Publisher Summary This chapter discusses the genetic variation in chromosome pairing. Alterations in the behavior of chromosomes at meiosis are known to result from genetic variation as well as from differences in several environmental factors. The operation of selection on genetically controlled variation in the chromosomal phenotype is responsible for the development of meiotic mechanisms adapted to the requirements of the breeding structures of species and of their populations. The fertility of an organism and consequently the fitness of the lineage to which it belongs are intimately related in sexual forms and in some apomicts, to the efficiency of the over-all meiotic process. The prime function, upon which the efficacy of the process depends, is the segregation of homologous chromosomes. Two distinct aspects of the process of meiotic chromosome pairing are therefore under genetic control. The extent to which synapsis is realized within the chromosome complement is determined by the activities of major genes and polygenes and by interactions within the genetic system comprised of relevant genes of both kinds. The specificity of synapsis is widened or narrowed by gene action to permit the pairing of chromosomes distantly or closely related genetically and evolutionarily.