158 GENETICS 



ture with consecutive thickenings (figure 37) ; in other 

 words, just such a structure as the results of experiments 

 show it to be. 



It may therefore be considered established that this con- 

 ception is correct. The arrangement of the genes may there- 

 fore be represented as in the diagram of figure 36. The 

 genes in any zygote form two series, one series derived 

 from the father, one derived from the mother. The two 

 series (P and M, figure 36) contain corresponding genes. 

 Thus the genes are in pairs, each pair containing one gene 

 of paternal origin, one of maternal origin. The two genes 

 of a pair have corresponding functions, though one may be 

 dominant, the other recessive; one normal, the other defec- 

 tive. 



This arrangement of the genes is one of the fundamental 

 facts of genetics, which must be thoroughly grasped if the 

 results of inheritance are to be understood. The arrange- 

 ment of the genes in chromosomes; the fact that those thus 

 together in the same chromosome usually go together into 

 the same germ cell and the same individual; the fact that 

 sometimes exchange of genes occurs between the two 

 chromosomes of a pair; the fact that the frequency of ex- 

 change is greater the farther apart the genes are in the 

 string of genes — all these have most important consequences 

 in inheritance and variation as generations pass. No one 

 who neglects them can have an adequate understanding of 

 the phenomena of heredity and variation, of similarity and 

 diversity among individuals. These relations play a most 

 Important role in the further account of genetics given in 

 the remainder of this book. 



Can the genes be seen under the microscope? During the 

 formation of germ cells, at the time when the crossing-over 

 of genes occurs, the chromosomes are seen to be linear 

 structures, with consecutive thickenings (chromomeres), 

 the two thickenings of the conjugating pair of chromo- 



