2 CELL HEREDITY 



expanded to handle more complex patterns of inheritance. From in- 

 formation obtained in this way, geneticists have identified a special class 

 of cell constituents — the hereditary determinants, or genes. Genes are 

 the units of inheritance with whose properties the science of genetics is 

 concerned. 



Genes have not been directly observed as yet, although recent experi- 

 ments to be described in this book are bringing this possibility closer and 

 closer to realization. However, the science of genetics was built by in- 

 ference and prediction about genes from experiments in which the unit of 

 observation was not the gene itself but its effect upon the organism. 

 Two important terms were designed to keep clear the distinction between 

 the postulated genes and their observed effects. As first proposed by the 

 Danish geneticist Johanssen in 1911, the genotype is the sum total of the 

 hereditary determinants or genes present in an organism and the pheno- 

 type is the sum total of observed characteristics of the organism. It is 

 the phenotype which is observed in breeding experiments, and inferences 

 about genes are made from the segregation of phenotypic differences. 



The phenotype is influenced by all the hereditary determinants and by 

 their interactions with environment as well. Consequently, the detection 

 of single phenotypic differences resulting from single gene differences can 

 be very difficult, and the choice of favorable experimental material is of 

 key importance. The science of genetics began with studies of complex 

 organisms such as the pea, the fruitfly, the corn plant, and the house 

 mouse. Observations were directed at traits — taillessness in mice for 

 example — far removed from the intracellular level where the genes are 

 located and where they exert their primary effects. The creation of a 

 powerful analytical science by inference from the results of these highly 

 indirect experiments represents a great intellectual achievement of the 

 pioneer geneticists. However, both the achievements and the limitations 

 of classical genetics derive from the same source: the remoteness of the 

 experimental observations from the primary actions of the hereditary 

 materials themselves. 



In an attempt to bridge this gap, some investigators began to employ 

 single-celled organisms such as bacteria, yeast, algae, fungi, and pro- 

 tozoa. By avoiding the complexities of tissue differentiation and integra- 

 tion, they were able to concentrate fully upon events at the cellular level. 

 In these systems the parent-progeny relation may be either sexual, in- 

 volving offspring derived from mating of two parents, or asexual, involving 

 daughter cells produced by fission of a single parental cell. 



The cellular approach has made possible a great refinement in concept 

 and in method, so that more precise questions can be asked and more 

 definitive experiments performed. The direction of advance has been 



