30 GENE RECOMBINATION 



ever, are not merely historical relics to be studied with the 

 techniques and attitudes of comparative anatomy and phylogeny 

 — they are rather dynamic genetic systems, concerned in most 

 cases with the recombination of genes. They should be studied 

 for what they are — basic mechanisms which determine the ge- 

 netic makeup of the individuals which comprise species popula- 

 tions. 



One of the major consequences of the particulate nature of the 

 hereditary material is the potentiality which automatically exists 

 for recombination. The sexual mode of reproduction, as had been 

 brought out so brilliantly in the work of Darlington ( 1939 ) and 

 White (1954), is biologically meaningful only when viewed 

 basically as a process of recombination of hereditary particles. 

 The analytical methodology of the whole science of genetics is 

 rooted in recombination; in fact, perhaps the most satisfactory 

 definition of a gene is a hereditary entity which acts as a unit 

 in recombination. 



Given a certain amount of hereditary diversity provided by 

 gene mutation, the sexual process, through its essential feature 

 of recombination, can generate an enormous number of diverse 

 genotypes. These genotypes, or recombinants, are exposed to 

 the action of natural selection in local populations. In the above 

 context, recombination occupies a focal position in modern 

 theory of evolutionary cause. For this reason, the devices whereby 

 sexual reproduction accomplishes recombination deserve close 

 scrutiny. 



Recombination in sexual reproduction is a function of three 

 processes. These are: crossing over between homologous chro- 

 mosomes, random segregation of chromatids from bivalent chro- 

 mosomes, and chance recombination of gametes. Thus the great- 

 est amount of recombination will result in an organism which 

 has intensive and extensive crossing over, high chromosome num- 

 ber and a high degree of interbreeding between relatively 

 unrelated individuals (outcrossing). Conversely, if exchanges 

 between homologous chromosomes are few, if the chromosome 

 number is small and if a considerable degree of inbreeding oc- 



