Short Answer
In referring to genes on a double-stranded DNA chromosome (the situation assumed in this question), the general and scientific usage of the term ‘gene’ includes both DNA strands.
The practical definition of ‘gene’ has come under scrutiny in recent years for reasons that will be discussed, especially in relation to the ENCODE project. However none of the discussions I have encountered considers restricting a gene to a single strand.
General and educational usage of the term ‘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.
A reputable general dictionary, Mirriam–Webster, suggests a modern concept of the term, comprehensible to non-specialized readers, to be:
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
e.g. “She inherited a good set of genes from her parents.”
And a similar definition is given to the medical profession in Morton and Spences Genetics for Surgeons.
Despite the knowledge of the complexity of genes that has accumulated in recent years, in two modern molecular biology text books the essential feature of the definition is that it is all-embracing rather than restrictive. Thus, the definition in Alberts et al. — Molecular Biology of the Cell is:
Region of DNA that controls a discrete hereditary characteristic,
usually corresponding to a single protein or RNA. This definition
includes the entire functional unit, encompassing coding DNA
sequences, noncoding regulatory DNA sequences, and introns.
And the definition in the glossary of Lodish et al. — Molecular Cell Biology is very similar.
More recent considerations of the term ‘gene’ in the context of the ENCODE project
There are several features of the structure and the regulation of the chromosomal information specifying proteins that has led to a reconsideration of the use of the term ‘gene’. This in not merely a semantic concern, as the major ENCODE Project, the purpose of which was to provide an “Encyclopaedia of DNA Elements”, had the practical task of naming the elements it describes.
I have found a couple of useful articles by others, considering the problem at length. One is by Smith and Adkinson (2010) and the other is by Portin and Wilkins (2017). A brief summary of the situation that they discuss is that there are two main problems. One problem is the deviation from the ‘one gene — one mRNA — one polypeptide chain’ concept caused by alternative splicing, multiple promoters and alternative translational initiations sites. A second is the finding of regulation of transcription by sequences greatly distant from the transcriptional initiation site.
The general thinking is to redefine the term ‘gene’ in terms of networks or integrated interactions. Thus, Portin and Wilkins own proposal is:
A gene is a DNA sequence (whose component segments do not necessarily need to be physically contiguous) that specifies one or more sequence-related RNAs/proteins that are both evoked by Gene Regulatory Networks and participate as elements in Gene Regulatory Networks, often with indirect effects, or as outputs of Gene Regulatory Networks, the latter yielding more direct phenotypic effects.
and that employed by the ENCODE project is defined by Gerstein et al. as:
The gene is a union of genomic sequences encoding a coherent set of potentially overlapping functional products
- a gene is a genomic sequence (DNA or RNA) directly encoding functional product molecules, either RNA or protein.
- In the case that there are several functional products sharing overlapping regions, one takes the union of all overlapping genomic
sequences coding for them.
- This union must be coherent — i.e., done separately for final protein and RNA products — but does not require that all products necessarily
share a common sub-sequence
(It is supplemented by an ontological diagram that I shall not reproduce here.)
This is complex, but in relation to the poster’s concern one thing is clear, at no time are the authors concerned with strandedness, and there is no proposal that a gene is confined to a single strand.
Two simple arguments
One might argue that nobody mentions strandedness because everyone assumes a gene is on only one strand. Really? Anyway, I‘ll finish with a couple of mundane arguments that strandedness has no place in defining genes, overlapping or not.
- The Transcription Factor/RNA polymerase binding site — the TATA box — is regarded as an integral part of a gene. Both strands of the TATA box are required for binding. Likewise other transcription factor binding sites. Hence the gene cannot be on only one strand.
- Single-stranded DNA viruses are found with what are called ‘positive sense’ and ‘negative sense‘ genomes. So clearly among these genomes there are heredity units which read ‘anti-sense’ as well as some that are ‘anti-sense’. On the ‘one-stranded gene’ thesis one of these could not be called genes. One would have to devise some term from them as progenitors of the gene in the complementary strand in the replicative DNA form!
