Polynucleic acid-encoded entity that, at a minimum, can be transcribed.
The concept of what is a gene has changed since the origin of the gene idea. What follows is a list of answers to the question of what is a gene as shown in approximate order of increasing sophistication and in terms of who's ideas the concept is based upon.
• "Mendelian" (classical genetical) concept: Discrete unit of inheritance
• "Morgan" (chromosome theory) concept: Locus on a chromosome
• "Watson and Crick" (DNA structure) concept: sequence of nucleotides
• "Beadle and Tatum" I (biochemical) concept: one gene-one enzyme
• "Beadle and Tatum" II (biochemical) concept: one gene-one protein
• "Beadle and Tatum" III (biochemical) concept: one gene-one polypeptide
• "Modern" I (transcriptional) concept: one gene-one RNA
• "Modern" II (post-transcriptional) concept: one gene-more than one type of RNA, polypeptide, or protein (especially in eukaryotic systems)
This latter point considers RNA processing, polypeptide posttranslational modification (such as cleavage and modification of amino acids), and various other posttranslational manipulations that can occur of proteins (including, for example, glycosylation).
Note that a gene is no longer necessarily considered to give rise to a protein product. Note also that an entity need not have a DNA-based genome to possess genes. That is, RNA viruses in particular too possess genes. As with DNA-based organisms, those genes give rise to an RNA product (though with the caveat that the virus genome itself can be the sole RNA product, serving as both chromosome and mRNA).
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