What is a gene?
The term gene was coined in 1909 “to describe the Mendelian unit of heredity”, long before it was suggested and established that these units resided in the chromosomal DNA of organisms.
I would suggest that this definition is as good as any that followed the latter discovery. Thus, modern general dictionary definitions such as the one below from Mirriam–Webster are an explanation to the layman of the chemical nature of genes, and the example given of lay usage bears no relation to this.
Definition of gene
a specific sequence of nucleotides in DNA or RNA that is located
usually on a chromosome and that is the functional unit of inheritance
controlling the transmission and expression of one or more traits by
specifying the structure of a particular polypeptide and especially a
protein or controlling the function of other genetic material
Examples of gene in a Sentence
She inherited a good set of genes from her parents.
I am most surprised that @WYSYWYG wrote:
A single gene is on one of the DNA strands, not both.
In my whole career as a molecular biologist I have never read this in any textbook or research paper, and challenge him or anyone else to produce one. So despite the votes for his accepted answer, I would maintain that it is incorrect and that in general scientific usage
In referring to genes encompassing regions of a double-stranded DNA
chromosome, the term includes both strands.
When it was found by sequencing that certain bacterial genes overlap (I remember it well) there was no redefinition of the word gene.
It is difficult to support my point of view just by saying “I have never heard anyone use the term that way”, so let us consider the implications of the ‘one-stranded gene’ thesis.
The concern would appear to be that only one strand of the DNA is transcribed into the precursor of the mRNA that is expressed as protein. On the ‘one-stranded gene’ thesis the gene would only be on the anti-sense strand — the strand complementary to the mRNA.
Let us now consider the genes of viruses — nobody would deny viruses have genes. However some viruses, both DNA and RNA viruses are single stranded. Single-stranded DNA viruses come in both ‘flavours’ — what are called ‘positive sense’ and ‘negative sense‘ genomes. And paroviruses package single-stranded DNA of both types. So clearly among these genomes are heredity units in the DNA that are not ‘anti-sense’ as well as some that are ‘anti-sense’. On the ‘one-stranded gene’ thesis these would not be genes, but would only become so when the double-stranded replicative form of the DNA was formed! This is patently absurd!
Likewise there are positive and negatively stranded RNA viruses. In the positive-stranded ones the genomic RNA is also the mRNA, but in the negative-stranded ones it is not. Presumably this latter genome has no genes at all, perhaps it is even wrong to say it has a genome.
Scientific terminology can be important for clarity of communication. However gene terminology is not of that type. Our knowledge of the structure of genes has developed, but rather than argue about what precisely is a gene (do we include introns?) we describe its different structural features and try to understand their function.