CHAPTER XXI 



REVIEW OF MENDELISM 



At this point it seems well to pause and to take stock of what we 

 have learned about heredity by following Mendel's lead. Let us first 

 enumerate some of the rules or laws of heredity discovered by Mendel. 

 These are commonly known as Mendel's laws. 



Mendel's first law: the law of dominance. — When two parent- 

 types differing from each other with reference to a single unit character 

 are crossed, the "hybrid-character resembles that of one of the parent 

 forms so closely that the other either escapes observation completely 

 or cannot be detected with certainty." The character that appears in 

 the first-generation hybrids is called "dominant" and that which ap- 

 parently becomes latent is called "recessive." The law of dominance 

 has been shown to be far from universal in its application. In fact, 

 complete dominance is relatively rare, and almost entire lack of domi- 

 nance is not uncommon. Evidently, then, dominance is not an essen- 

 tial feature of Mendelian heredity. 



Mendel's second law: the law of segregation, or purity of gam- 

 etes. — While the body cells and the germ cells of the F : parent, prior 

 to the reduction divisions involved in gamete formation, contain the 

 determiners (genes) of both alternative characters and are therefore 

 hybrid in character, a segregation of the alternative genes (allelo- 

 morphs) takes place during maturation so that only one or the other 

 gene comes to be present in any gamete. Thus gametes are pure for 

 any gene. A gamete has one or the other of a pair of allelomorphs 

 and is never hybrid with reference to any single character. This law 

 is by far the most important of Mendel's discoveries. In fact, it might 

 be called the discovery of Mendel, for it is almost unsurpassed among 

 biological generalizations on account of its far-reaching applicability. 

 The law has sometimes been called the law of the splitting of hybrids. 

 Whether dominance is present or not, the law of segregation always 

 holds. The second law, therefore, is much more important than the 

 first. 



Mendel's third law: the law of independent assortment of differ- 



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