It is impossible to imagine modern medicine without such a simple method of treatment as blood transfusion. At the dawn of the application of this method, doctors simply pumped fluid from one patient to another, without thinking about the consequences. Subsequent monitoring of the patient's condition led to the conclusion that not all blood is suitable for transfusion. This was the introduction to the discovery of blood types.
Definition
Multiple allelism is the existence of more than two variants of one gene. It acts as a means of natural selection, that is, it prevents the fusion of germ cells of organisms that cannot have viable offspring.
This explains the sterility of some plants and animals, as well as the impossibility of the appearance of "children" in animals of different species. A variety of characters is also called "multiple allelism." Examples of this condition are: different eye color and coat color, shape of ears or nose. But this same factor also affects what kind of blood type a person will have.
Multiple allelism has certain patterns:
- any gene can have more than two alleles;
- a variant of a gene can arise both because of a direct or reverse mutation of another allele, and because of a change in a “wild” gene;
- in an organism with a diploid set of chromosomes, two different alleles can be simultaneously located;
- alleles are in a dominant recessive relationship with each other;
- inheritance of alleles is subject to the laws of Gregor Mendel.
Blood type inheritance
Multiple allelism is one of the manifestations of the variability of the genetic material of living organisms. One example of such variability is the inheritance of a blood group according to the AB0 system.
This system has certain features:
- there are three alleles of gene I - these are A, B and 0;
- this gene is located on the ninth chromosome;
- alleles A and B dominate over 0, but they are equal to each other;
- according to the laws of Mendel, dominant genes manifest themselves both in a homo - and heterozygous organism, and recessive genes only in a homozygous one according to this characteristic.
Different combinations of alleles give four blood groups: 0, A, B, and AB. They differ among themselves with antigens (agglutinogens) present on the surface of red blood cells. Moreover, agglutinins constantly circulate in the liquid part of the blood . These are specific antibodies that do not coincide with agglutinogens.
The phenomenon of multiple allelism provides a variety of phenotypic manifestations of this trait in humans. The blood group is a good example of the norm of the reaction, since it does not change under the influence of any factors throughout life.
Antigenic systems
Multiple allelism is a gene mutation that persists in a population. One of the proteins that is affected by the mutation is the Rh factor. This protein is inherited in a dominant manner. With a blood transfusion or pregnancy, Rh incompatibility may occur. In this case, the blood begins to clot and DIC develops.
The blood of the mother and the fetus passes through the chorionic villi and does not mix. If during the first pregnancy a Rh-negative mother develops a Rh-positive baby, then in the process of childbirth his red blood cells with expressed antigens can enter the vascular system of a woman and cause an allergic reaction of a delayed type. That is, antibodies will accumulate in the body, but before repeated pregnancy they will not manifest themselves in any way. If the second child is also Rh-positive, a conflict will arise between antibodies and antigens. This can ultimately lead to fetal death.
Gene interaction
Multiple allelism is a manifestation of the same gene in different physiological variants. There are not only agglutinogens A, B and 0. Scientists distinguish more than two hundred agglutinogens, united in twenty groups. This determines the uniqueness of blood groups in each individual person on earth.
The difference between these groups from the AB0 system is that there are no agglutinins in them in the plasma. The most important are the agglutinogens Rh, MN, S, P, A, Levis, Duppi, Kell, Kidd and others. In the blood, various combinations of these antigens are possible.
Fortunately, with a single blood transfusion, it makes no sense to take into account all these protein molecules, since they will not cause a significant generalized allergic reaction. However, with frequent transfusions, it is not recommended to use the blood of the same donor.
Antigens of blood groups of the AB0 system
As we already know, in the human genotype there is always such a thing as multiple allelism. Blood groups are represented by antigens A and B, the appearance of which is due to the presence of gene I. They consist of protein and carbohydrates. The specificity of each antigen is determined by the terminal fragment of the carbohydrate chain.
For the formation of antigens A and B, an H-substance is required, for the presence of which the H gene is responsible. This structure is not an antigen, since all people have red blood cell membranes. People with the first blood group have only H-substance, but there are no antigens A and B.