Combinational variability is the main cause of the intraspecific diversity of all living organisms. But this type of gene modification leads only to the formation of a new combination of already present traits. And never combinational variability and its mechanisms do not cause the appearance of any fundamentally different gene combination. The emergence of completely new properties due to various gene variations is possible only in the case of intraspecific mutational changes.
Combinational variability is determined by the nature of the reproductive process. For this type of gene modification, the emergence of new genotypes based on newly formed gene combinations is characteristic. Combinational variation manifests itself already in the phase of formation of gametes (germ cells). Moreover, in each such cell there is only one chromosome from each homologous pair. It is characteristic that the chromosomes enter the germ cell randomly, as a result of which gametes in one organism can vary quite a lot in the set of genes. In this case, no chemical transformations are observed in the direct carrier of hereditary information.
Thus, combinational variation is due to a variety of recombinations of already existing genes in the chromosome set. This type of gene modification is also not associated with changes in gene and chromosomal structures. Sources of combinational variability can only be processes that occur during the reduction of cell division (meiosis) and fertilization.
The elementary (smallest) unit of various recombinations of hereditary material, causing the formation of new gene combinations, is called recon. Each such reconnection corresponds to two nucleotides (the building material of nucleic acids) in double-stranded DNA molecules and one nucleotide when it comes to the single-stranded structure of the nucleic acid of viruses. Recon is not divided during crossing over (the exchange process between paired homologous chromosomes during conjugation) and in all cases is transmitted in its entirety.
Combinational variability in eukaryotic cells is produced in three ways:
- Gene recombination in the process of crossing over, resulting in the formation of chromosomes with new combinations of alleles.
- An independent random divergence of chromosomes during anaphase of the first stage of meiosis division, as a result of which all gametes acquire their own genetic characteristics.
- Accidental encounters of germ cells during fertilization.
Thus, through these three mechanisms of combinational variation, each zygotic cell formed by the fusion of gametes acquires a completely unique set of genetic information. Such hereditary modifications explain the enormous intraspecific diversity. Genetic recombination is extremely important for the evolution of any biological species, because it creates an innumerable variety of genotypes. This makes any population heterogeneous. The appearance of organisms endowed with their individual traits determines the high efficiency of natural selection, giving it the ability to leave only the most successful combination of hereditary traits. Thanks to the inclusion of new organisms in the reproductive process, the genetic composition is constantly being improved.