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

BIOLOGY

SECTION 1. BIOLOGICAL FOUNDATIONS OF HUMAN VITAL ACTIVITY

1.4. ONTOGENETIC LEVEL OF LIFE ORGANIZATION

1.4.2. Basics of Human Genetics

Gene Doses

Gene action is dose-dependent. An allele present in a single copy within the body's Cells ensures The Development of the corresponding trait up to a certain quantitative limit. As the allele "doses" increase proportionally, the expression of the trait increases as well. For instance, the synthesis of vitamin A in corn endosperm cells is controlled by the dominant allele Y. Since endosperm cells contain a triploid chromosome set, four different genotypes are possible — yyy, YYy, YYy, YYY — corresponding to vitamin A levels (in activity units) of 0.05, 2.25, 5.00, and 7.50, respectively. As can be seen from these data, The Effect of a single dose of the dominant gene corresponds to approximately 2.25 units of vitamin A activity. With an increase in the gene dose, its effect is summed up, or cumulated. This type of action is referred to as cumulative or additive. Certain human Chromosomal Disorders arise from Changes in the "dose" of genes on specific Chromosomes. For example, Turner syndrome (45, X0) is caused by the loss of one of the sex chromosome pair in the body's cells, whereas Down syndrome is characterized by the presence of three gene "doses" for chromosome 21. A normal female karyotype contains two X chromosomes (46, XX), while a male karyotype contains one (46, XY). The Y chromosome localizes only a few genes, whereas the X chromosome carries about 150. Compensation (reduction) of the "doses" of genes located on the female X chromosomes is achieved through the genetic inactivation of one of the two X chromosomes, which occurs after 16 days of intrauterine development via its heterochromatization and transformation into a Barr body.