Credit: Christopher Chandler, Iowa State University, Michigan State University

Nematodes

One of the first multi-cellular organisms to have its genome decoded, way back in 1998, the nematode is a staple in many research labs. The nematode and its simple-minded cousins have about 20,000 genes. While similar in number to those of other animals, the nematode's genome contains only 100 million base pairs of DNA; one tenth the size of an average mammalian genome.

This is because more evolved organisms tend to have more non protein coding regions that regulate how, when and how much of a gene is expressed in different types of cells, and not necessarily have more genes. This fine-tuned regulation seems to play an important part in what makes mammals and other organisms unique.