MEDICAL BIOLOGY, ANATOMY, HUMAN 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

Basic Concepts and Terms of Modern Genetics

The subject of genetics is The Study of two fundamental yet opposing properties inherent in All living organisms.

Heredity is the ability of organisms to transmit their traits and developmental features to offspring, ensuring material and functional continuity across generations. Each species of Organism preserves and reproduces its own kind through a series of generations. However, heredity is invariably accompanied by Variability. Children always resemble their parents, yet they are never exact copies; they differ both from their parents and from one another.

Variability is the ability of organisms to acquire new traits or lose previous ones during development. It also refers to the differences among individuals of the same species. Variability ensures The Diversity of the living world and its adaptability to changing environmental conditions. Although it stands in contrast to heredity, variability is closely linked with it; together, they form the foundation of evolution.

Genotype is the complete set of genes within an organism. It serves as the hereditary foundation (genetic constitution) of a given organism as a whole or in relation to a specific inherited trait.

Phenotype is the sum total of all external and internal traits of an organism. The phenotype results from the interaction between the genotype and environmental conditions. It never serves as a complete reflection of the entire genotype, but rather reveals only that portion of it which is expressed under specific conditions. The terms "genotype" and "фенотип" were first introduced in 1909 by the Danish geneticist W. Johannsen.

Allelic genes, or alleles (from Greek *allelos* meaning mutually, opposite), are alternative forms of the same Gene located at the same locus on homologous Chromosomes, which determine alternative (mutually exclusive) states of a single trait. Like chromosomes, allelic genes come in pairs. In every Cell of a diploid organism, any given gene is represented by two alleles, one inherited from the father and the other from the mother. The only exception is Germ Cells (Gametes), which contain a single allele for a given gene. Alleles can be either dominant or recessive. A dominant gene (allele) is expressed in the phenotype of a heterozygous organism, whereas a recessive gene (allele) is not expressed in the phenotype of a heterozygous organism.

Homozygous organism is an organism in which both homologous chromosomes carry identical alleles of a specific gene (e.g., AA or aa); such an organism produces only one type of gamete for that gene (A or a).

Heterozygous organism is an organism in which the two homologous chromosomes carry different alleles of a specific gene (Aa); such an organism produces Two Types of gametes for that gene (A and a).

Hemizygous organism (from Greek *hemi* - half) is an organism in which a gene is represented by only one allele instead of two, and this single allele is always expressed phenotypically, even if it is recessive. A hemizygous state is characteristic of haploid organisms, as well as diploid heterogametic organisms regarding genes localized on sex chromosomes. For example, males (XY) are hemizygous for genes responsible for hemophilia and color blindness located in the non-homologous region of the X chromosome, since they possess only one X chromosome, whereas the Y chromosome lacks these genes.