Variability (genetics) - property of a living organism to respond to the impact of external or internal environment the acquisition of new biological characteristic.
Depending on the causes, nature and character of changes there are hereditary variability (mutation, or genotype) and non-hereditary (phenotypic or modification).
In the basis of genetic variability on changes of the genetic apparatus at any level of its organization - the gene, chromosome (see), the genome. The occurrence of any change copied and reproduced from generation to generation.
Hereditary, or genotypic, variability divided into combination gift and mutation.
Combination gift variability associated with obtaining new combinations of genes in the genotype (see Heredity), which is achieved in the result of two processes: 1) independent differences of chromosomes during meiosis (see) and their random combination during fertilization (see); 2) recombination of genes by crossing over; themselves hereditary factors (genes) are not changed, but a new combinations between them leads to the appearance of organisms with new phenotype.
Mutational variability is a result of sudden-onset genetic changes of the genetic material - mutations, not related in nature to the processes of splitting and recombination.
The mutation can occur in either the gene, every cell, at any stage of development. The ability of individual genes cells to mutate (mutability to different cells. In addition, the same changes genotypes may play out differently in different cells. The mutation occurs or in normal conditions of existence (spontaneous mutations), or under the influence of the special conditions, such as radiation, physical, chemical and other agents (induced mutations). The agent causing the mutation is called a mutagen, modified organism mutant.
Mutational process with equal probability can flow in either direction - from the source (wild) and mutant from mutant to the wild. In the latter case, say the opposite true or reverse mutation. On occurrence of a reverse mutation judge recovery (reversion) phenotype. However restore original phenotype is not an absolute indicator reversal genotype, for it may be due to a mutation in a completely different loci (part) of the genetic material. Such mutations called suppressor.
All changes to the genetic material are divided into gene and chromosome.
Gene, or point, mutations are all limited to a single gene and are caused by the replacement of one of the bases of nucleic acids (see)-other, their relocation or loss.
Chromosome mutations affect changes in the number of chromosomes or their structure. The latter can be limited to one chromosome - deletions or duplications (i.e. the loss or doubling parts of chromosomes), inversion (reversal of part of chromosome on 180 degrees), insertions, (swap genes) or can capture non-homologous chromosomes - translocation (change group of linked genes through exchange of land between the homologous chromosomes).
Changes in the number of chromosomes usually arise as a result of a breach of the normal processes of meiosis and are expressed in an increase or decrease of the number of complete sets of chromosomes (polyploidy, haploidy) or the number of individual chromosomes set (heteroploid, aneuploidy). Sometimes these changes indicate the generic term "genomic mutations".
The evolutionary significance of mutations of different cells varies and depends on the type of reproduction of the organism. From this point of view in individuals, reproducing sexually, there are mutations generative (cell mutation of the reproductive system) and somatic. Mutation somatic cells, if it is not detailed for it, will be played in the generation and will lead to the formation of cellular systems, consisting of normal and mutant cells (the so-called mosaic); number of mutant cells will be in proportion to the number of divisions, following the mutation.
This mechanism will make for generative mutations. Therefore, the earlier in relation to the timing of the development of germ cells will have the mutation, the more the number of changed gametes - sperm or eggs, and the more likely that fertilization will take part mutant sexual cell. Somatic mutations are not passed to the gametes and disappear with the death of the body. Thus, if for organisms that breed sexually, somatic mutation (from the point of view of its inheritance) has no value, then the value generative mutations in development of hereditary pathology is enormous. For organisms that breed asexually, a division of somatic and generative mutation is not essential.
Non-hereditary variability is not associated with the change in the genotype is observed as a change of morphological, physiological and biochemical characteristics of the organism in the process of development of the organism (the ontogenetic variability, phenotypic variability), or as a result of varying environmental conditions (modification variability). However, in both cases, all changes are monitored by genotype. In the first case, the time and manner of any changes, in the second - limits to these changes (fluctuation).
Specific phenotypic or modification change is not inherited, whereas the ability of a genotype to respond to a corresponding change of the environment hereditary.
Thus, if the role is not hereditary changes in the evolution of living nature is limited, genetic variability, regardless of its type, was the main source mechanism, in combination with artificial and natural selection, which resulted in the variety of forms of wildlife.