CHAPTER XI 



GENETICS 



WITH a clear understanding- of what has been said regarding 

 the division of chromosomes in maturation, in a former 

 chapter, and the discussion of interpretations in Chapter X, 

 we are in a position to understand the terminology of heredity, genetics, 

 and Mendelism, which is met with quite commonly in modern biological 

 literature. 



While genetics really means the "origin of things" it has come to 

 be used as the name of that science which studies the ways and means 

 by which minor inheritable characters can be judged. It must never be 

 forgotten that to inherit anything from one's parents in the biological 

 sense, means that the "something" which is inherited must already be 

 present in the egg of the mother, or the sperm of the father, or in both 

 these germ cells, at the time the egg is fertilized. Every factor that 

 may influence an organism, which is not already present in the gametes, 

 is due to environmental conditions and cannot be said to be inherited. 



At this point we must also remember the distinction made in a 

 former chapter that germplasm and somatoplasm are entirely separate 

 and distinct. 



Mendelism, or rather Mendel's "law," merely means that each char- 

 acteristic that we may inherit must be considered as a single unit. To 

 illustrate, we must not think of a child as inheriting its father's hair 

 because it has dark curly hair like its father, but we must think of dark- 

 ness in color as one character of inheritance and curliness as another; 

 for, a child may inherit the darkness in color from its mother and the 

 curliness from its father. 



Thinking in terms of unit characters will throw much light upon 

 many of the interesting problems of life. We may thus account for one 

 artist, for example, having a very decided sense of form and another of 

 color. 



It is now generally conceded by biologists that acquired character- 

 istics are not transmitted to the offspring. We know, however, that 

 brothers and sisters of the same family differ from each other in many 

 respects. We know that no two leaves of grass are exactly alike; in 

 other words, that all living things springing from the same parents vary 

 somewhat from each other. It is the purpose of genetics to find the 

 mechanism by which such variation takes place, and then to be able 

 to apply the knowledge thus gained toward bringing about the types of 

 variations one wishes. Every variation represents a single unit charac- 

 ter or a combination of these unit characters. One may use as an exam- 

 ple the various species of cattle. Cows of a certain breed may produce 



