Elimination of heterozygosity and efficiency of genetic systems.

One of the significant observations of recent years in the field of population genetics, highlighted by electrophoretic isozyme studies, is the presence of considerable heterozygosity within experimental and natural populations of highly inbred plants. This is found to be a general phenomenon, and is attributed to heterozygote advantage. In "Parthenogenetic Diploidy", where an organism develops without fertilization after doubling of the haploid egg nucleus, genetic heterozygosity is abolished altogether. Parthenogenetic diploidy, therefore, offers an excellent opportunity to examine the relevance of heterozygote advantage to the efficiency of genetic systems and maintenance of specific populations. In this paper, based on the study of comparative incompatibility behaviour in parthenogenetic diploids and parent plants, a hypothesis is proposed explaining the significance of persistent heterozygosity in inbred populations.The role of heterozygosity in genetic systems can be long-term, evolutionary, through segregation and recombination, termed here SEGREGATIONAL heterozygosity; or immediate, developmental, through allelic "co-action" or interaction, INTEGRATED heterozygosity. It is proposed here that a certain degree of genetic heterozygosity of the INTEGRATED type is incorporated in the regulatory polygenic components of various genetic systems involved in an organism, and may be essential for the normal development characterising a physiological system, an ecotype, a species, or a higher group. INTEGRATED heterozygosity is effective in overcoming the barriers of limited threshold regimes in physiological systems, and hence is particularly significant in extending the inherent plasticity in the physiological expression of genetic systems.