Interaction of genes
Plan.
1. Types of gene interaction.
2. Complementary effects of genes.
3. Epistasis effect of genes.
4. Modifier effect of genes.
1) During the study of Mendel's laws, we saw that one gene produces one trait. But in some cases, the result of crossbreeding did not turn out to be the same as in Mendelian conditions. Because in some cases, it was found that one character is the result of the interaction of two or more genes, and vice versa, several characters appear with the participation of one gene. This definitely affects the ratio of separation of characters in hybrids. Therefore, Har does not consider the genotype of an organism to be a set of genes obtained in unrelated alloX. The interaction of genes does not mean the physical interaction of genes, but the interaction of their primary and secondary products in the process of character development.
Gene interactions can be divided into allelic and non-allelic gene interactions.
total dominance half-dominance ultra-dominance codominance complementary epistasis poly meria
The interaction of allelic genes occurs in the forms of full dominance, partial dominance, super dominance and codominance. Complete dominance - in all heterozygous hybrids, the sign of only one allele is fully manifested, and the sign of the second allele does not appear.
In incomplete dominance, the dominant gene cannot fully express its trait. When the trait appears with predominance, the dominant trait does not appear in the heterozygous hybrids of the first generation.
Ultra-dominance is a stronger manifestation of the trait in the case of heterozygosity of the dominant allele - Aa, compared to homozygosity - AA.
Codominance is the appearance of signs specific to both alleles in a heterozygous organism.
Usually, each gene independently produces one trait. But in some cases, the allele of one gene is not found in the independent manifestation of the trait, and the second gene may have an effect. Three types of effects of nonallelic genes have been well studied: complementarity, epistasis, and polymorphism.
2) Complementarity is the fact that non-allelic genes each produce a trait individually, and together produce a different trait. In complementarity, the separation of characters in the hybrids of the second generation is 9:3:3:1, or 9:7 or 9:3:4.
Complementary effects of genes can be different. For example, when white-flowered chickpeas are crossed, red-flowered chickpeas appear, when a black ( AAvv ) mouse is crossed with an albino ( aaVV ) mouse, and finally, we see the emergence of different colored spherical hollows.
If different origins (AAvv and aaVV) but spherical pores are crossed, only flanged pores (AaVv) are formed in F1, and separation in F2 is 9:6:1, i.e. 9 flanged, 6 spherical, 1 Small holes are formed. In this case, each of the dominant complementary genes creates a spherical cavity in alloX, and two complementary dominant genes interact to create a flange-shaped cavity. As a result of interaction of recessive alleles of these genes, chuzinchok (aavv) cavity develops.
In the cells of mammals, a special oxil, i.e. interferon, is produced against the virus. Production of interferon depends on the complementary effect of two nonallelic genes. One of these genes is located on the second chromosome and the other is located on the fifth chromosome.
In the hemoglobin of adults, there are four polypeptide chains, each of which is controlled by the alloX gene. So, four complementary genes are involved in the synthesis of hemoglobin molecule.
3) Epistasis - dominance of the effect of one gene over the effect of another gene that does not have an allele to it. But in some cases, epistasis can occur under the influence of a recessive gene. Therefore, the effect of epistasis of genes is divided into two, i.e., dominant and recessive. In dominant epistasis, under the influence of one dominant gene, the second dominant gene cannot produce a trait (A > V). In recessive epistasis, the dominant gene under the influence of the recessive gene cannot produce a trait (a>D). A gene that manifests its trait by suppressing the influence of a gene that does not have an allele is called an epistatic gene, and a gene that cannot manifest its trait is called a hypostatic gene.
Epistasis effects of genes have been well studied in horses. If a gray horse (SSvv) is crossed with a black horse (ssVV), the genotype of the first generation hybrids will be SsVv, and all will be gray. The gray coloring of the hybrids indicates the dominance of the S gene over the V gene. When hybrids of the first generation are intercrossed, in the second generation, the characteristics of this phenotype are separated in the ratio of 12:3:1. All hybrids with dominant S gene are gray, hybrids with V genes are black, and hybrids with both recessive alleles are purple. In some cases, in dominant epistasis, the separation of characters in the second generation can be equal to 13:3. For example, this result is obtained when crossbreeding chickens (CCII and ccii). In recessive epistasis, separation is in the ratio of 9:3:4.
4) Polymerism is the appearance of one character under the influence of several non-allelic genes. The traits produced by such polymeric genes are considered polymeric genes. Polymeric genes are usually represented by a single letter, ie A1, A2, A3, A4, etc. As a result of the accumulation of such identical genes in one organism, their effect increases. If the total number of polymer genes is high, the characteristic of the organism develops strongly, if it is low, this characteristic develops weakly. At the beginning of our century, the emergence of polymer characters was made by the Swedish geneticist G. Nilson-Ele (1908) who cross-bred red and white wheat and obtained a 3:1 ratio of characters in their second generation, as in monohybrid crossing. But when some varieties of wheat with such signs were crossed, in the second generation the signs were not found in the ratio of 3:1 but in the ratio of 15:1, i.e. 15 colored (red) and 1 colorless (arrow). Colored wheat grains are produced in a uniform color, ranging from reddish to reddish brown. The dominant genes of the red allele produce the red color, and their recessive genes produce the white color. As the number of dominant genes decreases, the characteristic Ham (red color) becomes weaker. With an increase in the number of dominant genes, the characteristic Xam is manifested more strongly, this is called cumulative polymorphism.
In some cases, polymorphic genes can determine the quality of traits, not just the number. In this case, the appearance of symptoms depends on the presence or load of dominant genes, not on the number of dominant genes, and this score is called non-cumulative polymorphism. Many traits appear under the influence of polygenic genes. For example: growth rate, live weight, fertility of chickens, the amount of milk and fat in it, accumulation of vitamins, speed of biochemical reactions, characteristics of the nervous system in animals, etc.
The biological significance of gene-polymeric effect is that traits caused by polygenic genes are more stable and stable than those caused by a single gene. In the absence of polymer genes, the organism was easily susceptible to various influences, especially mutagenic factors.
5) Genes that increase or decrease the effect of basic genes are called modifier genes. They do not drastically change the sign, but cause its development to strengthen and weaken. Modifier genes can be dominant or recessive. For example, in some mice, it has been determined that this color depends not on the main gene that gives them such a color, but on the multiplicity (from 4 to 10) of modifier genes. The influence of modifier genes leads to the purple color of coramol, chuchka, kuy, horses, and the flowers of plants. It is known that the production of substances is regulated by the interaction of three types of genes: structural genes, operator genes and regulatory genes.
Genes that determine the sequence of amino acids in polypeptides are called structural genes. Such genes have the ability to produce a certain enzyme, and they can stop the synthesis of such an enzyme or continue it if needed. Such regulation is carried out by operator genes and regulatory genes. An operator gene increases or decreases the activity of a structural gene at the behest of a regulator gene.
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