HEREDITY 185 



we should expect if the hybrids of the first generation possessed two 

 kinds of sexual products in equal numbers; namely, those of the 

 paternal and those of the maternal species or variety. If a large number 

 of individuals of this kind be crossed, according to the law of probability, 

 in one fourth of the cases an egg cell with the determinant for the reces- 

 sive characteristic and a pollen cell of the same type would meet. This 

 would result in pure offspring with the recessive characteristic. In one 

 fourth of the cases egg cells with the determinant of the dominating 

 characteristic would meet with the pollen of the same type, and the re- 

 sult would be pure offspring with the dominant characteristic. In one 

 fourth of the cases an egg cell with the determinant for the dominant 

 characteristic would be met by pollen cells with the determinant for the 

 recessive characteristic, and this would give rise to individuals with the 

 dominant characteristic ; in the last fourth of the cases an egg cell with 

 the determinant for the recessive characteristic would be met by pollen 

 with the determinant for the dominant characteristic, and this again 

 would give offspring with the dominant characteristic. In this way 

 three fourths of the total offspring in the second generation would have 

 the appearance of the species or variety with the dominant, and one 

 fourth of the total offspring would have the appearance of the species 

 with the recessive, characteristic ; both kinds would therefore be in the 

 numerical relation of one to three, as stated. 



Through many cultures, Mendel has shown that his conclusions are 

 correct. Thus, e.g., his theory demanded that if the last-mentioned 

 experiment be continued, and the individuals of the first-mentioned 

 fourth of the offspring, namely, the ones that have reverted to the 

 recessive character, be bred among themselves, only pure breeds with 

 recessive character should be produced. The experiments proved that 

 this is entirely correct. 



Mendel raised also the question as to what would happen if varieties 

 of peas which differ in regard to two or more characteristics should 

 be hybridized. In this case, the hybrids of the first, as well as of the 

 later, generations behaved as if a specific hereditary substance existed 

 for each characteristic, and as if these substances did not irfluence each 

 other. For each one of the discriminating characteristics, the same laws 

 hold which existed where the varieties differed only in regard to one 

 characteristic. 



The epoch-making importance of Mendel's work lies in the fact that 

 he, for the first time, gave not a hypothesis but a theory of heredity, 

 which made it possible to predict the results of hybridizations numeri- 

 cally. His work forms the basis for all further work in this field which 

 is of equal theoretical and practical importance. 



