A human is a complicated organism, and it is considered unethical to do many kinds of experiments on human subjects. For these reasons, biologists often use simpler 'model' organisms that are easy to keep and manipulate in the laboratory. Despite obvious differences, model organisms share with humans many key biochemical and physiological functions that have been conserved (maintained) by evolution. Each of the following model organisms has its advantages and disadvantages in different research applications. This tool allows you to examine the similarities between different systems by comparing the proteins they share and the proportion of DNA they have in common. Choose a gene from the drop-down menu and select the species you want to compare. Rolling over the images will give you a more detailed description of each model. Clicking on a gene’s name will take you to the National Center for Biological Information, where you can explore the latest relevant scientific literature.

Choosing a model organism depends on the question being asked. Human Cells (Homo sapiens) When model organisms cannot provide the information needed to answer a particular research question, biologists can turn to cultured human cells – which are grown in Petri dishes, in the same way as bacteria. Since cultured cells carry out all basic biochemical functions, they are often used to model specific tissue functions and diseases. Primary cells are derived directly from living tissue – such as blood, muscle, or nerve – and can only be cultured outside the body for a limited number of cell divisions. Immortalized cells have acquired mutations that free them from a fixed life span and allow them to divide indefinitely in culture. Cultured human cells have been especially important in studying the genetic and biochemical events that convert a normal cell into a cancerous one. Embryonic stem cells have the ability to become any cell type, and therefore have enormous potential in terms of replacing damaged cells in the humans body. Chimpanzee (Pan troglodytes) As recently as five million years ago, humans and chimps shared a common ancestor. A chimp possesses approximately 20,000 to 25,000 genes… approximately the same number as humans. Of that total, there are only 50 human genes that have no homolog in chimps. Differences between the two species are, therefore, due more to changes in gene regulation. Identifying genes that are divergent between humans and chimps should be helpful in understanding disease susceptibility. For example, chimps do not suffer from some human diseases such as AIDS and malaria. A comparison of the two genomes may reveal that there are genetic reasons for this and result in new ways of treating and preventing human disease. Dog (Canis lupus) Man’s best friend is a valuable model organism for studying the genetics of complex traits. It is also an excellent model for researching numerous diseases requiring subtle phenotyping. The dog genome is similar in size to the genomes of humans and other mammals, containing an estimated 2.8 billion DNA base pairs. A comparison of the dog and human genomes could help scientists find the genetic roots of dog behavior and physiology and help to identify genes that cause diseases in both dogs and humans. Canine models have played an important role in advancing biomedical knowledge and techniques. Due to a long history of selective breeding, many breeds of dogs are prone to genetic diseases including cancer and autoimmune disorders that are difficult to study in humans. Mouse (Mus musculus) As a mammal, a mouse is rather closely related to a human being. However, it is small, easy to keep, and completes a generation in only ten weeks. It shares more genes, anatomy, and physiology with us than the simpler model systems – bacteria, worms, or flies. Many laboratory strains of mice have been inbred to be genetically identical, which makes it easier to see the effects of an experimental treatment or change in a single gene. A method called homologous recombination allows scientists to precisely replace virtually any mouse gene with a mutated copy of the same gene or a related gene from another organism. A “transgenic” mouse is usually created by injecting a foreign gene into embryonic stem cells and then implanting the manipulated embryos into a surrogate mother. Transgenic mice carrying human disease genes are models for Huntington’s disease, sickle cell anemia, Alzheimer’s disease, and many cancers. Rat (Rattus norvegicus) The rat is used extensively as a model organism for studying both normal and disease processes in the human. The rat is an ideal research model because scientists have a deep understanding of rat physiological mechanisms. There are numerous rat models that mimic human diseases. It is relatively easy to generate inbred congenic (nearly identical) and consomic strains. A consomic strain is an inbred strain with one of its chromosomes replaced by the homologous chromosome of another inbred strain via a series of marker-assisted backcrosses. Chicken (Gallus gallus) The domesticated chicken is a modern descendant of dinosaurs. It is the premier non-mammalian model organism with a large international research community dedicated to developing and sharing information. The chicken provides a new perspective on vertebrate genome evolution. Its genome is composed of approximately one billion base pairs and approximately 20,000-23,000 genes organized in 39 chromosome pairs. Although it shares a similar number of genes, the chicken’s genome is only about a third the size of mammalian genomes. This reflects a reduction in interspersed repeat content, pseudogenes and duplications. There are hundreds of mutant chicken stocks and inbred lines available for study. Research using the chicken as a model organism has resulted in important discoveries in virology, immunology, oncology, vertebrate development, genetics and evolution. Fruit Fly (Drosophila melanogaster) The fruit fly is a small invertebrate. Although it cannot be frozen like bacteria and worms, it is easy to maintain, has large numbers of offspring, and grows quickly – from embryo to adult in 12 days. The fruit fly shares with humans a number of so-called “master,” or homeotic, genes that control development of a complex, symmetrical body plan. Thomas Hunt Morgan and his students at Columbia University identified the first fruit fly mutations in the early 1900s. Since that time, scientists have developed a large library of genetic mutants. It is relatively straightforward to disrupt fruit fly genes and to introduce foreign ones. All of these features make Drosophila a powerful model organism for studying animal development and even elements of behavior – including learning and memory! Mosquito (Anopheles gambiae) Anopheles gambiae has become an important model organism for the study of insect – parasite interactions and innate immune responses to a protozoan parasite. When investigating the molecular responses of vertebrate epithelial cells to parasite invasion, the mosquito is an excellent model organism. Studies of innate immunity in insects and vertebrates are merging as new information confirms the extent of evolutionary conservation in the signaling pathways mediating immune responses. Round Worm (Caenorhabditis elegans) C. elegans is a microscopic roundworm. Although some roundworms are parasitic, C. elegans is free-living. Like E. coli, these worms grow quickly – from embryo to adult in three days – are easy to culture, and can be stored in a freezer. C. elegans is a simple animal with only about 1,000 cells, and scientists know exactly how each of these cells develops from the fertilized egg. C. elegans was the first multi-cellular organism to have its entire genome sequenced, with the surprising finding that 40% of its genes have human matches. Any of the organisms genes can be "knocked down" using the technique of RNA interference (RNAi). Mating animals, isolating genes, and introducing foreign DNA is much easier than in more complicated animals. All of these features make C. elegans a great model for understanding how cells divide, develop, and take on specialized tasks in higher (eukaryotic) organisms. Yeast (Saccharomyces cerevisiae) Yeast was the first eukaryote organism to have its entire genome sequenced. It has remained at the forefront of genetics research because it is quick and easy to grow with an 80 minute generation time. Unlike many other microorganisms, strains of S. cerevisiae have both a stable haploid and diploid state. This makes it easy to isolate recessive mutations. The cell cycle in yeast is very similar to the cell cycle in humans and is regulated by homologous proteins. The discovery in yeast of two close homologs of the mammalian ras proto-oncogene is evidence of conservation from the simple yeast organism to the complex human organism. By examining the yeast genome sequence, it is possible to estimate how many yeast genes have significant mammalian homologs.

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