Laws of independent inheritance of signs. Mendel's laws. Genetics
As a result of research by scientists K. Correns, G. de Vries, E. Cermak in 1900 were "rediscovered" the laws of genetics formulated as early as 1865 by the ancestor of the science of heredity - Gregor Mendel. In his experiments, the naturalist used a hybridological method, through which the principles of inheritance of traits and certain properties of organisms were formulated. In this article, we will consider the main patterns of heredity transmission, studied by a geneticist.
G. Mendel and his research
The use of the hybridologic method allowedthe scientist to establish a number of laws, subsequently called the laws of Mendel. For example, he formulated the rule of uniformity of hybrids of the first generation (Mendel's first law). He pointed to the fact that the hybrids F1only one characteristic that is controlleddominant gene. Thus, with the crossing of plants of the seed pea, the varieties of which differed in the color of the seeds (yellow and green), all hybrids of the first generation had only a yellow color of the seeds. Moreover, all these individuals also had the same genotype (they were heterozygotes).
The law of splitting
Continuing to interbreed among themselves the individuals taken from hybrids of the first generation, Mendel received in F2splitting of features. In other words, plants with a recessive allele of the test trait (green seed coloring) in a quantity of one-third of all hybrids were phenotypically identified. Thus, the established laws of independent inheritance of features allowed Mendel to trace the mechanism of transmission of both dominant and recessive genes in several generations of hybrids.
Di- and polyhybrid interbreeding
In subsequent experiments, Mendel complicatedconditions of their conduct. Now, for crossing, plants were taken that differed by both two and more pairs of alternative characters. The scientist traced the principles of inheritance of dominant and recessive genes and obtained splitting results, which can be represented by the general formula (3: 1)n, where n is the number of pairs of alternative characteristics by which the parent is different. So, for dihybrid crossing, the phenotype split in the second generation hybrids will look like: (3: 1)2= 9: 6: 1 or 9: 3: 3: 1. That is, hybrids of the second generation can observe four types of phenotypes: plants with yellow smooth (9/16 parts), with yellow wrinkled (3/16), with green smooth (3/16) and green wrinkled seeds (1/16 part ). Thus, the laws of independent inheritance of features received their mathematical confirmation, and poly hybrid hybridization was viewed as a few mono-hybrid - "superimposed" on each other.
Types of inheritance
In genetics there are several types of transmissionsigns and properties from parents to children. The main criterion here is the form of trait control, carried out by either one gene - monogenic inheritance, or several - polygenic inheritance. Previously, we considered the laws of independent inheritance of features for mono- and di-hybrid crosses, namely, Mendel's first, second and third laws. Now we will consider such a form as concatenated inheritance. His theoretical basis is Thomas Morgan's theory, called chromosomal theory. The scientist has proved that along with the traits transmitted to the offspring independently, there are such inheritance types as autosomal and sex-linked cohesion.
In these cases, several characteristics of one individualThey are inherited together, as they are controlled by genes localized in one chromosome and located next to each other. They form clutch groups, the number of which is equal to the haploid set of chromosomes. For example, a person has a karyotype of 46 chromosomes, which corresponds to 23 clutch groups. It was found that the smaller the distance between the genes in the chromosome, the less often between them there is the process of crossing-over, which leads to the phenomenon of hereditary variability.
How genes are localized in the X chromosome
We continue to study the patterns of inheritance,subject to the chromosome theory of Morgan. Genetic studies have established that both in humans and in animals (fish, birds, mammals) there is a group of features, the mechanism of inheritance of which affects the sex of the individual. For example, the color of hair in cats, color vision and blood coagulation in humans are controlled by genes located in the sex X chromosome. So the defects of the corresponding genes in humans are phenotypically manifested in the form of hereditary diseases, called gene. These include hemophilia and color blindness. The discoveries of G. Mendel and T. Morgan allowed the application of the laws of genetics in such important areas of human society as medicine, agriculture, the selection of animals, plants and microorganisms.
The relationship between genes and the properties they determine
Thanks to modern genetic studies,it was found that the laws of independent inheritance of attributes are subject to further expansion, since the ratio "1 gene - 1 attribute", underlying them, is not universal. In science, cases of multiple action of genes, as well as interactions of non-allelic forms, have become known. Such species include epistasis, complementarity, and polymorphism. So it was found that the amount of melatonin skin pigment responsible for its color is controlled by a whole group of hereditary deposits. The more in the human genotype of the dominant genes responsible for the synthesis of the pigment, the darker the skin. This example illustrates interaction such as polymer. In plants, this form of inheritance is inherent in the species of the cereal family, in which the color of the grains is controlled by a group of polymer genes.
Thus, each organism has a genotypeis represented by an integral system. It was formed as a result of the historical development of the biological species - phylogenesis. The state of most of the features and properties of the individual is the result of the interaction of genes, both allelic and non-allelic, and they themselves can influence the development of several features of the body.