HEREDITY 263 



The fact that in parthenogenesis the egg develops without the 

 intervention of a sperm shows that the egg alone may possess all 

 of the internal factors for development and for heredity. How- 

 ever, since it is common knowledge that paternal characters are 

 inherited in cases where the egg requires fertilization, it is clear 

 that hereditary factors are contributed by the sperm as well as 

 by the egg. The problem of heredity centers in explaining the 

 nature and location of herditary potencies of the germ cells and, 

 if possible, how the hereditary characters of the adult are con- 

 trolled or produced by them. 



We are accustomed to think of heredity as the cause or reason 

 for the reproduction in children of characters similar to those of 

 their parents; but while it is true that the reproduction of similari- 

 ties is due to heredity, it is also true that characters in children 

 differing from those of the parents are also the result of heredity. 

 Thus the fact that blue eyes as well as brown eyes may be 

 inherited from brown-eyed parents, means that blue eyes and 

 brown eyes, though classified as dissimilar hereditary conditions, 

 are really genetically related, and that the production of blue 

 eyes from brown-eyed parents is a result of this relationship. 

 Heredity may be defined as the production in successive genera- 

 tions of conditions not necessarily similar or identical, whose 

 specific character is determined by factors located in the germ 

 cells. The term hereditary factor or gene may be defined as a 

 substance or a condition in the germ cells that determines the 

 development of a hereditary character in the adult. Each 

 hereditary character presumably is represented by a factor or set 

 of factors in the germ cells. 



Chromosome Theory of Heredity. — The germinal factors of 

 heredity are generally thought to be located in the chromosomes 

 or chromatin rather than in the cytoplasm of the germ cells for a 

 number of reasons, some of which may be considered. The 

 basic argument for this view is that, since the chromatin of the 

 fertilized egg is the only substance contributed in practically 

 equal amounts by the parents to the fertilized egg, and since in 

 the long-run offspring inherit equally from the parents, it follows 

 that the chromatin must be the hereditary substance. The 

 cytoplasm of the fertilized egg is entirely maternal in origin, 

 save for the slight addition to its bulk made in those cases where 

 more than the head of the sperm enters the egg; yet the heredity 



