The progress of Genetics since the rediscovery of Mendel's papers. 



369 



A simple demonstration will illustrate this. If we imagine that 

 blackness and whiteness are an allelomorphic pair we may represent 

 a pure black parent by two black draughts, and a pure white parent 

 by two white ones (Fig. 1). The parent individuals may each be 

 fitly represented by two draughts, for it is well known that the somatic, 

 or zygote-cells as they may be called, are double structures. The 

 germ-cells however are single structures as judged by the constitution 

 of their nuclei. Consequently the germ-cells or gametes of the black 

 type may be represented by single black draughts, and those of the 

 white type by single white ones. The cross between the two produces 

 an individual represented by one black and one white draught. For 

 brevity this first cross-bred generation raised by the union of two 

 pure types is called Fj, the first filial generation. 



Fid. I 



F ^ 



»2 R 



jRatio 



RR 



ID 

 IR 



2DR 



3D 



IDD 



X 



all D 



ri0 3. 



Figs. 1 — 3. Diagrams showing the numerical consequences of Mendelian 



segregation. 



As each germ-cell carries either the one allelomorph or the other 

 — e. g., either blackness or whiteness, as in our diagram — there 

 are only three possible sorts of individuals or zygotes (1) those that 

 are pure to blackness (2), those that are pure to whiteness, and (3) those 

 that contain both characters. The pure types are spoken of as homo- 

 sygmiS] the impure or hybrids are called heterozygous. 



24* 



