Patterns of Inheritance

Abstract

SYNOPSIS. Every individual living organism on earth develops according to the specificationsof its individualized and unique DNA that is encoded as its genotype. The genotype consists of many genes and is established at the time of the individual's birth or separation from itsparent. Although offspring resemble parents, they are rarely if ever genetically identical to them. This result is a function of both the pattern of inheritance and the organization imposed by the genetic system embodied in the chromosomes. Genes are not just loose in the nucleus: they are organized into linear arrays on chromosomes. In the diploid, cross-fertilizing genetic system, the parents contribute about equally to the offspring's genotype through the haploidnuclei of their gametes. In most diploids, vast amounts of genetic variability are produced by the process of genetic recombination. This alone assures the genetic uniqueness of every individual of the new generation. The ultimate source of the variability is the process of gene mutation but the great storage capacity of the diploid system enhances recombinational variability. The powerful sources of recombination are: synapsis and crossing-over, processes that serve to scramble the genes. Independent assortment at meiosis provides unique gametes; this latter effect is enhanced by high chromosome numbers. Since two parents are involved in the formation of the individual, still another level of recombination is achieved at fertilization. Patterns of genetic systems vary greatly from species to species: man, mouse, maize and melanogaster are considered. In a significant number of cases, chromosome number reductions, balanced chromosomal aberrations and polyploidy are present and serve to restrict recombination potential. Even greater restrictions are imposed by the evolution, in natural populations, of patterns of inheritance that partially or even completely by-pass recombination. Thus, total dependence on vegetative reproduction, loss of meiosis, self-fertilization or parthenogenesis are examples. In organisms that have discarded the attributes that assure recombination, the formation of both new species and new adaptations is impaired. This emphasizes the key importance of the mode of inheritance for activating processes that adjust the genes of living things to their environments. Future studies of patterns of inheritance in relation to the evolution of life on earth are needed.