CHAPTER 24 • THE FACTS OF HEREDITY 



1 What makes living things resemble their parents? 



2 Why are not all the offspring of the same parents alike? 



3 Why do individuals of the same species differ from each other? 



4 Is inheritance due to something in the blood? 



5 Does one parent have more influence on inheritance than the 



other? 



6 How are new breeds of plants or animals produced? 



7 Are the effects of experience, training or injury passed on to the 



new generation? 



8 How can we tell whether a certain trait is due to outside in- 



fluence or to something that is inborn? 



9 Are mental qualities inherited, as well as physical? 

 10 Can anything be done to counteract heredity? 



We are so familiar with resemblances between parents and offspring that 

 they somehow seem "natural". But differences are also natural. They are 

 often obscured by the fact that in common species each individual has two 

 parents. For the individual, resembling both parents, seems to us somehow 

 "between" the two. Yet there are always characteristics that we cannot 

 trace to either parent or to other ancestors. Moreover, in the course of an 

 individual's growth he is being modified continuously by the surroundings — 

 nutrition, temperature, light, chemical factors, and so on. In human beings, 

 as well as in other species, experience and training, injury and disease — 

 various conditiojiings — all produce effects. Habits and skills, attitudes and 

 sentiments, likes and dislikes, become changed. Even under uniform condi- 

 tions there appear to be differences. 



What causes these differences? For that matter, what causes the resem- 

 blances? What happens when varieties are crossed? Are mental qualities 

 inherited in the same way as physical characters? 



How Can We Trace Inheritance through Successive Generations? 



Race Experience People everywhere seem to believe that "heredity 

 always runs in families" (see illustration opposite). Many ancient peoples had 

 strict rules regulating marriage. Some forbade the marriage of cousins and of 

 even more remote relatives. Some had no such restrictions. In some societies 

 brother-sister marriages were accepted as proper, although these are looked 

 upon with abhorrence by most peoples today. The reasons for the various 

 rules rest on what people assumed or believed about heredity or about "race". 



Breeders of race horses train the animals for swiftness, and then try to 



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