Gene mutations. Multiple Allelism

Multiple allelicism determines phenotypic heterogeneity in human populations. It, in turn, is one of the foundations of the diversity of the gene pool. Multiple allelicism is caused by gene mutations that alter the sequence in the nitrogenous bases of the DNA molecule at a site that corresponds to a particular gene. These mutations can be harmful, useful or neutral. Harmful transformations provoke hereditary pathologies, with which multiple allelism is associated. For example, a mutation is known that changes the structure in one of the chains of the hemoglobin protein by transforming the glutamic acid code into a valine (amino acid) code in the gene at the terminal site. As a result of this transition, a hereditary pathology develops, such as sickle cell anemia. Overdominization is due to the fact that in the heterozygous state there is a stronger manifestation of dominant cells than in the homozygous. This phenomenon interacts with the effect of heterosis and has a connection with such signs as total life expectancy, vitality and other. In man, like in other eukaryotes, multiple allelicism is revealed in various forms. There is also a large number of Mendelian characters, which are determined by different interactions. Allelic genes are different forms inherent in the same gene, located in homologous chromosomes, in the same areas.

When predicting inheritance using Mendelian laws, it is possible to calculate the probability of the appearance of children with this or that modeling feature. To analyze the transition of characteristics from generation to generation, the most convenient methodological approach is the genealogical method, which is based on the construction of pedigrees.

It should be noted that the phenotypic manifestation of a single gene is affected by other genes. As examples of a complex gene interaction, regularities in the inheritance of the Rh-factor system can occur : Rh negative and Rh plus. In the process of studying in 1939 the blood serum of a patient who had given birth to a dead child and had a blood transfusion of a husband compatible with the ABO group in her medical history, antibodies were detected. They were called Rh-antibody, and the patient's blood group was Rh-negative. Rhesus-positive blood group is determined by the presence on the surface of red blood cells of a separate group of antigens encoded by structural genes. They, in turn, carry information about membrane polypeptides.

They contribute to a significant decrease in viability and lead to the death of the body lethal genes. They are divided into two groups. The first include recessive genes. They provoke the death of the body only in the homozygous state. The second group contains dominant genes. Their effect is also possible in the heterozygous state.

The manifestation of lethal genes can be noted at different stages of the development of the individual. These are the earliest stages (embryonic mortality), and very late stages (fetal death, provoking spontaneous abortion, the birth of unviable monsters). In addition to lethal, half-lethal genes are also known. They do not provoke the rapid death of the body, but significantly reduce its viability.

Dominant lethal genes are present in a much smaller number in comparison with recessive ones. The Merle factor belongs to the dominant category. Carriers of dominant genes, if desired, can be easily removed from breeding, since they have a characteristic phenotype. Part of the lethal genes provokes serious anomalies, the other part - the breakdown of physiological processes.