What traits to pick The reaffirmation of a known phenomenon 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 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 http www mendelweb org CollText homepage html MCB140 01 22 07 1 Qualitative simple trait MCB140 01 22 07 2 Quantitative complex trait AA aa AA yellow green short MCB140 01 22 07 3 aa tall MCB140 01 22 07 4 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 01 22 07 5 http www mendelweb org CollText homepage html MCB140 01 22 07 6 1 MCB140 01 22 07 7 Inverting the direction of the cross does not alter the phenotype of the hybrid MCB140 01 22 07 8 What to do with the hybrid i e the F1 plants 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 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 9 MCB140 01 22 07 10 The data A bit of metaanalysis Expt 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 seed coats giving the proportion 3 15 1 Expt 4 Form of pods Of 1181 plants 882 had them simply inflated and in 299 they were constricted Resulting ratio 2 95 1 Expt 5 Color of the unripe pods The number of trial plants was 580 of which 428 had green pods and 152 yellow ones Consequently these stand in the ratio of 2 82 1 Expt 6 Position of flowers Among 858 cases 651 had inflorescences axial and 207 terminal Ratio 3 14 1 Expt 7 Length of stem Out of 1064 plants in 787 cases the stem was long and in 277 short Hence a mutual ratio of 2 84 1 In this experiment the dwarfed plants were carefully lifted and transferred to a special bed This precaution was necessary as otherwise they would have perished through being overgrown by their tall relatives Even in their quite young state they can be easily picked out by their compact growth and thick dark green foliage In this generation there reappear together with the dominant characters also the recessive ones with their peculiarities fully developed
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