There are many mechanisms that influence the gene transcription and/or availability of gene products the keyword is "gene expression regulation". One way that

There are many mechanisms that influence the gene transcription and/or availability of gene products the keyword is "gene expression regulation". One way that happens for instance is packing/unpacking of the chromatin (genes in densely packed chromatin are not accessible). The trick is that chromatin can easily pack or unpack depending on what is going on around. Another simpler possibility is that there is a molecule that binds (stochastically) near promoter region (the place from which the gene is being transcribed) and hence influences the rate at which the gene is transcribed.