230 Mechanism of Mendelian Heredity 



character only but also if they differ for two or more 

 characters. The statement made is Mendel's law of 

 heredity, or, more correctly, MendeFs law of the 

 segregation of the hereditary characters of the parents 

 in the sex cells of the hybrids.' Mendel's law allows 

 us to tabulate and calculate beforehand the relative 

 number of different forms which appear if the offspring 

 of a mating of two varieties are bred among themselves. 

 In order to do this it must be remembered also that 

 while in some cases the hybrid is an intermediate 

 between the two parent forms, in other cases it can- 

 not be discriminated from one of the two parent forms. 

 In such cases the character which appears in the 

 hybrid was called by Mendel the dominant character 

 and the one which disappeared the recessive character. 

 According to Bateson, who was the first to systematize 

 the phenomena of Mendelian heredity, recessiveness 

 means generally the absence of a character which is 

 present in the dominant type. When, e, g.j the cross 

 between a tall and a dwarf form of pea gives in the 

 first generation only tall peas, on the basis of the 

 presence and absence theory the dominant form con- 

 tains a factor for growth which is lacking in the dwarf 

 form. While this theory fits many cases it meets with 

 difficulties in others. Thus the presence of a factor 



' Mendel, G., "Experiment in Plant-Hybridization," translated in 

 W. Bateson's classical book on MendeVs Principles of Heredity. 

 Cambridge, 1909. 



