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

BIOLOGY

CHAPTER 1. BIOLOGICAL FOUNDATIONS OF HUMAN VITAL ACTIVITY

1.4. ONTOGENETIC LEVEL OF LIFE ORGANIZATION

1.4.2. Basics of human genetics

Crossing-over

Crossing-over (chromosomal crossover) is the reciprocal EXCHANGE OF GENETIC material between homologous Chromosomes. It occurs regularly during prophase I of Meiosis at the pachytene stage during gametogenesis (Spermatogenesis and oogenesis). During prophase I, homologous chromosomes (maternal and paternal) lie in the closest possible alignment (synapsis). Each chromosome consists of two chromatids. Before separating, the chromatids form X-shaped figures known as chiasmata. At the points of intersection, chromatids break and rejoin, resulting in the exchange of segments between maternal and paternal chromatids rather than between sister chromatids. As a rule, genetic material is exchanged between non-sister chromatids of homologous chromosomes, though it can occasionally occur between sister chromatids as well. Crossing-over disrupts Gene linkage, resulting in recombinant chromosomes with novel gene combinations and new linkage groups, which gives rise to offspring displaying new combinations of parental genes. Crossing-over plays a significant role in evolution as a primary mechanism driving combinatorial Variability.

In addition to meiotic crossing-over, which regularly takes place during gamete formation in meiosis, genetic exchange between chromatids of homologous chromosomes is occasionally observed in somatic Cells (Mitotic crossing-over). For example, an autosomal recessive human mutation known as Bloom syndrome exhibits cytological features resembling the synapsis of homologous chromosomes and even The formation of chiasmata.

The frequency of crossing-over is determined by The ratio of recombinant offspring to the total number of progeny from a test cross, expressed as a percentage. Recombination frequency between genes is directly proportional to the distance separating them. The closer genes are located to one another, the fewer potential crossover points can occur between them; conversely, the greater the distance between genes, the higher the likelihood of multiple crossover points. Crossing-over between two genes can occur not only at a single point, but also at two (double crossover) or even more points.

Crossing-over frequency reflects the degree of gene linkage and remains constant for any given pair of genes under identical conditions. For other genes on the same chromosome, the recombination frequency will necessarily differ, yet it remains constant within a range from a fraction of a percent up to nearly 50%. The constancy of the crossover percentage between genes serves as an indicator of their relative map distance. One centimorgan (morganide) is adopted as the unit of map distance, corresponding to a crossing-over frequency of 1%. In Morgan's experiments, recombinants accounted for 17% of the progeny. Consequently, the crossing-over frequency between genes B and V is 17%, corresponding to a distance of 17 centimorgans. Based on these findings, T. Morgan and his colleagues established METABOLISM/2.html">THE CONCEPT OF the linear arrangement of genes on chromosomes and proposed the principles of chromosome mapping.