270 BIOLOGY: GENERAL AND MEDICAL 



The accompanying diagram will assist the reader in 

 forming a concept of these allelomorphs, their occur- 

 rence in pairs, their possible separation, and their appear- 

 ance in new combinations. 



It is difficult to find a satisfactory concise definition of 

 what is known as Mendel's law. Indeed, he did not 

 formulate it in words himself. 



The facts upon which the law is based are displayed 

 in the following tabulation taken from Mendel's own 

 paper, and show that interbreeding among hybrids 

 results in the progressive separation of the combined 

 characters and in increasing number of reversions to the 

 pure specific types. Before the tabulation can be under- 

 stood, however, it becomes necessary to say a few words 

 about the terms dominant and recessive as applied to 

 the Mendelian characters. The characters, which it 

 will be remembered go in pairs, are opposed to one another 

 in quality and take precedence over one another in 

 character. Thus, whiteness and blackness are opposed 

 Mendelian characters, of which blackness takes prece- 

 dence over the whiteness, and is, therefore, dominant; i.e., 

 whenever blackness is present it can be seen whether 

 there be some whiteness in the individual or not, but 

 whiteness may be present and completely concealed by 

 blackness. Mendel's tabulation of what happens when 

 only two sets of characters are concerned is shown thus: 



A. Dominant character; a, recessive character; Aa, 

 hybrid. For brevity's sake it is assumed that each 

 plant produces only four seeds (peas). 



Generation A Aa a Ratio 



2 1 : 2 n : 21 



