As polymorphism ( Greek πολυμορφισμός Polymorphismos , diversity ') is referred to in the field of genetics the occurrence of multiple genetic variants within a population . The different variants of a particular gene on the same locus ( locus ) can also alleles mentioned.
Genetic variations do not have to result in distinguishable gene products in every case and can be restricted to the genotype . A variation of the base sequence of the DNA alone , a sequence variation , is called a polymorphism in molecular biology if there are several variants that can be converted into one another by mutation . Sequence variations within the coding DNA segments, however, usually have an effect on the gene product , and different proteins are then formed according to the variant templates .
A different gene product can lead to characteristic variations in the appearance of organisms. If several clearly distinguishable phenotypes occur as a result of variant genes , this is referred to as a polymorphism in biology . For this, by definition, the rarer gene variant within a population must have an occurrence frequency ( allele frequency ) of over one percent; otherwise, a sequence variation or a mutation is also used here .
An enzyme or protein polymorphism exists when a polymorphism of genetic information - in the genome - also means a distinctness of the respective gene product, through the transcription in RNA - in the transcriptome out through - Translation then also of the synthesized protein - in the proteome - causes , for example a particular enzyme .
Polymorphism , heteromorphism and discontinuous diversity can also be used as synonyms for polymorphism .
Three types of sequence variations can be distinguished:
- Single nucleotide polymorphisms ( single nucleotide polymorphisms , SNPs)
- small insertion and deletion polymorphisms (usually <50 nucleotides ), also called INDELs .
- Structural variants (or copy number variants, CNVs).
The most common sequence variations are the single nucleotide polymorphisms (SNP), which are based on the exchange of a nucleotide in the DNA molecule . An SNP per 200 to 1000 base pairs in the human genome is assumed. If the SNP is in the coding region of a gene sequence, this can result in the replacement of an amino acid in the resulting protein . However, since several different base triplets often stand for the same amino acid in the genetic code , not every base change in the DNA necessarily results in an amino acid change in the protein .
An insertion or - on the contrary - a deletion is understood to mean the incorporation or loss of at least one nucleotide. This can mean (if both alleles are affected) that no activity can be detected for the gene product (an enzyme ). Smaller insertions and deletions (<50 nucleotides ) are called INDELs .
CNVs ( gene duplications , duplications or deletions ) can ultimately affect the entire gene . This can lead, for example, to a considerable increase in the metabolic capacity of the gene product concerned.
Effects of sequence variations
It is observed again and again that in a group of people who live under comparable conditions and are jointly exposed to certain environmental influences, only a few individuals are affected by health effects as a result of these environmental influences: In rare cases, for example, chain smokers get comparatively old, while occasional smokers with With a corresponding genetic predisposition, they can die early from lung cancer. When dealing with the environment and the foreign substances absorbed from it, enzymes always play an important role, which can break down such foreign substances. Sequence variations can, as studies of accidents and drug side effects show, lead to significant differences in the breakdown of foreign substances in the body of different people. However, these findings have not yet been used in practical applications. Considerable ethnic differences were also observed in the occurrence of sequence variations, the reasons for which are still unclear.
The increasing accuracy with which sequence variations can be detected on the basis of genetic testing raises growing ethical problems: Should people with known risk factors be excluded from certain occupations ? What consequences will the knowledge of a certain sequence variation have for the conclusion of a life insurance ? On the other hand, better information about risks (for example an increased risk of heart attack with certain polymorphisms) could give an affected person an opportunity to actively counteract a possible disease - for example through healthy nutrition, sporting activity and targeted preventive measures - and thus possibly improve their own quality of life .
Polymorphism in externally visible features
Originally, the term polymorphism stood for a term that almost exclusively recorded externally visible features, i.e. the relationship between phenotypes of different shapes (morphs). The best-known examples therefore also correspond to this conception, which is limited to features of external appearance:
- If there are different occurrences of appearance within a species , one speaks of a phenotype polymorphism. Many species show at least one sexual dimorphism , provided that males and females differ from one another. Another form of polymorphism is the social or caste polymorphism, as can be observed, for example, in state-forming insects, especially ants and termites . A temporal or seasonal polymorphism is present when the generations of a population that occur at different times in the year develop different morphs, as occurs, for example, in some butterflies. In addition, for example, snails have several color variants next to each other (color polymorphism). Behavioral polymorphism has also been described (polyethism).
- Single Nucleotide Polymorphism (SNP)
- Restriction Fragment Length Polymorphism (RFLP)
- Genetic variation (human)
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