BIOLOGY Lecture Notes - Golden Pages 2003

7. FUNDAMENTALS OF VARIABILITY

Variability is a universal property of living organisms to acquire new traits or novel combinations of traits. Variability is one of the main driving forces of evolution and the primary source for The Emergence of new species. It is subdivided into non-heritable (modificational or phenotypic) and heritable (genotypic) variability.

Modificational variability is driven by the direct IMPACT OF ENVIRONMENTAL factors (Nutrition, Temperature, humidity, light) on a developing Organism, which induces phenotypic changes. Every organism exhibits specific limits to the variation of its traits, known as the reaction norm in response to a particular environmental influence. These changes affect only somatic Cell traits. While their genotype may undergo changes (somatic Mutations), these alterations are not transmitted to the genotype of offspring during sexual reproduction.

Ontogenetic variability is a type of phenotypic variation associated with changes in an organism's phenotype at various stages of individual development, while the genotype remains unchanged. This variability results from the differential expression of genes at different Stages of Ontogeny.

Genotypic variability is associated with alterations in the genotype of an organism's Germ Cells (the phenotype may remain unchanged), which subsequently manifest as trait modifications in the progeny. It is divided into combinatorial and mutational variability. Genotypic variability driven by Gene recombination is termed combinatorial variability. This process results in novel gene combinations within the genotype, driven by the independent assortment of Chromosomes during meiotic division, the random combination of Gametes upon Fertilization, and gene recombination via Crossing Over. The underlying Structure OF THE genes themselves remains unaltered; however, their novel combinations give rise to organisms with new phenotypes. Mutational variability involves alterations in the genetic material of a cell caused by changes in gene structure, chromosomal structure, or deviations in chromosome number. Mutations are classified into several types: gene (or point) mutations, chromosomal mutations, and genomic mutations.

Gene Mutations occur at the level of individual NUCLEOTIDES and are categorized into:

1) transitions (substitution of a purine for a purine or a pyrimidine for a pyrimidine);

2) transversions (substitution of Purines for Pyrimidines or vice versa);

3) nucleotide deletions;

4) nucleotide insertions.

Chromosomal mutations are structural alterations of chromosomes (aberrations, rearrangements) resulting from disruptions in the DNA nucleotide sequence or the loss of chromosomal segments following breaks or improper end-to-end rejoining. Structural rearrangements can occur within a single chromosome as well as between homologous and non-homologous chromosomes. Chromosomal mutations may be spontaneous or induced. The loss of genetically inert heterochromatic regions may have no phenotypic effect, whereas the loss of euchromatic regions disrupts the gene balance and manifests phenotypically as various pathological conditions (e.g., cri du chat syndrome is associated with a deletion of the short arm of chromosome 5).

Distinctions are made between intrachromosomal and interchromosomal rearrangements. Intrachromosomal mutations include:

✵ deletions — the loss of internal chromosomal segments;

✵ duplications — the doubling of chromosomal segments;

✵ inversions — the rotation of a chromosomal segment by 180°.

Interchromosomal rearrangements refer to translocations involving The transfer of a detached chromosomal segment to a new Location on a non-homologous chromosome (a chromosome from a different pair).

Genomic mutations involve A change in the chromosome number within the karyotype of individuals; they arise from mitotic or meiotic irregularities and are subdivided into several types.

Polyploidy is characterized by a proportional (multiple) increase in the number of complete genome sets in The Nucleus.

Haploidy refers to The Genome being present in a single copy.

Aneuploidy is an alteration in chromosome number resulting from the gain or loss of individual chromosomes.

Monosomy is the loss of a single chromosome from the genome. An example of monosomy in humans is Turner syndrome (45, X0), where females possess 45 chromosomes.

Polysomy is The addition of one or more chromosomes to the genome. Human Examples of polysomy (trisomy) include Klinefelter syndrome (47, XXY) observed in males, as well as Triple X syndrome described in females.

Nullisomy occurs As a result of the loss of both homologues of a chromosome pair. This type of chromosomal number variation typically leads to an arrest of embryonic development.