Polymorphism (biology): distinct forms within a population

Three types of sequence variation can be distinguished:

• Single Nucleotide Polymorphisms (SNP)
• 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. It is assumed that there is one SNP in 200 to 1000 base pairs in the human genome. If the SNP is located in the coding region of a gene sequence, this can result in the exchange 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 exchange in the DNA necessarily results in an amino acid exchange in the protein.

An insertion or - the opposite - a deletion is the insertion or loss of at least one nucleotide. This can lead (if both alleles are affected) to no activity being detectable for the gene product (an enzyme). Smaller insertions and deletions (<50 nucleotides) are referred to as INDELs.

CNVs (gene duplications, multiplications or deletions) can eventually affect the entire gene. This can lead, for example, to a significant increase in the metabolic capacity of the affected gene product.

It is repeatedly observed that in a group of people living under comparable conditions and jointly exposed to certain environmental influences, only some individuals are affected by health effects as a consequence of these environmental influences: For example, in rare cases, chain smokers live to a comparatively old age, whereas occasional smokers with a corresponding genetic predisposition may die early from lung cancer. Enzymes that can break down such foreign substances always play an important role in dealing with the environment and the foreign substances absorbed from it. Sequence variations can lead to significant differences in the degradation of foreign substances in the bodies of different individuals, as shown by studies of accidents and drug side effects. However, these findings have no practical utility applications to date. Considerable ethnic differences in the occurrence of sequence variations have also been observed, the reasons for which are still unclear.

The increasing accuracy with which sequence variations can be detected using genetic tests raises growing ethical problems: for example, should individuals with known risk factors be excluded from certain professions? What consequences will knowledge of a particular sequence variation have for the purchase of life insurance? On the other hand, improved information about risks (for example, an increased risk of heart attack with certain polymorphisms) could give an affected person reason to try to actively counteract a possible disease - for example, through a healthy diet, exercise and targeted preventive care - and thus possibly improve their own quality of life.