Phenomena of Inheritance 99 



from both parents, pink flowers when it is derived from one parent, 

 white flowers when it is derived from neither parent (Fig. 28). 



5. Summary of Mendelian Principles. Since the rediscovery 

 in 1900 of Mendel's work many investigators have carried out 

 similar experiments on many species of animals and plants and 

 have greatly extended our knowledge of the principles of inheri- 

 tance discovered by Mendel, but in the main Mendel's conclu- 

 sions have been confirmed again and again, 'so that there is no 

 doubt that they constitute an important rule of inheritance among 

 all sexually produced organisms. 



In brief the "Mendelian Law of Alternative Inheritance" or of 

 hereditary "splitting" consists of the following principles: 



(a) The Principle of Unit Characters. The heritage of an 

 organism may be analyzed into a number of characters which are 

 inherited as a whole and are not further divisible; these are the 

 so-called "unit characters" (deVries). 



(b) The Principle of Dominance. When contrasting unit 

 characters are present in the parents they do not as a rule blend 

 in the offspring, but one is dominant and usually appears fully 

 developed, while the other is recessive and temporarily drops out 

 of sight. 



(c) The Principle of Segregation. Every individual germ cell 

 is "pure" with respect to any given unit character, even though 

 it come from an "impure" or hybrid parent. In the germ cells 

 of hybrids there is a separation of the determiners of contrasting 

 characters so that different kinds of germ cells are produced, 

 each of which is pure with regard to any given unit character. 

 This is the principle of segregation of unit characters, or of the 

 "purity" of the germ cells. Every sexually produced individual 

 is a double being, double in every cell, one-half of its determiners 

 having been derived from the male and the other half from the 

 female sex cell. This double set of determiners again becomes 

 single in the formation of the germ cells only once more to be- 

 come double when the germ cells unite in fertilization. 



