Brilliant in Br nn part II 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 If it chance that an egg cell unites with a dissimilar pollen cell we must then assume that between those elements of both cells which determine opposite characters some sort of compromise is effected The resulting compound cell becomes the foundation of the hybrid organism the development of which necessarily follows a different scheme from that obtaining in each of the two original species 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 also occurs a compromise in so far that the formation of a cell as the foundation of the hybrid becomes possible but nevertheless the arrangement between the conflicting elements is only temporary and does not endure throughout the life of the hybrid plant Since in the habit of the plant no changes are perceptible during the whole period of vegetation we must further assume that it is only possible for the differentiating elements to liberate themselves from the enforced union when the fertilizing cells are developed In the formation of these cells all existing elements participate in an entirely free and equal arrangement by which it is only the differentiating ones which mutually separate themselves In this way the production would be rendered possible of as many sorts of egg and pollen cells as there are combinations possible of the formative elements Brilliant isn t strong enough This is swish central Nothing but net Mendel s data showed to him that in a heterozygote the two alleles A and a remain DISTINCT and SEPARATE They reach a compromise for the life of the plant but then during gametogenesis they go their separate ways unchanged To describe this incredibly simple idea as influential would be akin to calling Michael Jordan a pretty good shooting guard Sadly this idea sat on the bench for the entire 1865 1900 season More on why that happened shortly MCB140 01 23 07 1 Charles Darwin 1859 The Origin of Species by Means of Natural Selection 1 2 3 4 MCB140 01 23 07 2 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 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 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 01 23 07 3 Sergei Chetverikov 1926 MCB140 01 23 07 4 Archibald Garrod 1902 On certain aspects of the evolutionary process from the standpoint of modern genetics mutational load 1 Trap 239 Drosophila melanogaster near Moscow 2 Self their offspring brother sister 3 32 recessive loci 186 in humans A species like a sponge soaks up heterozygous mutations while remaining phenotypically homozygous Higher frequency of children with alkaptonuria urine turns dark on standing and alkalinization from consanguineous marriages Why There is no reason to suppose that mere consanguinity of parents can originate such a condition as alkaptonuria in their offspring and we must rather seek an explanation in some peculiarity of the parents which may remain latent for generations MCB140 01 23 07 5 http www esp org foundations genetics classical ag 02 pdf MCB140 01 23 07 6 1 Ah It has recently been pointed out by Bateson that the law of heredity discovered by Mendel offers a reasonable account of such phenomena Garrod 1902 Lancet 2 116 MCB140 01 23 07 7 Garrod 1902 Lancet 2 116 MCB140 01 23 07 8 A useful term An awful awful term monogenic trait If a trait follows in its inheritance Mendel s first law that phenomenon is described as simple Mendelian inheritance SMI Examples cystic fibrosis sickle cell anemia hemophilia A If a trait follows SMI what does that say about its genetic architecture A highly pernicious school of lack of thought in biological instruction uses the term monogenic trait It is most unfortunate MCB140 01 23 07 9 MCB140 01 23 07 10 MCB140 01 23 07 11 MCB140 01 23 07 12 2 MCB140 01 23 07 13 MCB140 01 23 07 14 In what sense is blood clotting a monogenic trait X In no sense at all What is simple is the GENETIC difference between an unaffected individual and an individual who has hemophilia It can be as small as a single base pair change In other words what the phenotype tracking allows us to do is look at the genetics of the difference If a particular phenotype follows SMI then all that says is The difference in genotype between an organism with that phenotype and without it is due to a genetic difference at a single position in the genome a single locus Nothing can be learned from this analysis about the number of genes that are required for that phenotype to develop or about the role this particular gene plays in having this phenotype develop PKU for example has to do with the breakdown of aromatic amino acids and its lack causes mental retardation What is the specific role of phenylalanine breakdown in cognition MCB140 01 23 07 15 MCB140 01 23 07 16 Manhood a monogenic trait Note SRY directly causes the conversion of a female embryo to a male one XY XX Sry transgene Motherhood a monogenic trait A Defect in Nurturing in Mice Lacking the Immediate Early Gene fosB Brown et al Cell Vol 86 297 309 July 1996 Koopman et al 1991 Nature 351 117 Prof Cline lecture 25 MCB140 01 23 07 17 MCB140 01 23 07 18 3 MCB140 01 23 07 19 In other words
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