Genetics with Basics of Selection - M.P. Myhun - 2008

CHAPTER IV. Variability and Its Genetic Foundations

4.4. Homologous Series in Hereditary Variability

N.I. Vavilov (1920), based on studying hereditary Variability in species populations and comparing the discovered regularities with similar data obtained for other taxonomic plant species, concluded that the hereditary variability of organisms is subject to the law of homologous series.

Genetically close species and genera are characterized by similar series of hereditary variability with such precision that, knowing a series of forms within one species, one can predict the existence of parallel forms in other species and genera. The closer genera and species are genetically positioned in the general system, the more complete the similarity in their variability series.

Entire plant families are generally characterized by a specific cycle of variability that runs through all the genera and and species forming the family.

M.I. Vavilov's formulation shows that the law of homologous series in the hereditary variability of organisms is based on The Emergence of series of similar Mutations in genetically related species of organisms. This is evidenced by the formula of the law through which it operates. If we designate a Linnaean species by the symbol L, and the series of traits characterizing this species by the symbols a, b, c..., this can be written as L (a+b+c...). Since series consist of species, by designating the genus as G, we can write G (L₁+L₂+L₃...). This constitutes the formula of homologous series in the hereditary variability of organisms. It provides a rather clear and visual way to express the complete similarity of these series.

Example. Let us designate the coloration of spikelet glutes in cereal plants as a, awnedness as b, and the number of flowers in a spikelet as c; then for wheat L₁, barley L₂, and rye L₃, we can write L₁(a+b+c...), L₂ (a+b+c...), L₃ (a+b+c...).

The formulas of these species demonstrate the existence of parallelism in the series of traits that characterize the taxonomic species of the grass family (Poaceae). Since each trait is characterized by its inherent variability, it is possible to compile a General scheme of varietal plant variability for a given family.

Today, it can be stated that evolutionarily related species of organisms generally give rise to mutually similar mutations.

Based on the law of homologous series in hereditary variability, if spontaneous or induced mutations are discovered in one taxonomic species of Organism, a similar pattern of variability for the exact same traits can be expected in organisms of genetically close species.

The law of homologous series in hereditary variability expresses the general regularity of the Mutational Process and organismal form-generation.

As a result of extensive work conducted by geneticists from various countries, key theoretical and practical issues regarding the utilization of the mutational process have been identified.

1. Mutations occur under natural conditions in all organisms, ranging from Bacteria to humans.

2. Mutations can be induced artificially, with their frequency of occurrence increasing manifold in the process.

3. Mutations are the result of complex physiological processes taking place within Cells and based on physicochemical reactions.

4. Mutations resulting from Changes in the Cell's genetic structures cause various phenotypic changes that affect diverse external and internal Features of the organism: morphological, physiological, and biochemical. These changes are stably transmitted to subsequent generations.

5. Under Modern Methods of Treatment, the mutational process is undirected, with the exception of obtaining polyploid forms.

6. The hereditary variability of organisms of a given species or form cannot proceed in any arbitrary direction; it is materially constrained by the capacities of its genetic structures to change.