What traits to pick?The reaffirmation of a known phenomenonQualitative (“simple”) traitQuantitative (“complex”) traitNot Mendel’s faultThe genesis of the famous termSlide 7Slide 8Inverting the direction of the cross does not alter the phenotype of the hybridWhat to do with the hybrid (i.e., the F1 plants)?The dataA bit of metaanalysisBrilliant in BrünnAnalysis of the grandchildrenSlide 15Mendel, humble son of a peasant family, pollinating textbooks all over the world for the rest of time with his nomenclatureAnd now (drum roll) – the first lawSlide 18Slide 19Slide 20Slide 21Slide 22Brilliant in Brünn, part IIA useful termSlide 25Slide 26Slide 27An awful, awful term: “monogenic trait”Slide 29In other wordsMCB140 01-22-07 1What traits to pick?“Experiments which in previous years were made with ornamental plants have already affording evidence that the hybrids, as a rule, are not exactly intermediate between the parental species. With some of the more striking characters, those, for instance, which relate to the form and size of the leaves, the pubescence of the several parts, etc., the intermediate, indeed, is nearly always to be seen; in other cases, however, one of the two parental characters is so preponderant that it is difficult, or quite impossible, to detect the other in the hybrid.”http://www.mendelweb.org/CollText/homepage.htmlMCB140 01-22-07 2The reaffirmation of a known phenomenonMendel is pointing out the distinction between two “types” of traits.1. The hybrid plant is “intermediate” in phenotype between two parents. For instance, the offspring of a tall and a short plant would be intermediate in height.2. The hybrid plant has the phenotype like one of the parents. For instance a green x yellow cross yields only yellow-seeded plants.Mendel chose to study “type 2 traits” – a judicious decision. We now know that the laws he discovered in doing so also apply to “type 1” traits, but that fact is considerably more difficult to observe.MCB140 01-22-07 3yellow greenQualitative (“simple”) traitaaAAMCB140 01-22-07 4short tallQuantitative (“complex”) traitAAaaMCB140 01-22-07 5Not Mendel’s faultIn retrospect, we see that the overwhelming majority of traits in humans, other animals, and plants – traits that are of most interest and importance from a public health, and other societally relevant perspectives (height, weight, body plan, facial appearance, skin color) – are quantitative. Mendel – wisely – chose to study a set of “qualitative” traits because he was a skilled reductionist. As a consequence, he discovered two fundamental facts about the functioning of the genetic material. The teaching of genetics, however, always begins with Mendel’s work, and this creates two erroneous impressions:1. … that the traits he studied are “controlled by a single gene.” That, of course, is not true (all traits are controlled by multiple genes) – he simply worked with plants that were genetically different from each at only one locus out of the many required for the development of the trait. We shall return to that point shortly.2. … that simple Mendelian relationships of recessivity and dominance between alleles, and “one gene-one trait” correlations he – supposedly – observed are ubiquitous in Nature. “She has her mother’s eyes.” “He gets his brains from his Dad.” Neither trait – eye color nor “intelligence” – exhibit simple Mendelian inheritance, yet most people assume otherwise.MCB140 01-22-07 6The genesis of the famous term“… in other cases, however, one of the two parental characters is so preponderant that it is difficult, or quite impossible, to detect the other in the hybrid.This is precisely the case with the Pea hybrids. In the case of each of the 7 crosses the hybrid-character resembles that of one of the parental forms so closely that the other either escapes observation completely or cannot be detected with certainty. This circumstance is of great importance in the determination and classification of the forms under which the offspring of the hybrids appear. Henceforth in this paper those characters which are transmitted entire, or almost unchanged in the hybridization, and therefore in themselves constitute the characters of the hybrid, are termed the dominant, and those which become latent in the process recessive. The expression "recessive" has been chosen because the characters thereby designated recede or entirely disappear in the hybrids, but nevertheless reappear unchanged in their progeny, as will be demonstrated later on.”“In der weiteren Besprechung werden jene Merkmale, welche ganz oder fast unverändert in die Hybride-Verbindung übergehen, somit selbst die Hybriden-Merkmale repräsentiren, als dominirende, und jene, welche in der Verbindung latent werden, als recessive bezeichnet. Der Ausdruck "recessiv" wurde desshalb gewählt, weil die damit benannten Merkmale an den Hybriden zurücktreten oder ganz verschwinden, jedoch unter den Nachkommen derselben, wie später gezeigt wird, wieder unverändert zum Vorscheine kommen.”http://www.mendelweb.org/CollText/homepage.htmlMCB140 01-22-07 7MCB140 01-22-07 8MCB140 01-22-07 9Inverting the direction of the cross does not alter the phenotype of the hybrid“All experiments proved further that it is entirely immaterial whether the dominating trait belongs to the seed plant or to the pollen plant; the form of the hybrid remains identical in both cases.”Recall Leeuwenhoek’s “proof” using grey rabbits that sperm provides all the genetic material, and the egg solely provides nourishment.Eeeh, what’s up with THAT, doc?MCB140 01-22-07 10What to do with the hybrid (i.e., the F1 plants)?Mendel decided to let them self (which Pisum sativum does naturally, thank you very much). He then grew the progeny (the F2) and did precisely what he promised: he counted the number of phenotypic classes in this F2, and measured the ratio.MCB140 01-22-07 11The dataExpt. 1. Form of seed. -- From 253 hybrids 7324 seeds were obtained in the second trial year. Among them were 5474 round or roundish ones and 1850 angular wrinkled ones. Therefrom the ratio 2.96:1 is deduced. Expt. 2. Color of albumen. -- 258 plants yielded 8023 seeds, 6022 yellow, and 2001 green; their ratio, therefore, is as 3.01:1. Expt. 3. Color of the seed-coats. -- Among 929 plants, 705 bore violet-red flowers and gray-brown seed-coats; 224 had white flowers and white
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