62 The Source of Variability 



tion of the genetic determinants and the mechanisms by which 

 they change and cause character change. 



THE GENETIC MECHANISM 



Typically all organisms begin as single cells. This is obviously true 

 of single-celled organisms such as Amoeba, and is equally true of 

 the multicellular organisms. Oak trees and horses alike start from 

 a zygote, the single cell resulting from two gametes. Incredible 

 though it seems, the single cell initiating a new generation of the 

 most complex organism carries within it a physico-chemical code 

 of some sort which sets in operation and maintains a precise set 

 of complex reactions resulting in a daughter organism essentially 

 like the parent. If an alternation of generations occurs in the life 

 history, the coded information in the spores or eggs originates the 

 series of physiological reactions that produce the members of the 

 life cycle in their correct order. These happenings always presup- 

 pose an environment suitable for the existence of the organism. 

 New types of organisms, or genetic variants, presumably arise be- 

 cause changes occur in the coded information called the genetic 

 material or genome. 



According to present knowledge the great proportion of this 

 genetic material occurs in the chromosomes, situated in the nucleus. 

 A great volume of published information chronicles the existence 

 of literally thousands of genetic determinants or genes on the 

 chromosomes and explains the manner and proportions in which 

 the various determinants are passed on from one generation to the 

 next. A lesser proportion of genetic material is apparently located 

 outside the chromosomes, and this proportion is classed as cyto- 

 plasmic inheritance. The units of cytoplasmic inheritance are some- 

 times called plasmagenes but more frequently are simply called 

 genes. Relatively few instances of cytoplasmic inheritance have 

 been established, but this may be due more to the pioneering state 

 of the field than to the existence of only a small number of cyto- 

 plasmic genes. 



Chromosomal Inheritance 



The chromosome number varies from 2 in some plants of the 

 family Compositae (Jackson, 1957) to over a hundred in some 

 animals (White, 1954) and over 200 in some plants (Manton, 

 1950). 



Chromosomes are relatively similar in structure and behavior 



