As a result of research by scientists K. Correns, G. de Vries, E. Cermak in 1900, the laws of genetics were "rediscovered", formulated in 1865 by the founder of the science of heredity - Gregor Mendel. In his experiments, the naturalist applied the hybridological method, thanks to which the principles of inheritance of traits and some 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 hybridological method allowed the scientist to establish a number of patterns, later called Mendel's laws. For example, he formulated the rule of uniformity of hybrids of the first generation (Mendel's first law). He pointed to the factmanifestations in F1 hybrids of only one trait controlled by the dominant gene. So, when crossing plants of seed peas, the varieties of which differed in seed color (yellow and green), all hybrids of the first generation had only yellow seed coloration. Moreover, all these individuals also had the same genotype (they were heterozygotes).
Split Law
Continuing to cross between individuals taken from hybrids of the first generation, Mendel received splitting of characters in F2. In other words, plants with a recessive allele of the studied trait (green seed color) were phenotypically identified in the amount of one third of all hybrids. Thus, the established laws of independent inheritance of traits allowed Mendel to trace the mechanism of transmission of both dominant and recessive genes in several generations of hybrids.
Di- and polyhybrid crosses
In subsequent experiments, Mendel complicated the conditions for their implementation. Now, plants were taken for crossing, differing both in two and in a large number of pairs of alternative traits. The scientist traced the principles of inheritance of dominant and recessive genes and obtained splitting results that can be represented by the general formula (3:1) , where n is the number of pairs of alternative traits that distinguish parental individuals. So, for a dihybrid crossing, the splitting by phenotype in hybrids of the second generation will look like: (3:1)2=9:6:1 or 9:3:3:1. That is, hybrids of the secondgenerations, four types of phenotypes can be observed: plants with yellow smooth (9/16 parts), with yellow wrinkled (3/16), with green smooth (3/16) and with green wrinkled seeds (1/16 part). Thus, the laws of independent inheritance of traits received their mathematical confirmation, and polyhybrid crossing began to be considered as several monohybrid - "superimposed" on each other.
Types of inheritance
In genetics, there are several types of transmission of traits and properties from parents to children. The main criterion here is the form of control of the trait, carried out either by one gene - monogenic inheritance, or by several - polygenic inheritance. Earlier, we considered the laws of independent inheritance of traits for mono- and dihybrid crosses, namely the first, second and third laws of Mendel. Now we will consider such a form as linked inheritance. Its theoretical basis is the theory of Thomas Morgan, called the chromosome. The scientist proved that along with traits that are transmitted independently to offspring, there are such types of inheritance as autosomal and sex-linked linkage.
In these cases, several traits in one individual are inherited together, as they are controlled by genes localized on the same chromosome and located side by side in it - one after another. They form linkage groups, the number of which is equal to the haploid set of chromosomes. For example, in humans, the karyotype is 46 chromosomes, which corresponds to 23 linkage groups. It was found that whatthe smaller the distance between the genes in the chromosome, the less often the process of crossing over occurs between them, which leads to the phenomenon of hereditary variability.
How genes located on the X chromosome are inherited
Let's continue to study the patterns of inheritance, subject to Morgan's chromosome theory. Genetic studies have established that both in humans and in animals (fish, birds, mammals) there is a group of traits, the mechanism of inheritance of which is influenced by the sex of the individual. For example, coat color in cats, color vision, and blood clotting in humans are controlled by genes located on the sex X chromosome. Thus, defects in the corresponding genes in humans phenotypically manifest themselves in the form of hereditary diseases called gene diseases. These include hemophilia and color blindness. The discoveries of G. Mendel and T. Morgan made it possible to apply the laws of genetics in such important areas of human society as medicine, agriculture, breeding of animals, plants and microorganisms.
The relationship between genes and the properties they define
Thanks to modern genetic research, it was found that the laws of independent inheritance of traits are subject to further expansion, since the “1 gene - 1 trait” ratio underlying them is not universal. In science, cases of multiple action of genes, as well as the interaction of their non-allelic forms, have become known. These types include epistasis, complementarity, polymeria. So it was found that the amount of skin pigmentmelatonin, which is responsible for its color, is controlled by a whole group of hereditary inclinations. The more dominant genes responsible for pigment synthesis in the human genotype, the darker the skin. This example illustrates an interaction such as polymer. In plants, this form of inheritance is inherent in species of the cereal family, in which the color of the grain is controlled by a group of polymeric genes.
Thus, each organism's genotype is represented by an integral system. It was formed as a result of the historical development of a biological species - phylogenesis. The state of most traits and properties of an individual is the result of the interaction of genes, both allelic and non-allelic, and they themselves can influence the development of several traits of the organism at once.
