Билет
Interaction of non-allelic genes. Heritability, methods and importance of its assessment in medicine.
Population polymorphisms and their causes. Mutations in population genetics, frequency of mutations
Nonallelic genes are genes located in different regions of chromosomes and encoding unequal proteins. Non-allelic genes can also interact with each other.
In this case, either one gene causes the development of several features, or, conversely, one feature is manifested under the action of a set of several genes.
There are three forms and interactions of non-allelic genes:
Complementarity;
An epistasis;
polymerism.
Complementarity
Complementary (additional) action of genes is a kind of interaction of non-allelic genes whose dominant alleles, when combined in a genotype, determine a new phenotypic manifestation of traits. The splitting of F2 hybrids according to the phenotype can occur in the ratios of 9: 6: 1, 9: 3: 4, 9: 7, sometimes 9: 3: 3: 1.
An example of complementarity is the inheritance of the shape of the pumpkin fruit. The presence of dominant A or B genes in the genotype determines the spherical shape of the fruit, and the recessive genes are elongated. If the genotype of the simultaneously dominant A and B genes is present, the shape of the fetus will be discoid. When crossing clean lines with varieties that have a spherical fruit shape, in the first hybrid generation F1 all fruits will be disc-shaped, and in the F2 generation there will be a phenotype splitting: out of every 16 plants 9 will have discoid fruits, 6 - spherical and 1 - elongated.
Epistasis
Epistasis is the interaction of non-allelic genes, in which one of them is suppressed by another. The suppressing gene is called epistatic, suppressed - hypostatic. If the epistatic gene does not have its own phenotypic manifestation, it is called an inhibitor and is indicated by the letter I. The epistatic interaction of non-allelic genes can be dominant and recessive. With a dominant epistasis, the hypostatic gene (B, b) is inhibited by the dominant epistatic gene (I> B, b). Splitting by phenotype with a dominant epistasis can occur in a ratio of 12: 3: 1, 13: 3, 7: 6: 3. Recessive epistasis is suppression by the recessive allele of the epistatic gene of the alleles of the hypostatic gene (i> B, b). The phenotype splitting can take place in a ratio of 9: 3: 4, 9: 7, 13: 3.
Polymeria is the interaction of nonallelic multiple genes that uniquely affect the development of the same trait; The degree of manifestation depends on the number of genes. Polymeric genes are denoted by the same letters, and the alleles of one locus have the same subscript.
Polymerism
The polymeric interaction of non-allelic genes can be cumulative and non-cumulative. With cumulative (accumulative) polymer the degree of manifestation of a sign depends on the summing action of the genes. The more dominant alleles of genes, the more pronounced this or that sign. The splitting of F2 according to the phenotype occurs in the ratio 1: 4: 6: 4: 1.
With non-cumulative polymorph, the sign is manifested in the presence of at least one of the dominant alleles of polymeric genes. The number of dominant alleles does not affect the degree of expression of the trait. Splitting by phenotype occurs in a ratio of 15: 1.
Example: the skin color of people, which depends on four genes.
Heritability is the proportion of the variability of a certain trait, which is due to hereditary factors. The notion of heritability is widely used in breeding, as well as in quantitative and population genetics.
There are several types of heritability: inheritance is realized - this is a concept used by breeders to assess the appropriateness of further selection work, it is approximate to the true heritability. True inheritance is divided into two types: heritability in a broad sense, and heritability in the narrow sense (this is only that part of the variability caused by the additive action of genes).
Methods for assessing heritability
Heritability can be assessed in several different ways: in the first place, as described above, you can calculate the realized heritability; Secondly, the contribution of heredity and environment to variability can be determined by minimizing one of these factors; Third, heritability can be investigated by comparing relatives and twin studies. Recently, the method of segregation of marker genes has also been used.
Minimizing the components of the total variance
If one of the factors affecting the variability of the trait is reduced to zero, then it is possible to determine the share of variance due to another factor.
In research organisms it is possible to establish whether a certain attribute is hereditary by the following method: in the parental population, select two groups of organisms with extreme values of the trait and cross between each group. After this, the offspring obtained from such hybridization is grown under the same conditions. In this case, the ecological component of the variance will be zero, and any differences between the two classes of offspring will be resembled by genetic factors. So, if no special differences are observed, this will indicate that the trait is not hereditary, otherwise, we can talk about the heritability of the trait.
A similar approach can be used for quantitative assessment, namely, determining the value of heritability in a broad sense. The genetic component of variability can be eliminated using clean lines or clones. For example, F. Robertson studied the variability of breast length in Drosophila: he found that the dispersion in a genetically heterogeneous population is 0.366; And in genetically homogeneous (inbred lines were used) - 0.186. Based on these data, we can calculate V G:
V G = V Ph - V E = 0.366 - 0.186 = 0.180
So heritability in the broad sense for the length of the breast in a fruit fly is:
H 2 = 0.180 / 0.366 = 0.49
On the other hand, heritability can be calculated and minimized the influence of the environment on the variability of organisms: for example, by growing plants in greenhouses. The obtained data are compared with the variance in the population, is in natural conditions.
Comparison of relatives
Comparison of a certain quantitative trait in relatives can allow to determine heritability in the narrow sense. For this, the correlation coefficient for this attribute (r obs) is determined, as well as the relationship between relatives (r exp). The level of kinship reflects the proportion of joint polymorphic genes in individuals, and is known for any pair of relatives (for example, for monozygotic twins, it is 1, for parents and their children, and also for siblings - 0.5). The ratio of these two quantities gives the inheritance of inheritance in the narrow sense:
H ^ 2 = frac {r_ {obs}} {r_ {exp}}
However, comparing phenotypes of relatives, especially in humans, can often give very distorted information about heritability, because in families, usually not only genes, but also the environment are common. So family signs are not necessarily hereditary signs. Sometimes it is difficult to distinguish them, for example, in 1910, the US Health Commission came to the conclusion that pellagra is a genetic disease, on the basis of the fact that whole families were affected by it. Now it is known that this disorder is associated with a deficiency of vitamin B 3, and is completely dependent on the diet.
Comparison of relatives is also used to determine the inheritance of personal qualities, such as the intelligence factor, propensity to alcoholism or behavioral disorders. However, the most correlation among all such characteristics is observed for political and religious views, which, obviously, are not hereditary. Therefore, to distinguish family traits from hereditary, for research, often adopted children and their biological parents are used