Genetics - A. V. Sivolob 2008

Variability of Genetic Material
Consequences of Mutational Variability

The overall consequence of mutational Variability is the disruption of the hereditary programs of Cells and organisms. At the same time, however, mutational variability drives an increase in biological diversity by generating new genomic variants and, consequently, new genotypic and phenotypic forms. Most resulting genome variants are relatively neutral. A large proportion of newly formed phenotypes are either unviable (manifesting as lethal Mutations) or exhibit reduced viability (semi-lethal mutations). Nevertheless, new variants occasionally acquire adaptive advantages. Adaptive and neutral variants become fixed within populations, laying the foundation for phenomena such as multiple allelism and genetic polymorphism (see Chapters 3 and 8).

It is important to note that point mutations do not necessarily alter the hereditary program only when they occur within coding Regions of the genome. Nucleotide substitutions in non-coding regions can also impact Gene Expression: substitutions in the 5'- or 3'-untranslated regions of mRNA can affect mRNA half-life; substitutions in introns can influence splicing efficiency; and alterations in regulatory gene regions can modify expression levels. Consequently, DNA polymorphism in non-coding regions is not always neutral.

When discussing the CONSEQUENCES OF MUTATIONAL variability, a distinction must be made between effects on individual organisms (individual consequences) and impacts on populations and species as a whole (evolutionary consequences). Mutations in somatic cells often produce negative effects in single organisms; for instance, all Selection/21.html">Types of mutations—from point mutations to genomic aberrations—can trigger malignancies. However, such mutations are not inherited by subsequent generations. Germline mutations give rise to individuals in whose cells every single one will carry the given alteration. A typical manifestation of such mutations is various hereditary disorders (see Chapter 7).

Naturally, for populations and groups of organisms—and consequently for speciation—germline mutations are by far The most significant. Throughout the evolution of various vertebrate classes (from jawless fish to mammals), a substantial number of chromosomal and genomic rearrangements have occurred, predominantly consisting of translocations, inversions, and changes in chromosome number. For example, humans differ from great apes by a Robertsonian translocation (the second human chromosome is a translocation product of two acrocentric Chromosomes found in apes, see Chapter 7).

Genomic mutations (polyploidy and aneuploidy) play a crucial role in the evolution of flora and fauna. However, as a rule, the absence of one chromosome from a homologous pair leads to reduced individual viability or, in some cases, proves lethal. This effect can be explained either by a deficiency in specific protein products (since genes from only a single chromosome are expressed) or by the presence of lethal allele variants for certain genes on the remaining chromosome in the aneuploid Nucleus. Notably, the Phenotypic effect of monosomy depends on which specific chromosome is lost. In humans, for example, monosomy for the sex X chromosome (Turner syndrome, see Chapter 7) has a less severe impact on the Organism compared to autosomal monosomies, which are lethal. In Drosophila, individuals lacking the fourth chromosome are unviable, whereas the loss of the second, third, or X chromosome is not lethal. In plants (such as wheat, corn, and tobacco), monosomic forms may be indistinguishable from normal diploid organisms or exhibit only minor differences in the size of individual plant parts.

Regarding the presence of an extra chromosome in the karyotype, reduced viability or lethality resulting from trisomy is generally characteristic of the animal kingdom. Trisomic plants, by contrast, are often viable and display only minor deviations from normal organisms; in the Jimson weed (Datura stramonium), for instance, viable trisomics have been described for each of the 12 chromosome pairs.

Polyploidy is most widespread in the plant kingdom. An increase in chromosome sets in plants leads to greater vegetative biomass and enhanced resistance to adverse environmental conditions. The prevalence of polyploid forms in plants is largely due to their capacity for vegetative propagation and self-pollination. In contrast, polyploid forms in animals are most often unviable because polyploidy is fundamentally incompatible with sexual reproduction. Polyploidy in animals frequently results in sterility, particularly in allopolyploid forms. Accordingly, polyploidy is more characteristic of animals that reproduce via parthenogenesis.