Consider gene is countable, can anyone give a concrete example of "a gene"? gene is a countable noun but people always say genes, so what is A gene? for instance, Each chromosome contains many genes, so, which part of the chromosome of Escherichia coli could be `A gene`?

There are about 20,000 genes in the human genome. Here are the first 5 genes (following alphabetical order) on chromosome 1:

* AADACL3: Arylacetamide deacetylase-like 3
* AADACL4: Arylacetamide deacetylase-like 4
* ACADM: acyl-Coenzyme A dehydrogenase, C-4 to C-12 straight chain
* ACTL8: Actin-like 8
* ADGRL2 (1p31.1): adhesion G protein-coupled receptor L2