134 MENDELISM chap. 



are resident in the chromosomes, whence they 

 influence the nature of the cells and of the organism 

 which the cells go to build up. How the phenomena 

 of segregation can be brought into line with the 

 phenomena exhibited by the chromosomes in the 

 formation of the gametes will necessitate a; brief 

 digression. 



The number of the chromosomes in the cells of 

 an animal or plant is normally a constant number 

 which is some multiple of two. This is because 

 one half of the chromosomes is derived from each 

 gamete of which the new individual is found. When 

 the sperm and the egg fuse, the number of chromo- 

 somes in the zygote so formed is double that con- 

 tained in either of the gametes. The chromosomes 

 of a gamete often differ in size and shape, and when 

 this is so every gamete, whether egg or sperm, 

 contains a corresponding series of chromosomes.^ 

 The zygote therefore possesses a paired series of 

 chromosomes corresponding to the single series in 

 the gamete. Let us now take a simple imaginary 

 case in which the gamete contains two chromosomes, 

 one long and one short one (Fig. 39). Further, we 

 will distinguish the maternal and paternal chromo- 

 somes, I.e. those brought into the zygote by the egg 

 and the sperm respectively, by different methods of 

 shading in the diagram. A zygote results with 4 

 chromosomes, a pair of long ones and a pair of short 

 ones. By repeated divisions this zygote grows into 

 a plant or animal as the case may be. At every 

 division each chromosome is halved, and each of the 



^ Apart from the peculiar sex-chromosomes already alluded to 

 on p. 102. 



