Chapter 1 

 GENETICS, CELLS, AND CHROMOSOMES 



Genetics is one of the numerous branches of the biological 

 sciences. It attempts to discover the laws which determine why 

 certain individuals related by descent resemble one another or 

 why they differ from one another. It is the science of heredity 

 and it attempts to discover how and why certain resemblances 

 "run in families" and why many differences are also found among 

 members of the same family. It is one of the biological sciences 

 for it includes both plants and animals in its investigations, and, 

 especially in its more recent aspects, it borders upon physics 

 and chemistry. It is, furthermore, a relatively new science, not 

 estabhshed on a scientific basis before 1900. 



The science of genetics is intimately related to another bio- 

 logical science, cytology. Cytology is a study of those minute 

 living units, the cells, of which plants and animals are constructed. 

 Among the many structures found in cells are certain bodies, the 

 chromosomes, which have been shown to be of the greatest im- 

 portance to students of genetics because in them are located the 

 hereditary units. In other words, the physical basis for the laws 

 of heredity is to be found in the chromosomes; therefore, a 

 knowledge of cytology or at least of chromosomal cytology is 

 absolutely necessary for an understanding of the principles of 

 the science of genetics. 



The intimate relationship between the sciences of genetics and 

 cytology was not realized during their early development. How- 

 ever, as more information became available in both fields of 

 knowledge, a striking parallelism became evident which soon 

 suggested that they were in reality not two separate studies but 

 merely two pliases of one. Further experiments only served to 

 corroborate this unity until it became evident that the close 

 relationship between genetics and cytology was incontrovertible. 



During the earher years of scientific investigations into the 

 field of heredity, data were obtained by methods that are con- 



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