244 BIOLOGY: GENERAL END MEDICAL 



principle of much importance in studying the problems 

 of heredity. 



The disposition of these characteristics traceable 

 from either parent into the hybrid shows that each 

 hybrid is not, so to speak, composited of two factors, 

 but compounded of many units, which must occur as 

 such in the germ plasm. To such of these units as are 

 found to undergo segregation in the germ plasm of the 

 hybrid, and consequent separation in its offspring, Men- 

 del has given the name Allelomorphs. The allelomorphic 

 units are found to go in pairs, the individuals being 

 separable and therefore capable of varying combinations. 

 Germinal cells in which the allelomorphic units are of 

 the same kind are said to be homozygous; those in which 

 they are of opposed kinds, heterozygous. 



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, seeing that 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 



