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

BIOLOGY

SECTION 1. BIOLOGICAL BASES OF HUMAN VITAL ACTIVITY

1.4. ONTOGENETIC LEVEL OF LIFE ORGANIZATION

1.4.2. Fundamentals of Human Genetics

Human Variability as a Property of Life and a Genetic Phenomenon

Individuals of any species differ from one another in many traits, and humans are no exception in this regard. People vary in Skin, eye, and Hair color, body and organ mass and linear dimensions, Blood Cell counts, arterial and venous blood pressure values, erythrocyte Antigens (Blood Groups, Rh system), enzyme activities, and histocompatibility complexes. These and other differences determine human individuality, which must be taken into account in transplant surgery, transfusiology, forensic medical practice, and Medical-Genetic Counseling.

These differences are driven by both genetic and environmental factors, both of which are equally important. However, their relative roles in trait development are not identical. All traits are genetically determined, but some are exclusively genetic in nature—meaning that under all environmental conditions compatible with life, a given genotype produces the exact same phenotype (e.g., blood groups). Other differences are also genetically determined, but the degree of their phenotypic expression is influenced by the environment. For instance, the erythrocyte count in peripheral blood is often related to the altitude above sea level, although the Organism's inherent ability to adjust this count within certain limits, depending on atmospheric oxygen partial pressure, is dictated by its genotype. Many differences are shaped by both HEREDITY AND ENVIRONMENT. Human height is controlled by several dominant non-allelic genes (polygenes) and also depends on nutritional habits.

Differences among individuals of the same species are referred to as variability. Two forms of variability are distinguished: hereditary and non-hereditary. Non-hereditary variability encompasses phenotypic (modification) variability, whereas hereditary variability includes genotypic variability. Genotypic variability is further subdivided into mutational and combinative variability. While the phenotype changes in all forms of variability, in genotypic variability these changes are driven by alterations in the genotype, whereas in phenotypic variability they are caused by environmental factors.