What is combinational variability?
Combinative variability is a process based on the formation of recombinations. In other words, such combinations of genes that are absent from the parents are formed. Further, combinational variability and its mechanisms will be discussed in more detail.
The reasons for the process
Combinative variability due to sexual reproduction of organisms. As a result, a large variety of genotypes is formed. Certain phenomena appear as practically unlimited sources of genetic variation. Sources that will be listed below, acting independently, simultaneously provide continuous "shuffling" in the genes. This provokes the appearance of organisms with a different pheno and genotype. In this case, the genes themselves are not subject to change. At the same time, it is noted that new combinations quite easily break up in the process of transmission from generation to generation.
- An independent discrepancy occurs in the homologous chromosomes of the first meiotic division. Each such pair diverges, regardless of the others.Actually, the independent combination of chromosomes in the process of meiosis is the basis of the third law of Mendel. When crossed with smooth yellow green wrinkled pea seeds, yellow smooth and green wrinkled seeds appear in the second generation. Thus, combinational variability manifested itself.
- With meiosis, at the very beginning, the so-called crossing-over occurs. In the course of this process, homologous chromosomes are closely converging and changing their position. When they line up opposite each other, their sections begin to intersect, break away. Then they join again, but on a different chromosome. As a result, the formation of new elements. In particular, 4 chromosomes are formed. They have a different combination of genes. Recombinant chromosomes carry new combinations that were absent in the original elements. Thus, in the process of mutual exchange of plots, new linking groups are formed in homologous chromosomes. All this is an important source of genetic allelic recombination. Once in the zygote, new chromosomes contribute to the development of signs that are atypical for parents.
- During fertilization, a random combination of gametes occurs.
Combination variability is considered to be the most important source of all the existing enormous diversity, which is characteristic of organisms. The above sources, however, do not generate any significant changes for the survival of stable changes in the genotype, which, according to the theory of evolution, are necessary for the emergence of new species. Changes of this kind may occur with mutations.
Combinative variability may explain, for example, why a new combination of signs of kinship along paternal and maternal lines was found in a child. Moreover, it is possible to carry out a study of certain and specific options that are not peculiar to parents or previous generations. Combinative variability contributes to the generation of genotype diversity in the offspring. This is of particular importance for the entire evolutionary process. First of all, the species diversity of the material for natural selection increases without lowering the viability of individuals. In addition, the empowerment of adaptation of organisms to regularly changing environmental conditions is noted. This ensures the survival of the species (population, group) as a whole.
Combination variability is used in breeding to obtain more valuable in the economic value of complexes of hereditary traits. So, the phenomena of increasing vitality, heterosis, growth rate and other properties are used in the process of hybridization between representatives of different varieties or subspecies, which, in turn, causes a definite and significant economic effect. The opposite result is observed when closely related crossing (inbreeding) - the connection of organisms with common ancestors. This kind of origin increases the likelihood of the presence of identical alleles of genes. Consequently, the risk of homozygous organisms increases. The highest degree of closely related crossbreeding is observed during self-pollination in plants, as well as self-fertilization in animals. At the same time, homozygosity increases the likelihood of manifestation of allelic genes of the recessive type. Their mutagenic changes provoke the appearance of organisms with different hereditary anomalies.
Medical Genetic Counseling
The results obtained in the study of combinational variability are actively used in predicting offspring and explaining the meaning of genetic risks. In the process of advising future spouses, it is used to establish the possible presence of each individual alleles that have passed from the same ancestor and are of identical origin. In this case, the coefficient of relationship. It is expressed in fractions of a unit. For homozygous twins, the coefficient is 1, for children and parents, sisters and brothers - 1/2, for nephew and uncle, grandson and grandfather - 1/4, for second cousins and brothers - 1/32, for cousins - 1/8.
Consider the flower "night beauty." He has a gene of petals of red (A) and white (a) color. The organism Aa petals have a pink color. The flower does not have the original pink gene. It appears in the process of combining white and red elements. One more example. A person has sickle cell anemia. Death is aa, and AA is the norm. Sickle cell anemia is AA. With this pathology, a person is not able to endure physical exertion. But at the same time he is not sick with malaria, that is, the causative agent of this disease - Plasmodium - cannot consume the wrong hemoglobin.This feature is significant in the equatorial belt. This combinational variability appears with a combination of genes a and A.
Some occurring mutations begin to coexist with others, to be part of genotypes. In other words, many allelic combinations appear. For any individual characteristic of genetic uniqueness. The only exceptions are identical twins and individuals, which resulted from asexual reproduction in a clone that has one cage as an ancestor. If we assume that for each pair of homologous chromosomes there is only one pair of allele genes, then for a person whose haploid set is 23, the number of possible genotypes can be 3 to the 23rd degree. This colossal number is 20 times the population of the Earth. But in reality, the difference between homologous chromosomes takes place on several genes. The calculations do not take into account the phenomenon of crossing over. In this regard, the number of probable genotypes is expressed in astronomical numbers, and it can be said with certainty that the appearance of two completely identical people is impossible in practice.The exceptions are identical twins that have emerged from a single egg. All this makes it possible to reliably determine the identity of the remnants of living tissue and to refute / confirm paternity / motherhood.