CHROMOSOMES AND MENDELIAN HEREDITY 313 



groups of linked genes are segregated, each gamete (or spore) carrying 

 one chromosome of each homologous pair and hence one linked group 

 of each kind. Investigation of the individuals of the next generations 

 shows that two corresponding gene groups may have exchanged some of 

 their members, this recombination being accomplished by an exchange of 

 portions between homologous chromosomes. It was this phenomenon 

 which first made it possible to determine the relative order of the genes 

 in the linear series in the chromosome. 



Because of the distribution of multiplying generations of chromosomes 

 through successive life cycles and the activity of genes carried by them, 

 the characters appear in successive generations according to definite 

 rules first discovered by Mendel, who was not, however, aware of the 

 cytological reasons for the phenomena he observed. Such recombinations 

 of genes and Mendelian characters with occasional mutation are plainly 

 responsible for much of the diversity of type observed among species in 

 nature. It is accordingly inferred that the prevalence and high special- 

 ization of that form of reproduction involving gametic union and meiosis, 

 with its random assortment and crossing-over of chromosomes, are 

 evidences of its important evolutionary role in producing frequent and 

 diverse recombinations of hereditary elements. Its full value in this 

 respect has not been measured. 



