CHAPTER XXV 
GENETICS AND EVOLUTION 
GENETICS is the science which deals with the coming into. 
being _ of organisms. It deals with heredity, variation, sex 
determination, and the origin of individual plants and animals. 
Herepiry is the faithful transmission of characters from 
parent to offspring. While similar to genetics it is not entirely 
the same, since the heredity complex, representing what the 
_ offspring receives at the time of fertilization, may be changed by 
environment. Genetics deals with both the parental complex 
and the environmental “complex. “Modern genetics is based 
upon the law that the chromatin material of germ plasm is 
inherited. 
All organisms are described in the literature in terms of 
characters. These are the units employed to distinguish each 
individual, variety or species from another. Characters are 
evident in the outlines of leaves, color of leaves, size of seeds or 
plants, etc., and are the result of hereditary material acquired 
from the parent, modified by environment. They are mainly 
hereditary. Hereditary substance is.chromatin material in the 
nucleus of the cell. 
MENDEL’s LAWS OF ALTERNATIVE INHERITANCE 
The methods of heredity which have been confirmed in the 
main by the experiments and observations of many biologists 
were discovered by Gregor Johann Mendel (1822-1884), an 
Austro-Silesian monk of Briinn, Austria.t_ Mendel was the first 
to determine that the genes or “unit characters,” as he ‘termed 
them, can separate from one another and recombine in such a 
way that the characters which they represent appear to act as 
_ Separate units that can often be removed from one strain and 
- introduced into another. 
1 Although Mendel’s Laws were published in 1866, they were disregarded until 
1900 when DeVries unearthed them and brought them to the attention of or. 
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