gemmules were produced by the cells and them migrated through the body and some of them would be retained in the gonads EA Carlson Mendel s Legacy The most unfortunate of the assumptions underlying Darwin s mechanism of evolution was that of blending heredity i e that parental differences are merged in the offspring of bisexual reproduction so that variation is constantly being diminished The basis for this assumption wsa the so called intermediacy of hybrids which Koelreuter regarded as a law for all true hybrids R Olby Origins of Mendelism Copyright 2006 by Sidney Harris MCB140 9 3 08 17 Charles talk to Gregor PLEASE pretty please With Pisum it was shown by experiment that the hybrids form egg and pollen cells of different kinds and that herein lies the reason of the variability of their offspring With regard to those hybrids whose progeny is variable we may perhaps assume that between the differentiating elements of the egg and pollen cells there occurs a compromise but nevertheless the
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