combinative variability is a major cause of intraspecific diversity of all living organisms.But this kind of genetic modification leads only to the formation of new combinations of already present symptoms.And never combinative variability and its mechanisms do not cause the appearance of a fundamentally different gene combinations.The emergence of entirely new properties due to different gene variations, only possible in the case of intraspecific mutational changes.
combinative variability determined by the nature of the reproductive process.For this type of genetic modification peculiar to the emergence of new genotypes based on the newly formed gene combinations.Combinative variability manifests itself in the phase of formation of gametes (sex cells).Moreover, in each such cell is only one of each homologous chromosome pair.Characterized by the fact that the chromosomes fall into germ cell randomly, resulting gametes in one organism can vary considerably in a set of genes.It does not appear to be any changes in the chemical nature of the direct carrier of hereditary information.
Thus, combinative variability due to a variety of recombination of genes are already present in the chromosome set.This type of genetic modification also does not involve changes in gene and chromosome structure.Sources combinative variability can only be processes in cells during meiosis (meiosis) and fertilization.
elementary (smallest) unit of the different recombination of hereditary material, which causes the formation of new gene combinations, called Recon.Each recon corresponds to two nucleotides (the building material of the nucleic acid) in the double-stranded DNA molecules, and one nucleotide, when it comes to the structure of single-stranded nucleic acid viruses.Recon is not divided at the crossing-over (exchange process between paired homologous chromosomes conjugation) and in all cases transmitted in full.
combinative variability in eukaryotic cells in three ways:
- Genetic recombination during crossover, resulting chromosomes are formed, with new combinations of alleles.
- independent random chromosome segregation at anaphase of the first stage of meiotic division, resulting in all gametes acquire their own genetic characteristics.
- a chance encounter of gametes during fertilization.
Thus, through these three mechanisms combinative variability of each cell zygote formed by the fusion of the gametes, becomes a completely unique set of genetic information.That such modifications hereditary explains the huge diversity within species.Genetic recombination is extremely important for the evolution of any species, as it is created due to the innumerable variety of genotypes.This gives any population heterogeneity.The appearance of organisms endowed with its own individual characteristics, determines the high efficiency of natural selection, giving him the opportunity to leave only the most successful combination of hereditary traits.Thanks to the incorporation of new organisms in the reproductive process, the genetic makeup is continuously improved.
