306 EVOLUTION AND GENETICS 



the literature of genetics and evolution. It has been shown that 

 the course of development after fertilization results in the early 

 separation of those cells which develop into the body proper and 

 those which remain undifferentiated for the production of new 

 germ cells. In Ascaris, for example, it has been found that one 

 cell of the sixteen-cell stage develops into germ cells alone, while 

 the remaining fifteen develop into the body proper. Within the 

 germ cells the genes never gain expression; in the differentiated 

 cells of the body they reach their fullest expression. 



The body has been termed the soma or somatoplasm, and the 

 germ cells are commonly called the germplasm. It is obvious that 

 in all cases of sexual reproduction the former originates anew 

 with every generation from the latter while the germplasm con- 

 tinues without evident change from generation to generation. 

 The association of inherited genes with their respective characters 

 is akin to this relation of soma and germplasm. Whether or not 

 those transmitted to the next generation may have been influenced 

 by the generation which has carried and nurtured them is one 

 of the great unsettled problems of heredity. Mechanism for such 

 influence has not been demonstrated, but neither has the lack 

 of such mechanism. 



Unit Characters and Their Factors. The behaviour of allelo- 

 morphic characters in hybrids indicates that each is represented 

 by at least one factor. Evidently then a given unit character in a 

 homozygous individual is also represented by two genes which 

 segregate in the germ cells with the synaptic mates containing 

 them. 



Complication of Mendelian ratios has disclosed several other 

 relationships which are valuable both from the practical and 

 the purely scientific points of view. These include linkage and 

 sex-linkage, crossing over, multiple allelomorphs and multiple 

 genes. 



Linkage. Different unit characters, when combined in hybrids, 

 usuall}^ separate from each other in varying combinations in the 

 F2 generation. Thus the two characters of shape and color in 

 Mendel's peas, with their allelomorphs, give a 9:3:3: 1 dihybrid 

 ratio. Occasionally however two such characters remain nor- 

 mally associated and give only the monohybrid 3: 1 ratio. This 

 phenomenon was briefly mentioned in Chapter XV. It is called 

 linkage. 



