no Heredity and Eugenics 



ducing a slight tendency toward a sessile leaf. Factors B 

 and C may ultimately arise as modifications of factor A. 

 Each alone may produce the same result as A; yet if they 

 are transmitted independently, are not allelomorphic to 

 each other and are cumulative in their action, a sessile leaf — 

 a new character — is produced. 



The remainder of this chapter will be devoted to a con- 

 sideration of Johannsen's genotype conception of heredity. 

 It may seem as if Mendelism has been dropped abruptly to 

 take up a new subject. This is not the case. The genotype 

 conception of heredity is merely an acknowledgment of the 

 universality of Mendelism. 



Johannsen, who developed the genotype idea, found 

 that some variations were not inherited. These were the 

 general variations in relative perfection of development of 

 parts, caused by varying physical and chemical conditions 

 of environment. This is only an acceptance of Weismann's 

 theory that inheritance is from germ cell to germ cell, and 

 that ordinary environmental influences affecting the body 

 only are not transmissible. Of course both Weismann and 

 Johannsen acknowledge that certain changes in environ- 

 ment may produce structural modification of the germ 

 plasm and therefore a heritable variation, but whether the 

 heritable variation produced is ever identical with the 

 adaptive response of the parent organism is still in dispute. 



If this contention be true, it follows that the hereditary 

 characters of an organism are determined by the consti- 

 tution of the fertilized egg from which it came. Johannsen 

 denotes the sum total of the gametic factors making up a 

 zygote by the word genotype. If two individuals possess 

 identical gametic factors, they are members of the same 

 genotype. Of course no one can describe a genotype in 



