CHAPTER XXXIII 

 HEREDITY 



Thomas Hunt Morgan 



Gregor Mendel 



What are Mendel's laws of inheritance ? How were they formulated f 

 Do these laws of inheritance hold true for all plants and animals f 



It has been found that truly heritable traits or characteristics 

 of an individual are comparatively independent of each other and 

 may be inherited independently. These characters are called 

 unit characters. If the members of a pair of unit characters meet 

 in an offspring, each member retains its identity and each char- 

 acter of the pair may be separated in subsequent generations. The 

 following illustration is an example of the behavior of a pair of unit 

 characters. 



Incomplete dominance. There are two distinct kinds of Anda- 

 lusian fowls, one having white feathers splashed with black ; and 

 the other, black feathers. The black fowls are known as pure black 

 because, for generations, their ancestors have been black, have 

 mated, and have given rise to only black descendants. The 

 splashed-white fowls are known as pure white; their ancestors 

 were white, they bred with white, and have always produced white 

 offspring. A fowl from each group was selected as a parent. 

 They were mated, fertile eggs were laid and incubated, and the 

 outcome of the experiment awaited. Would the chickens be black, 

 or white, or a mixture of the two colors ? Although the experi- 

 ment was repeated several times, the results were always the same. 



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