Genes

In biology, the word gene (from γένος, ; meaning generation or birth or gender) can have several different meanings. The Mendelian gene is a basic unit of heredity and the molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protein-coding genes and noncoding genes.

During gene expression, the DNA is first copied into RNA. The RNA can be directly functional or be the intermediate template for a protein that performs a function. The transmission of genes to an organism's offspring is the basis of the inheritance of phenotypic traits. These genes make up different DNA sequences called genotypes. Genotypes along with environmental and developmental factors determine what the phenotypes will be. Most biological traits are under the influence of polygenes (many different genes) as well as gene–environment interactions. Some genetic traits are instantly visible, such as eye color or the number of limbs, and some are not, such as blood type, the risk for specific diseases, or the thousands of basic biochemical processes that constitute life.

Genes can acquire mutations in their sequence, leading to different variants, known as alleles, in the population. These alleles encode slightly different versions of a gene, which may cause different phenotypical traits. Usage of the term "having a gene" (e.g., "good genes," "hair color gene") typically refers to containing a different allele of the same, shared gene. Genes evolve due to natural selection / survival of the fittest and genetic drift of the alleles.

The concept of gene continues to be refined as new phenomena are discovered. For example, regulatory regions of a gene can be far removed from its coding regions, and coding regions can be split into several exons. Some viruses store their genome in RNA instead of DNA and some gene products are functional non-coding RNAs. Therefore, a broad, modern working definition of a gene is any discrete locus of heritable, genomic sequence which affect an organism's traits by being expressed as a functional product or by regulation of gene expression.

The term gene was introduced by Danish botanist, plant physiologist and geneticist Wilhelm Johannsen in 1909. From p. 124:  "Dieses "etwas" in den Gameten bezw. in der Zygote, … – kurz, was wir eben Gene nennen wollen – bedingt sind."  (This "something" in the gametes or in the zygote, which has crucial importance for the character of the organism, is usually called by the quite ambiguous term Anlagen [primordium, from the German word Anlage for "plan, arrangement ; rough sketch"].  Many other terms have been suggested, mostly unfortunately in closer connection with certain hypothetical opinions.  The word "pangene", which was introduced by Darwin, is perhaps used most frequently in place of Anlagen.  However, the word "pangene" was not well chosen, as it is a compound word containing the roots pan (the neuter form of Πας all, every) and gen (from γί-γ(ε)ν-ομαι, to become). Only the meaning of this latter [i.e., gen] comes into consideration here ; just the basic idea – [namely,] that a trait in the developing organism can be determined or is influenced by "something" in the gametes – should find expression. No hypothesis about the nature of this "something" should be postulated or supported by it. For that reason it seems simplest to use in isolation the last syllable gen from Darwin's well-known word, which alone is of interest to us, in order to replace, with it, the poor, ambiguous word Anlage. Thus we will say simply "gene" and "genes" for "pangene" and "pangenes". The word gene is completely free of any hypothesis ; it expresses only the established fact that in any case many traits of the organism are determined by specific, separable, and thus independent "conditions", "foundations", "plans" – in short, precisely what we want to call genes.) It is inspired by the Ancient Greek: γόνος, gonos, that means offspring and procreation.