MEDICAL BIOLOGY, HUMAN ANATOMY, PHYSIOLOGY AND PATHOLOGY - Y.I. Fedonyuk 2010

BIOLOGY

SECTION 1. BIOLOGICAL FOUNDATIONS OF HUMAN VITAL ACTIVITY

1.4. ONTOGENETIC LEVEL OF LIFE ORGANIZATION

1.4.2. Fundamentals of Human Genetics

Combinative Variability

This is a hereditary type of variability caused by new combinations of alleles in the offspring's genotypes. It occurs naturally during sexual reproduction. There are three mechanisms underlying its emergence, two of which are associated with Meiosis: 1) Crossing-over during prophase of the first meiotic division; 2) independent segregation of non-homologous Chromosomes in anaphase of the first meiotic division; and 3) random combination of Gametes during Fertilization. These three mechanisms serve as an inexhaustible source of diverse Gene combinations (genetic recombinations) and, consequently, of the traits controlled by these genes. The genes themselves remain unchanged, and no new traits are created, yet the genotypes and phenotypes of the offspring vary in every generation. Regardless of the parents' specific union, children exhibit combinative variability. This type of variability is exceptionally widespread in nature. Although microorganisms lack meiosis, genetic recombinations still occur in them through transformation, Transduction, and conjugation. Combinative variability enhances the hereditary diversity of populations and plays a significant role in evolution. Nevertheless, Mutations remain the primary and sole source of novel hereditary changes.