POSTULATES OF GENETICS 245 



duced, the permanence is only potential. Potential permanence has 

 already been shown to be a universal property of the chromosomes 

 (Ch. II). 



2. Qualitative Differentiation, Every organism has various and 

 specific properties of variation (mutation) which are also permanent ; 

 the permanent material must therefore be of various specific kinds, 

 i.e., must be qualitatively differentiated. 



3. Segregation. All organisms with sexual reproduction have 

 two stages in their life cycle, the one in which two corresponding 

 elements or " factors " in each cell affect its breeding properties (one 

 from the mother, the other from the father), and the other in which 

 only one of these genes affects each character. There must therefore 

 be a point in the life cycle at which corresponding elements separate 

 or segregate to different daughter-cells having half the number 

 present in the mother-cells, as well as a point at which the elements 

 recombine. 



A separation such as is required occurs at meiosis. Since the 

 chromosome pairs which segregate at random at meiosis give 

 corresponding products yet are qualitatively differentiated, it follows 

 that the members of each pair correspond. The recombination is 

 seen to take place at fertilisation (Ch. I). 



4. Linkage. The elements or factors having different capacities 

 for mutation are arranged in groups which segregate at random as 

 between groups but with restricted freedom within the group. The 

 restriction, known as linkage, is fixed in degree as between particular 

 factors. It can be represented as fixed by their arrangement in 

 linear order in a thread which has a fixed chance of exchanging 

 segments (crossing-over) with the corresponding thread. The 

 hereditary materials, as we have seen, give evidence of this character 

 and behaviour. The linear character of the chromosomes has 

 already been shown in considering the prophase of mitosis and 

 meiosis. 



It therefore becomes possible to refer the " factor " which deter- 

 mines by its change a mutation or hereditary difference to a particle 

 lying in the thread. Such a particle is described by Johannsen's 

 term as a gene. It is analogous in its behaviour to the chromomere 

 seen in the cell. It might be thought that the observed chromomeres 



