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

Gene Expressivity and Penetrance, Pleiotropy

Genes controlling specific traits may not manifest in the phenotype of all carriers or may exhibit varying degrees of phenotypic expression. The quantitative measure of a gene's phenotypic expression is characterized by penetrance, while the qualitative measure is characterized by expressivity.

Penetrance is the frequency of phenotypic manifestation of a gene within a population of individuals carrying that gene. It is quantified as The ratio of individuals in whom the gene is phenotypically expressed to the total number of individuals carrying the gene (%). A dominant gene manifests in both homozygous (AA) and heterozygous (Aa) states, whereas a recessive gene manifests only in the homozygous state (aa). If a gene is expressed in all individuals carrying it, penetrance is termed 100%; otherwise, it is considered incomplete, and the percentage of individuals expressing the gene is specified. For example, with a penetrance of 20%, out of 100 individuals carrying the gene, it will manifest phenotypically in only 20 of them. Several Hereditary diseases are inherited with incomplete penetrance: Gout (20% in men), congenital hip dislocation (25%), and retinoblastoma (60%).

Expressivity (from Latin expressus – clear, distinct) is the degree of phenotypic expression of a gene, or the intensity of gene action. The same gene can be expressed more strongly or weakly under different conditions. For instance, polydactyly may appear on one or both hands or feet, and the number of digits may be six or more. The hereditary disease phenylketonuria exhibits varying severity of manifestation, ranging from mild mental retardation to profound idiocy.

Penetrance and expressivity depend on The Nature of the given gene, METABOLISM/18.html">The Influence of modifier genes, and environmental conditions. Modifier genes enhance or suppress the action of the primary gene controlling the trait.

Pleiotropy. The ability of a single gene to control multiple traits is called pleiotropy or multiple gene action. For example, Marfan Syndrome is a hereditary human disease typically characterized by a triad of features: 1) dislocation of the lens of the eye; 2) aortic aneurysm; 3) musculoskeletal abnormalities presenting as "spider fingers" (arachnodactyly), chest deformity, and joint hypermobility. All these Connective Tissue disorders are caused by a mutation in a single gene responsible for the Synthesis of the connective tissue fiber protein, fibrillin. The fibrillin gene is located on chromosome 15. The gene is dominant.