MEDICAL BIOLOGY, HUMAN ANATOMY, PHYSIOLOGY AND PATHOLOGY - Ya.I. Fedoniuk 2010

BIOLOGY

CHAPTER 1. BIOLOGICAL BASES OF HUMAN LIFE ACTIVITY

1.4. ONTOGENETIC LEVEL OF LIFE ORGANIZATION

1.4.2. Basics of human genetics

Blood Group Genetics; Inheritance of the ABO blood group system

The human ABO blood group system is inherited via multiple alleles of a single autosomal Gene, the locus of which is designated by the letter I (derived from isohemagglutinogen). There are three multiple alleles: I^a, І^в, І (the allele i is also denoted as I°). The alleles I^а and І^В are dominant to the allele i, while being codominant with each other. The allele I^А controls the synthesis of antigen A, the allele І^в controls antigen B, and the allele i produces neither. These Antigens are found On the surface of erythrocytes as well as other Cells (leukocytes, platelets, and tissue cells). Every individual inherits any two alleles out of the three possible ones. Depending on their combination, there are 4 Blood Groups (4 phenotypes), the differences between which are associated with the presence or absence of specific substances: agglutinogens (antigens) A and B on the erythrocyte surface, and agglutinins (Antibodies) a and P in Blood Plasma. These four phenotypes correspond to six genotypes (Table 1.7).

Class="center">Table 1.7.

ABO blood groups

Antigen A and antibody a never occur together, nor do antigen B and antibody β. When antigens interact with their corresponding antibodies, erythrocyte agglutination (clumping) occurs, which indicates incompatibility between donor and recipient blood. During blood transfusion, it is essential that the donor's antigens do not encounter the corresponding antibodies of the recipient. Since group I has no antigens,

individuals with this blood type are referred to as universal Donors, whereas those with group IV are considered universal recipients.

The inheritance of two alleles out of three possible follows Mendelian patterns. Blood groups II (A) and III (B) are inherited in an autosomal dominant manner, while group I (0) is autosomal recessive. If both parents have blood group II (A), their children may have group II (A) or I (0), but not III (B) or IV (AB). The fourth blood group (AB) is inherited not according to G. Mendel's rules, but via codominance. Since blood groups are genetically determined and remain unchanged throughout life, their determination can be helpful in cases of disputed paternity. It must be kept in mind, however, that blood typing cannot definitively prove that a specific man is the child's biological father; it can only indicate that he could be the father or that paternity is excluded.

In individuals with blood group IV (AB), a special gene arrangement known as the cis-position is observed in 0.1–0.2% of cases, where both genes I^А and I^В are located on the same chromosome. In such cases, if a person marries someone with blood group I (0), the birth of children with blood group I (0) is possible, which must be taken into account in MEDICAL Genetic Counseling and forensic medical examinations.

Over 20 different blood systems are distinguished based on the presence of specific antigens, including the Rhesus factor.