Mendel anticipated by thirty-five years the 

 practical establishment of the art of breed- 

 ing new species of plants and animals, 

 kinds that had never existed in nature. It 

 is commonly assumed that an organism 

 transmits its distinctiveness entirely, or else 

 not at all. This means that a Jersey cow, 

 for example, transmits to her offspring all 

 her "jerseyness" or that a cherry transmits 

 all its "burbankness" — or none. Mendel 

 analyzed the "character" of a strain into its 

 many separate qualities. His basic scien- 

 tific and practical contribution was the 

 working out experimentally of a method 

 for ascertaining the essential facts as to 

 just what is inherited — just what particular 

 characteristics appear in successive gen- 

 erations, what ones fail to appear, what 

 ones reappear later 



GREGOR MENDEL (1822-1884) 



Historical Pictures Service 



between the corresponding characters of the parents, but partly in having 

 some characters just Uke those of the mother and other characters just like 

 those of the father (see illustration, p. 476). 



The results which Mendel obtained he generalized as the law oi dominance. 

 His idea was that where the two "factors" causing the contrasting characters 

 meet in an individual, one of them dominates over, or masks, the other one, 

 which Mendel called the recessive. The recessive is not destroyed, as we shall 

 see. Of course it is impossible to tell in advance which of two characters in a 

 contrasting pair will be dominant and which will be recessive. The cross has 

 to be tried out (see the tables on pages 480 and 481). 



The Law of Segregation^ The yellow seeds of a hybrid plant are not 

 distinguishable from the yellow seeds of the pure yellow-seeded parent — just 

 as you cannot tell whether a brown-eyed person has two brown-eyed parents 

 or only one. With plants grown from hybrid yellow seeds, Mendel carried 

 out three classes of cross-pollenation (see illustration, p. 477): (1) He crossed 

 hybrids with plants of the parent (pure) yellow-seeded variety. (2) He 

 crossed hybrids with plants of the parent (pure) green-seeded variety. (3) He 

 crossed yellow-seeded hybrids with yellow-seeded hybrids. 



From these experiments, which have now been made with hundreds of 

 species of plants and animals, it is seen that the hybrid does not reproduce itself 

 in offspring having uniform characteristics. Some of the offspring resemble the 

 grandmother's type, and some the grandfather's type. This general fact of 

 "splitting" is called the law oi segregation. It agrees with the past experience 



iSee No. 3, p. 503. 

 475 



