Combinative variability is the main reason for the intraspecific diversity of all living organisms. But this kind of genetic modification only leads to the formation of a new combination of traits already present. And combinative variability and its mechanisms never 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.
Combinative variability is determined by the nature of the reproductive process. This type of gene modification is characterized by the emergence of new genotypes based on newly formed gene combinations. Combinative variability manifests itself already in the phase of formation of gametes (sex cells). Moreover, in each such cell only one chromosome from each homologous pair is represented. It is characteristic thatchromosomes enter the germ cell randomly, as a result of which gametes in one organism can vary quite a lot in terms of the set of genes. At the same time, no chemical transformations are observed in the direct carrier of hereditary information.
Thus, combinative variability is due to various 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 combinative variability can only be the processes occurring during the reduction cell division (meiosis) and fertilization.
The elementary (smallest) unit of various recombinations of hereditary material, causing the formation of new gene combinations, is called a recon. Each such recon corresponds to two nucleotides (the building blocks 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. The 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.
Combinative variation 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.
- Independent random divergencechromosomes during the anaphase of the first stage of meiosis division, as a result of which all gametes acquire their own genetic characteristics.
- Random encounters of germ cells during fertilization.
Thus, through these three mechanisms of combinative variability, each zygote cell formed by the fusion of gametes acquires a completely unique set of genetic information. It is these hereditary modifications that explain the enormous intraspecific diversity. Genetic recombination is extremely important for the evolution of any biological species, since it creates an incalculable variety of genotypes. This is what makes any population heterogeneous. The appearance of organisms endowed with their own individual traits predetermines the high efficiency of natural selection, giving it the opportunity to leave only the most successful combination of hereditary traits. Through the inclusion of new organisms in the reproductive process, the genetic makeup is continuously improved.
