Homozygous and Heterozygous Balance
The concepts of homozygous and heterozygous balance were advanced by Mather to explain the varied responses of different species to inbreeding. The species that reproduce by cross- fertilization are highly heterozygous. These species carry a large number of lethal, subvital and other unfavourable recessive genes, which are of little value to the species. The sum total of these unfavourable genes constitutes genetic load of these species. The harmful effects of such recessive alleles are masked by their dominant allele as result of which they are retained in population. The population, therefore, develops a genetic organisation, which favours heterozygosity as a result , homozygosity leads to detrimental effects. This type of genetic organisation in known as heterozygous balance, because it promoted heterozygosity.
The self fertilized species are naturally homozygous. They have no genetic load and are prompty eliminated (from the population). These species therefore develop a genetic organisation, which is adapted to homozygosity i.e which does not produce undesirable effects in the homozygous state. This type of genetic organisation is known as homozygous balance. The self – pollinated species are believed to have evolved from cross fertilized species. It has been suggested that the self- fertilized species retain sufficient heterozygous balance to show the beneficial effects of out crossing i.e heterosis. The cross- fertilized species that is generally grown in very small populations’ e. g Cucurbits would show some degree of homozygosity due to inbreeding. This would leads to the development of homozygous balance in such cross fertilized species.
The homozygous and heterozygous balances are concepts of genetic organisation of populations. These concepts are neither very clear nor very specific in terms of the physical bases of this genetic organisation of the types of gene combination involved.