494 ZOOLOGY 



cells and primordial germ cells (see 49, 60), has no part in 

 developing the body. The recessive determiner cannot develop 

 in the presence of the dominant. Whenever the dominant deter- 

 miner is absent, the alternative recessive qualities will appear. 

 This explains why the inbred recessives always breed true. 

 They would not show at all if there were any taint of the 

 dominant quality. 



c. The principle of segregation, or purity of the gametes. 

 This principle asserts that, while both dominant and recessive 

 determiners are to be found in every primordial germ cell, both 

 determiners cannot go into one sperm or one ovum. In the 

 process of maturing ova and sperm by nuclear division the two 

 kinds of determiners separate, and thus an equal number of 

 sperm and ova carry the dominant and the recessive characters. 

 In other words, while the body of an organism may be a hybrid, 

 the eggs and sperm are always pure. 



In the peas, for example, if T represents the determiner for 

 tallness, and t that for dwarf ness, one-half the egg cells of the 

 first hybrid generation will carry T and one-half will carry the 

 recessive t. The same will be true of the male nuclei of the pollen 

 tube. In mating the chances are even that each kind of male 

 cell will fertilize each kind of female. The "checker-board" 

 diagram (Fig. 254) will show the probable number of offspring of 

 each possible type. 



494. The Relation of MendePs Conclusions to the Process of 

 Maturation of Ova and Sperm. The student will realize that 

 these views of Mendel fit in remarkably with the facts that were 

 brought out (49 and 50) relative to the behavior of chromo- 

 somes in nuclear division, and especially in the divisions in the 

 forming of sperm and ova. It is not possible to develop the sub- 

 ject at length here. These two lines of study, experiments in 

 breeding, and studies of the structure and behavior of the minute 

 parts of the germ cells, supplement one another; and both are 

 throwing increasing light upon the problems of inheritance. It 

 only adds to the appreciation of Mendel's results to recall that 

 nothing was known in his time of these remarkable facts of 

 cell behavior. 



