What were these principles? Before we answer that question let us identifywhat we regard as the principles of the science we know as genetics. Clearlythe experimental approach is that of cross-breeding, but this method as usedby the practical breeder is not enough. The forms crossed must be of provenconstancy of type, the character differences need to be recorded for allparents and offspring so that their hereditary transmission can be followedfrom generation to generation, and the numbers of offspring raised need to besufficient to yield statistically significant results. For anyone who has readMendel's paper of 1865 there is no question that Mendel did enunciate theseprinciples. Over the years 1856 to 1861 he raised some 10,000 plants ofvarieties of the edible pea (Pisum sativum) which he had tested forpurity of type over a two year period. He followed the transmission of sevenspecially selected traits - round-seeded plants with wrinkled-seeded plants,tall plants with short, green-seeded with yellow-seeded, etc. Heself-fertilized their progeny to discover if they returned or reverted to oneor other of their originating forms. He found that they did, but in a quiteremarkable manner. When he surveyed the total number of hybrid offspringcarrying one or other of the contrasted characters - tall or short, etc. - theyshowed an integral relationship, one contrasted character being present inthree times as many plants as the other. This 3 : 1 ratio [and its resolutioninto the three-fold ratio 1 : 2 : 1] formed the key to the genetic analysis ofall hereditary traits conforming to Mendelian heredity.


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