As a result of research by scientists C. Correns, G. de Vries, E. Chermak in 1900, the laws of genetics, formulated back in 1865 by the founder of the science of heredity - Gregor Mendel, were "rediscovered". In his experiments, the natural scientist used the hybridological method, thanks to which the principles of the 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, subsequently called the laws of Mendel. For example, he formulated the rule of uniformity of hybrids of the first generation (the first law of Mendel). He pointed to the fact that only one trait controlled by the dominant gene manifested itself in F 1 hybrids . So, when crossing plants of sowing peas, the varieties of which differed in the color of the seeds (yellow and green), all hybrids of the first generation had only yellow staining of seeds. Moreover, all these individuals also had the same genotype (they were heterozygotes).
Law of splitting
Continuing to interbreed individuals taken from first-generation hybrids, Mendel received in F 2 a splitting of characters. In other words, plants with a recessive allele of the studied trait (green color of seeds) in the amount of one third of all hybrids were phenotypically identified. Thus, the established laws of independent inheritance of characters allowed Mendel to trace the mechanism of transmission of both dominant and recessive genes in several generations of hybrids.
Di- and polyhybrid crossbreeding
In subsequent experiments, Mendel complicated the conditions for their implementation. Now, for breeding, plants were taken that differ in both two and a large number of pairs of alternative characters. The scientist traced the principles of inheritance of dominant and recessive genes and obtained the results of cleavage, which can be represented by the general formula (3: 1) n , where n is the number of pairs of alternative characters that distinguish the parental individuals. So, for dihybrid crossbreeding , phenotype cleavage in hybrids of the second generation will have the form: (3: 1) 2 = 9: 6: 1 or 9: 3: 3: 1. That is, in hybrids of the second generation, 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 green wrinkled seeds (1/16 part ) Thus, the laws of independent inheritance of characters received their mathematical confirmation, and polyhybrid crossbreeding began to be considered as several monohybrid ones - “overlapping” one another.
Types of inheritance
In genetics, there are several types of transmission of signs 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 several - polygenic inheritance. Earlier, we examined the laws of independent inheritance of characters for mono- and dihybrid crosses, namely, the first, second, and third Mendel law. Now we will consider such a form as linked inheritance. Its theoretical basis is the theory of Thomas Morgan, called chromosome. The scientist proved that along with traits passed on to offspring independently, there are types of inheritance such as autosomal and sex-related linkage.
In these cases, several traits in one individual are inherited together, as they are controlled by genes located on the same chromosome and located next to it - one after another. They form cohesion 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 the smaller the distance between the genes in the chromosome, the less often the crossing-over process occurs between them, which leads to the phenomenon of hereditary variation.
How genes localized on the X chromosome are inherited
We continue to study the laws of inheritance that obey Morgan's chromosome theory. Genetic studies have found that both humans and animals (fish, birds, mammals) have a group of traits whose inheritance is influenced by the sex of the individual. For example, hair color in cats, color vision, and blood coagulation in humans are controlled by genes located on the sex X chromosome. Thus, defects in the corresponding genes in humans are phenotypically manifested in the form of hereditary diseases called genes. 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 their properties
Thanks to modern genetic studies, it was found that the laws of independent inheritance of traits are subject to further expansion, since the ratio “1 gene - 1 trait” underlying them is not universal. In science, cases of the multiple action of genes, as well as the interaction of non-allelic forms, have become known. These types include epistasis, complementarity, polymerization. So it was found that the amount of skin pigment melatonin, 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 polymer genes.
Thus, in each organism, the genotype is represented by a holistic system. It was formed as a result of the historical development of a biological species - phylogenesis. The state of most of the traits and properties of an individual is the result of the interaction of genes, both allelic and non-allelic, and they themselves can affect the development of several traits of the organism.