32 

 The Principles of Heredity 



The influence of heredity is universally recognized. A child will 

 show physical characteristics, mannerisms, and abilities of a father that 

 he may have never even seen. Fine race horses transmit their qualities 

 for speed and endurance to hundreds of offspring which develop into 

 champions of the race tracks. Good hunting dogs are bred together to 

 produce offspring with the same fine qualities for seeking out and re- 

 trieving game. Any plant or animal breeder knows the importance of 

 heredity — he would never hope to produce prize-winning beef cattle 

 from scrub ancestry or champion egg-layers from ordinary barnyard 

 chickens. 



In spite of the recognized importance of heredity, however, its 

 simple principles are often not fully understood. When a child shows 

 some striking difference from its parents, such as blue eyes from brown- 

 eyed parents, it may seem as if heredity is a capricious thing, working 

 at some times and suspending its operations at other times. Heredity 

 may seem to be unstable, unpredictable, and of a mysterious nature. 

 Mystery cannot long withstand the cold light of scientific research, 

 however; and intensive investigations in the field of genetics have 

 shown us that heredity is stable and that its results can be predicted with 

 mathematical precision. A blue-eyed child with brown-eyed parents 

 is not an exception to heredity — it is a normal result of the laws of 

 heredity under certain circumstances. It would be an exception only if 

 we did not get a certain proportion of blue-eyed children from brown- 

 eyed parents of this nature. In this chapter we will study some of the 

 principles underlying heredity which explain many of our everyday 

 observations on this subject. 



Dominance and Recessiveness 



The method of gene action in heredity can probably best be under- 

 stood if we first consider the influence of a single gene on a particular char- 

 acter and learn how it is transmitted and expressed. Then, since other 

 genes follow a similar pattern we will have a general understanding of 

 them all. We are naturally intensely interested in human heredity, but 



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