In genetics, the gene density of an organism's genome is the ratio of the number of genes per number of base pairs, usually written in terms of a million base pairs, or megabase (Mb). The human genome has a gene density of 12-15 genes/Mb, while the genome of the C. elegans roundworm is estimated to have 200.
Seemingly simple organisms, such as bacteria and amoebas, have a much higher gene density than humans. Bacterial DNA has a gene density on the order of 500-1000 genes/Mb. This is due several factors, including that the fact that bacterial DNA has no introns. There are also fewer codons in bacterial genes.