Chapter 34 

 HEREDITY— GENETICS 



1. Definitions and Methods of Studying Genetics. — Genetics (je-net' 

 iks) (Gr. genesis, descent) is the science of heredity (L. her edit as, heir- 

 ship) in which this branch of biology attempts to discover how hereditary 

 materials are transmitted through succeeding generations of plants and 

 animals and, as a consequence, how similarities and differences develop 

 from these hereditary materials subject to the influences of various inter- 

 nal and external environmental conditions. 



In general, genetics may be studied (1) by the experimental crossing 

 method in which organisms of known genetic composition are crossed 

 and the results interpreted, (2) by the cytogenetic method which is a 

 study of those structures and functions of cells responsible for the trans- 

 mission and development of hereditary traits, and (3) by the pedigree 

 method whereby the differences and similarities of individuals in large 

 populations are properly recorded and scientifically analyzed. Much 

 valuable information regarding the inheritance of certain human traits 

 has been secured by this method. Sometimes a combination of all three 

 methods is used in an attempt to solve certain problems in genetics. In 

 some instances, the hereditary mechanism is subjected to physical influ- 

 ences, such as x-rays, atomic radiations, etc., or chemicals, such as 

 colchicine, and the results on the inheritance observed. 



2. Chromosomes, Polyploidy, and Mitosis. — Chromosomes carry a 

 linear series of genes or determiners by^ means of which hereditary mate- 

 rials are borne from parents to offspring and through which the expres- 

 sion of traits is controlled (Figs. 334 and 335). Chromosomes within 

 the nucleus are always closely related and dependent upon the cyto- 

 plasm which surrounds the nucleus, either for their normal activities or 

 for the normal and proper development and expression of their inherent 

 hereditary factors. Chromosomes occur in even numbers (in pairs) in 

 most organisms. Species which apparently are closely related may differ 

 widely with respect to the number of their chromosomes, while species 

 of unquestionably remote relationship may have an identical number of 

 them in each of their cells (see accompanying table). 



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