Genotype and its variability, mutation and recombination

Genotype

Genotype is a specific combination of hereditary dispositions, a set of all alleles (forms of genes) of an individual. It could also be defined as the genetic constitution of an individual, or more specifically: a specific set of alleles at a specific locus or multiple loci.

• represents the genetic constitution of an organism represented by a set of alleles specifically arranged in the genome (therefore, they are all DNA or RNA molecules (in the case of RNA viruses) of a living system that are characterized by replication and are passed on to offspring)
• determines the extent and degree of phenotypic (set of all characters) possibilities of an individual

Mutation

The DNA sequence is subject to changes that are caused by the action of chemical, physical and biological agents or arise as rare errors during replication (endogenous mutations) - resulting in allelic variations.

• random hereditary changes in the genotype (genetic information) are called mutations
• mutations resulting from errors in DNA replication are called spontaneous mutations and occur without intervention from the external environment
• DNA polymerase is not only very accurate, but also has a self-correcting function – the probability of an error is in the order of about 10^-7
• the frequency of mutations is therefore very low, moreover, cells are to some extent capable of eliminating these errors thanks to repair enzymes
• most mutations are therefore so-called induced, i.e. induced by external mutagenic factors (mutagens) – these can be, for example, radiation (UV, X-rays), chemical substances (arenes, heavy metals, peroxides...) etc.
• mutations can occur to varying extents:

Gene point mutation

They take place at the level of the DNA molecule, affecting 1 gene. The result is a damaged nucleotide sequence, due to which triplets (codons) are changed and an error occurs in proteosynthesis (completely different amino acids are synthesized).

If the gene regulating the multiplication and differentiation of the cell is damaged, it can lead to uncontrollable growth (tumorous diseases).

It can happen:

• substitution - the exchange of one nucleotide for a nucleotide carrying another base, is among the most frequently occurring mutations
  • transition – exchange of purine-purine, pyrimidine-pyrimidine
  • transversion – exchange of purine – pyrimidine and vice versa
• deletion - loss of one or more nucleotides in a DNA sequence
• insertion – one or more nucleotides are inserted into the DNA sequence

Distribution of gene mutations in terms of effect on the gene product: both coding and non-coding sequences of the human genome can be subject to mutations, mainly mutations related to coding DNA have serious consequences

• synonymous (silent) – does not change the sequence of the gene product
• nonsynonymous - changes the sequence of a gene product (protein or RNA) with all possible consequences

Chromosomal mutation (chromosomal aberration)

There is a change in the number or structure of chromosomes - for example, during crossing over: chromosome fragments do not connect properly; but the entire block can also be lost.

These mutations disrupt the course of meiosis and cause gametes to become non-functional.

Types:

1. deletion (loss of part of a chromosome)
2. inversion (turning over part of a chromosome)
3. duplication (doubling of part of a chromosome)
4. translocation (attachment of part of the chromosome to the wrong chromosome)
5. fragmentation (breakdown of the chromosome into fragments)

Genomic mutations

Inversion of part of a chromosome

There is a change in the genome itself. Mostly it is either a multiplication of the entire chromosome set (euploidy = polyploidy = an individual has 3n or more...), or a change in the number of individual chromosomes from the set (aneuploidy).

Mutations can be distinguished according to the type of affected cells:

1. somatic – affect the offspring of the mutated cell, but are not transmitted between individuals
2. gametic - affect cells of the germline, can be transmitted from parents to offspring

Mutations can be distinguished according to the mechanism of origin:

1. spontaneous
2. induced

Recombination

Recombination is the name for various changes in DNA, consisting in splitting it and connecting it to another chain, which gives rise to new properties

1. Meiotic recombination – i.e. crossing – over
   • 

     deletion of part of a chromosome
     the process during which two homologous chromosomes paired in prophase I of meiosis exchange part of their DNA, the result of a correctly performed crossing-over is the exchange of part of the alleles between chromosomes - i.e. disruption of gene linkage
   • the consequence is a considerable increase in the variability of the offspring

Crossing-over is one of the main sources of genetic variability, in addition to mutations and the random separation of chromosomes into gametes, it does not create new alleles, but allows the creation of new combinations of already existing alleles of genes located on the same chromosome (see gene linkage)

• we distinguish between simple crossing-over (one crossing over occurs, chromatids swap ends) and multiple crossing-over (several crossed over, sections "inside" chromatids are also swapped)
• multiple crossing-overs disturb the calculations determining the strength of linkage and the distance of genes on the chromosome - a three-point test is used to filter them out

1. Mitotic recombination – very rare
   • recombinant DNA technology - a procedure in genetic engineering in which simple genes are isolated from cells and then introduced back into cells of the same or different species of organism

• these are biotechnological procedures that make it possible to create new combinations of DNA molecules that do not occur together in a natural organism
• recombinant DNA technology is used, for example, in gene therapy or genetic modification (GMO - e.g. modified corn or soy)
• The prerequisite for evolution is variability. Genetic variability is increased by mutation, recombination and gene flow. Recombination is the rearrangement of DNA material between already existing alleles (forms of genes) to create new alleles. This process creates new individuals with combinations of alleles different from those of their parents. This happens due to free combinability or crossing-over.

Links

Related articles

• Genotype
• Mutation
• Crossing-over
• Gametogenesis

Sources

• ŠTEFÁNEK, Jiří. Medicína, nemoci, studium na 1. LF UK [online]. [cit. 11.02.2010]. <http://www.stefajir.cz>.