Phenotype and genotype

Phenotypic (non-hereditary) variability

Changes in the phenotype under the influence of environmental factors occur without intervention in the genetic apparatus. This is an adaptive reaction that is group-specific and is not inherited.

The limits of such changes are called the reaction norm. It can be narrow (for qualitative traits, such as blood groups) or broad (for quantitative traits, such as weight or milk yield).

In bacteria, phenotypic (modification) variability occurs with high frequency due to the temporary blocking of enzyme activity by external factors. When normal conditions are restored, reversion to the original traits occurs. Main manifestations:

• Morphological: streptomycin causes elongation of Salmonella cells, low doses of penicillin turn anthrax bacilli (B. anthracis) into spherical forms, and ethyl alcohol inhibits flagella growth.
• Cultural: dissociation of colonies into S-forms (smooth — smooth, encapsulated, with a complete antigenic structure and high virulence) and R-forms (rough — rough, polymorphic, with reduced virulence).
• Biochemical: production of adaptive enzymes (for example, the synthesis of the enzyme for lactose breakdown in Escherichia coli begins only in the presence of this carbohydrate in the nutrient medium).

Genotypic (hereditary) variability

Unlike phenotypic variability, genotypic variability affects the DNA structure itself. The changes are passed on to offspring, occur with low frequency, and are practically irreversible (reversions are rare). In bacteria, it is realized through two main mechanisms: mutations and recombinations.

In eukaryotes, combinative variability also plays a crucial role, creating unique genotypes (the basis of biodiversity) through crossing over, independent assortment of chromosomes in meiosis, and random gamete fusion during fertilization.

Mutations as the basis of genotype alteration

Mutations are sudden and stable changes in the genetic material. By their origin, they are divided into spontaneous and induced. Induced mutations are caused by mutagens: physical (UV rays, X-rays), chemical (heavy metal salts, base analogues), or biological (measles and rubella viruses, transposons).

Classification of mutations by the level of damage:

1. Gene (point) mutations: nucleotide substitution (transition or transversion), insertion, or deletion leading to a reading frame shift.
2. Chromosomal aberrations: breaks and redistribution of segments. They include deletions (loss), duplications (doubling), inversions (180° rotation), and translocations (exchange of segments between non-homologous chromosomes).
3. Genomic mutations: changes in chromosome number. This can be haploidy, polyploidy (multiple increase, lethal for humans), or aneuploidy (change in the number of individual chromosomes due to non-disjunction in meiosis or mitosis).

To protect against mutations, there is DNA repair — the process of restoring damaged DNA by special enzymes (light and dark repair, SOS repair).

Recombinations in bacteria

Recombination involves the transfer of DNA segments from a donor cell to a recipient, resulting in the acquisition of new properties. Main pathways:

• Conjugation: direct transfer of DNA through a conjugation bridge. The ability to undergo conjugation depends on the presence of F-plasmids encoding sex pili.
• Transformation: uptake of an isolated fragment of chromosomal or plasmid DNA from the external environment.
• Transduction: transfer of genetic material from one bacterium to another using temperate bacteriophages.

Class and exam questions

What else to study on this topic

• Types and functions of bacterial plasmids (F, R, Ent, Col, Hly)
• DNA repair mechanisms (excision, photoreactivation, SOS repair)
• Classification of mutagenic factors (physical, chemical, biological)
• Types of transduction (abortive, generalized, specialized)
• Combinative variability and meiosis mechanisms

Other topics in the "Additional Topics" section

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