The starting material Gregor Mendel Versuche ber Pflanzenhybriden The First Law MCB140 29 08 07 1 In all 34 more or less distinct varieties of Peas were obtained from several seedsmen and subjected to a two year s trial All the varieties yielded perfectly constant and similar offspring at any rate no essential difference was observed during two trial years For fertilization 22 of these were selected and cultivated during the whole period of the experiments They remained constant without any exception http www mendelweb org CollText homepage html MCB140 29 08 07 2 An SCN9A channelopathy causes congenital inability to experience pain Nature Dec 14 2006 Pure breeding line An awkward phrase that is best retired but never will be It refers to an organism that exhibits a particular trait e g seed color and all progeny of that organism whether it is selfed or outcrossed to another such organism also exhibit that trait Pure breeding lines are best made by selfing or brother sister crosses like Nefertiti The index case for the present study was a tenyear old child well known to the medical service after regularly performing street theatre He placed knives through his arms and walked on burning coals but experienced no pain He died before being seen on his fourteenth birthday after jumping off a house roof MCB140 29 08 07 3 MCB140 29 08 07 4 William Ernest Castle founder of mouse genetics UCB 1936 1962 Mendel picked the pea as the system What traits to pick 1 Inbreeding as a tool for making genetically uniform strains of mice that are homozygous for every allele in the genome 2 Brother sister matings makes 12 5 of all loci in the genome homozygous Clarence Little Experiments which in previous years were made with ornamental plants have already provided 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 Why homework After 40 generations of brother sister mating 99 98 of genome is homozygous By F60 mice are considered genetically identical to one another MCB140 29 08 07 5 http www mendelweb org CollText homepage html MCB140 29 08 07 6 1 The reaffirmation of a known phenomenon Mendel 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 29 08 07 7 Qualitative simple trait MCB140 29 08 07 8 Quantitative complex trait AA aa AA yellow green short MCB140 29 08 07 9 aa tall MCB140 29 08 07 10 Not Mendel s fault The genesis of the famous term In 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 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 unver ndert in die Hybride Verbindung bergehen somit selbst die Hybriden Merkmale repr sentiren als dominirende und jene welche in der Verbindung latent werden als recessive bezeichnet Der Ausdruck recessiv wurde desshalb gew hlt weil die damit benannten Merkmale an den Hybriden zur cktreten oder ganz verschwinden jedoch unter den Nachkommen derselben wie sp ter gezeigt wird wieder unver ndert zum Vorscheine kommen MCB140 29 08 07 11 http www mendelweb org CollT ext homepage html MCB140 29 08 07 12 2 Inverting 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
View Full Document
Unlocking...