6 GENETIC VARIATIONS 



thus exists in the nucleus as a large number of separable 

 particles or genes, grouped into the chromosomes.^ 



These genie materials are extremely active in the life of the 

 organism. From within the nucleus the genes send out prod- 

 ucts of their action into the remainder of the cell — the cyto- 

 plasm. Here the genie products interact with each other and 

 with the cytoplasm itself, so becoming modified in many 

 ways, and producing many different things. In this way they 

 produce the various tissues of the body, and give rise to its 

 structural and physiological peculiarities; to what we call its 

 characteristics. 



But of each of these genie materials there is retained an 

 unmodified part, a reserve portion, enclosed within the 

 nucleus. It is these unmodified reserves that are called the 

 genes. When cell division occurs, each of these unmodified 

 reserves, or genes, divides, so that each of the cells produced 

 receives a half of each gene; this grows again to form an 

 entire gene. Thus every cell of the body has the entire stock 

 of reserve materials or genes. 



Genie materials therefore have two most important prop- 

 erties. First, they affect development, and thus help to deter- 

 mine the characteristics of the individual produced. And 

 second, each genie material assimilates, grows, and repro- 

 duces true to type; each gene, in other words, produces addi- 

 tional material of the same kind as itself. 



Half of the genie materials within the nucleus come to the 

 fertilized egg from one of the two parents, half from the other 

 parent. They are built up in the nucleus into a complex 

 structural system, just as the nervous material is built up into 

 a complex nervous system. This system of genie materials we 

 may conveniently call the genetic system. Its peculiarities play 

 a great role in determining the nature of genetic variations, so 

 that we must examine them. 



