2 Genetics, Cells, and Chromosomes 



sidered purely genetical. AVhen the physical basis of genetic 

 phenomena was realized numerous studies were undertaken using 

 the methods of both genetics and cytology and correlating data 

 obtained by genetic procedures with observations determined by 

 cytological techniques. This dual approach to the problems of 

 heredity has given us the term cytogenetics, a term which em- 

 phasizes the correlation of information obtained by the two 

 diverse techniques. Many of the methods of cytogenetics make 

 use of chromosomal aberrations, for it is by an intense study 

 of exceptional chromosomal behavior that we obtain our best 

 information in regard to the normal conditions. Although cyto- 

 genetics is frequently concerned with aberrations, che term is a 

 broad one and includes all situations in which data from cytology 

 and genetics are studied with reference to each other. 



A study of the chromosomes and of their behavior in related 

 species and genera has sometimes aided in a better understanding 

 of the evolutionary relationships of taxonomic groups. Many 

 difficult problems of classification and of relationships have been 

 clarified in whole or in part by supplementing taxonomic studies 

 with those of chromosomal cytology. A study of phylogenetic 

 relationships by the methods of both systematic botany or 

 zoology and chromosomal cytology is sometimes called cyto- 

 taxonomy. 



Resting Cells 



As part of the biological background for a study of heredity 

 we must realize that all living organisms are composed of minute 

 structures called cells. In the higher animals and plants the body 

 is made up of many cells which may differ greatly in both shape 

 and function. 



When a cell is not dividing, it is usually referred to as a 

 resting or, more properly, a metabolic cell, and it is in this con- 

 dition that most of the cells of both plants and animals are to 

 be found. 



The living part of all cells, whether in the resting stage or 

 dividing, is a very complicated mixture of a number of different 

 chemical substances, known as protoplasm. Under the micro- 

 scope, protoplasm, while alive, appears as a colorless, optically 

 homogeneous fluid containing granules, crystals, and droplets; 

 l)ut, when killed, fixed, and stained, it appears to have a finely 



