*Escherichia coli* is a model gram-negative prokaryotic organism
*Drosophila melanogaster*, one of the most famous subjects for experiments in genetics
A **model organism** is a non-human species that is studied over many years, building up a lot of knowledge about it in order to understand fundamental biological phenomena. The hope is that discoveries made in the model will give insight into how other organisms work.

In particular, model organisms are widely used in animal testing to explore potential causes and treatments for human disease when experimenting on humans would be impossible or less ethical.

This strategy is made possible by the similarities of all living organisms. They are similar because of their common descent and the conservation of metabolic and developmental pathways and genes over the course of evolution.

Model organisms in genetics
---------------------------

### *Drosophila melanogaster*

*Drosophila melanogaster* was one of the first animals used for genetics. Today it is one of the most widely used and genetically best-known of all eukaryotic organisms.

All organisms use common genetic systems; understanding transcription and replication in fruit flies helps to understand these processes in other eukaryotes, including humans.

Studies of X-linked traits confirmed that genes are found on chromosomes. Studies of linked traits led to the first maps of genetic loci on chromosomes. The first maps of *Drosophila* chromosomes were done by Alfred Sturtevant.

*Drosophila melanogaster* is one of the most studied organisms in biological research, particularly in genetics and developmental biology. Its complete genome was sequenced and first published in 2000.

Because great deal is known about its development from egg to larva to adult, it is a key model for developmental genetics, or evo-devo. The hox genes, or homeobox, which control development in metazoa, were worked out first of all in *Drosophila*.

### *Escherischia coli*

In 1946, Joshua Lederberg and Edward Tatum first described the phenomenon known as bacterial conjugation using *Escherichia coli* as a model bacterium.

*E. coli* was an integral part of the first experiments to understand phage genetics, and early researchers, such as Seymour Benzer, used *E. coli* and phage T4 to understand the topography of gene structure. Before Benzer's research, it was not known whether the gene was a linear structure, or if it had a branching pattern.

*E. coli* was one of the first organisms to have its genome sequenced; the complete genome of *E. coli* K12 was published by *Science* in 1997.

The long-term evolution experiments using *E. coli*, begun by Richard Lenski in 1988, have allowed direct observation of major evolutionary shifts in the laboratory.

Other model organisms
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* Viruses
	+ Phage Lamda
	+ Tobacco Mosaic Virus
* Protists
	+ *Emiliania huxleyi*: coccolithophore alga, extensively studied as a model phytoplankton species
	+ *Chlamydomonas reinhardtii*
* Fungi
	+ *Aspergillus nidulans*
	+ *Neurospora crassa*
* Plants
	+ *Arabidopsis thalliana*
	+ *Zea mays* Maize
	+ *Pisum sativum*, Pea
* Animals
	+ *Caenorhabditis elegans*, a nematode
	+ *Tribolium castaneum*, flour beetle
	+ *Drosophila melanogaster*
	+ *Mus musculus*, the mouse
	+ *Rattus norvegicus*, the Norwegian rat