The term “gene” is normally used to denote the ordered sequence of nucleotides located in a particular position – that codes for biologically active RNA – on a particular chromosome. The gene contains information for the production of a protein or for the execution of a regulatory function. A gene occurs in two alternative forms known as alleles. Of these alleles, which code for example for eye colour, one is inherited from the father and one from the mother. 

The synthesis or production of RNA, which is responsible for passing on information from DNA to the cell, takes place in the cell nucleus and is referred to as transcription. The subsequent protein biosynthesis (i.e. the building of proteins from amino acids) occurs in ribosomes within the cell in a mechanism referred to as translation. 

In cells of eukaryotic living beings (all living beings except for bacteria and archaea) there are coded sections (exons) and uncoded sections (introns) in the genes. The latter are not transcribed to RNA and do not code for protein. Introns have no direct function in protein structure and are thus cut out from the translation.

The “genetic code” is a form of encryption by means of which the “information” stored in DNA is translated into a protein. It follows the same principle in all living organisms, bacteria and plants, animals and humans, and is therefore universally named. Specific amino acids, which serve as the building blocks for the formation of proteins, are always encoded by three bases (triplets). For example, the triplet C-G-G codes for the amino acid arginine. 

Given the fact that with four different bases the number of possible triplet combinations (altogether 64) exceeds the number of amino acids (20), different triplets can encode the same amino acid; this is referred to as degeneration of the genetic code.

A genetic defect is a lasting mutation of the DNA in a gene that has damaging consequences for the organism. The defect can affect areas of the gene of varying size, from a single incorrect base (point mutation) to a lack or duplicate of complete gene segments. Genetic defects can occur accidentally, through erroneous DNA transcription, or as a result of an external influence of mutagens such as radioactivity or UV radiation.