Phenotypic (non-hereditary) variability
This type of variability is also called modification variability. Its essence is that under the influence of environmental factors, only the phenotype of the organism changes, while the genotype itself remains the same.
Main features of phenotypic variability:
- Adaptivity. Changes help the organism adapt to current conditions.
- Group character. Identical external factors cause similar changes in an entire group of individuals.
- Reversibility. Most often, when the factor disappears, the trait returns to its original state.
- Non-heritability. Traits acquired in this way are not passed on to offspring.
The limits within which a trait is able to vary are called the norm of reaction. It is strictly determined by the genotype. A distinction is made between a narrow norm of reaction, characteristic of qualitative traits (e.g., blood type), and a broad norm, which is typical of quantitative parameters (body weight, milk yield).
Hereditary (genotypic) variability
Hereditary variability affects the genetic apparatus and is passed on to subsequent generations. It is divided into two major categories: combinative and mutational.
Combinative variability does not create new genes, but forms unique genotypes by reshuffling existing ones. This is a crucial mechanism for maintaining biodiversity. It is based on three processes:
- Crossing over — the exchange of chromosomal segments occurring in prophase I of meiosis.
- Independent assortment of homologous chromosomes during anaphase of meiosis I.
- Random meeting of a spermatozoon and an ovum (gametes) at the moment of fertilization.
Mutational variability is associated with sudden and stable changes in the genetic material itself, which inevitably affect the phenotype. The biological significance of mutations is twofold: on the one hand, they are most often harmful and reduce adaptation, and on the other hand, it is they that create a hidden reserve of hereditary variability, accumulating in heterozygotes.
Classification of mutations
Mutations can be divided into several groups according to various criteria. By cell type, somatic mutations (arising in body cells and not transmitted during sexual reproduction) and generative mutations (occurring in gametes and inherited) are distinguished. By localization, they can be nuclear and cytoplasmic, and by cause, spontaneous and induced.
The most important classification is based on the level of organization of the hereditary material:
- Gene (point) mutations. A change in the nucleotide sequence within a single gene occurs (substitution, insertion, or deletion). This can lead to a reading frame shift, missense, or nonsense mutations. Anti-mutation mechanisms combat them — DNA repair systems (excision repair, photoreactivation, SOS repair), where enzymes and DNA polymerases correct errors.
- Chromosomal aberrations. Associated with breaks and redistribution of chromosomal segments. Within a chromosome, loss of a segment (deletion), its duplication (duplication), or rotation by 180 degrees (inversion) can occur. Exchange of segments between nonhomologous chromosomes (translocation) is also possible.
- Genomic mutations. A change in the number of chromosomes. These include haploidy (reduction of the set to a single one), polyploidy (multiple increase, e.g., 3n or 4n, which is lethal for humans), and aneuploidy/heteroploidy (change in the number of individual chromosomes, e.g., 2n ± 1, arising due to their nondisjunction).
Mutagenesis factors (Mutagens)
Stable changes in the genotype are caused by specific factors, which are generally divided into three groups:
- Physical: ionizing radiation (gamma and X-rays), ultraviolet light, extreme temperatures.
- Chemical: nitrogen base analogues, heavy metal salts, alkylating agents, and a number of drugs.
- Biological: mobile genetic elements (transposons), cellular metabolism products, as well as viruses (e.g., pathogens of measles and rubella).
Briefly: answers to topical questions
Which specific enzymes carry out nucleotide excision repair of DNA?
Nucleotide excision repair of DNA is carried out by a complex of specific enzymes, each performing its function at a specific stage.
This process involves:
- DNA-N-glycosylase — cleaves the bond between the damaged nitrogenous base and deoxyribose, participating in base excision repair.
- Endonuclease (repair endonuclease) — recognizes the damage and makes a cut in the DNA strand.
- Exonuclease — removes the damaged strand segment containing the defect (e.g., a thymine dimer).
- DNA polymerase — synthesizes a new strand fragment in place of the removed one, filling the resulting gap.
- DNA ligase (ligase) — restores the phosphodiester bond, joining the ends of the inserted segment to the main DNA strand.
Which human hereditary diseases are caused by aneuploidy?
Clinical examples of aneuploidies in humans include:
- Down syndrome — trisomy for chromosome 21, karyotype 47, 21+.
- Patau syndrome — trisomy for chromosome 13.
- Edwards syndrome — trisomy for chromosome 18.
- Turner syndrome — monosomy for sex chromosomes, karyotype 45, X0; the only monosomy compatible with life.
- Klinefelter syndrome — polysomy for sex chromosomes, karyotype 47, XXY.
- Triple X syndrome — polysomy for sex chromosomes, karyotype 47, XXX.
To memorize the types of chromosomal mutations, use the abbreviation DDIT: Deletion, Duplication, Inversion (intrachromosomal) and Translocation (interchromosomal).
Class and exam questions
Are somatic mutations inherited?
No, during sexual reproduction, somatic mutations are not passed on to offspring. They remain only in the organism in whose body cells they arose.
What is the difference between polyploidy and aneuploidy?
Polyploidy is a multiple increase in the entire chromosome set (3n, 4n). Aneuploidy affects only individual chromosomes, increasing or decreasing their number (e.g., the appearance of a single extra chromosome — trisomy).
How does the body protect itself against gene mutations?
Cells use DNA repair systems, such as excision repair, photoreactivation, and SOS repair. Special enzymes find damage and restore the correct nucleotide sequence.
What else to study on this topic
- Molecular mechanisms of DNA repair
- Reading frame shift: missense and nonsense mutations
- The effect of biological mutagens on the genome
- Crossing over and independent assortment of chromosomes in meiosis
- Aneuploidy and its consequences using human diseases as an example
Master the entire topic in the "Biology" course
This is a summary. The course covers everything tested in classes and exams.
- Complete study guide on «Variability» and another 3,793 pages of the subject
- Assignments and quizzes after every topic — just like in a colloquium
- AI tutor answers questions on the topic anytime
- 129 video lessons on the subject
Other topics in the "Medical Genetics" section
Free video lessons: BiologyShort lessons with study notes — no registration required.
Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.