We have seen already that many of the great advances in genetics were made using species that are not especially important from a medical, economic, or even ecological perspective. Geneticists, from Mendel onwards, have used model organisms for their experiments. Today, a small number of species are widely used as model genetic organisms. All of these species have characteristics that make them easy to grow in large numbers in laboratories: they are small, have a short generation time and produce lots of progeny from matings that can be easily controlled. Genetic model organisms also usually have small genomes (small c-value), and are diploid (i.e. chromosomes are present in pairs). Yeast (Saccharomyces cerevisiae) is a good general model for the basic functions of eukaryotic cells. The roundworm, Caenorhabditis elegans is a useful model for the development of multicellular organisms, in part because it is transparent throughout its life cycle, and its cells undergo a well-characterized series of divisions to produce the adult body. The fruit fly (Drosophila melanogaster) has been studied longer, and probably in more detail, than any of the other genetic model organisms still in use, and is a useful model for studying development as well as physiology and even behaviour. As a mammal, mouse (Mus musculus) is the model organism most closely related to humans. However, some of the practical difficulties of working with mice led researchers more recently to develop zebrafish (Danio rerio) as a genetic model for vertebrates. Unlike mice, zebrafish embryos develop externally to their mothers and are transparent, making it easier to study their development. Finally, a small weed, Arabidopsis thaliana, is the most widely studied plant genetic model organism.
The study of genetic model organisms has greatly increased our knowledge of genetics, and biology in general. Model organisms also have important implications in medical research. For example, at least 75% of the approximately 1,000 genes that have been associated with specific human diseases have highly similar sequences in both humans and D. melanogaster. Information learned from model organisms about particular biochemical pathways can usually be applied to other species, since the main features of many biochemical pathways tend to be shared between species.
It is also possible, and sometimes necessary, to study biological processes in non-model organisms. Humans, for example, have none of the characteristics of a model organism, and there are some diseases or other traits for which no clear analog exists in other organisms. Some of the tools of genetic analysis can be applied to non-model organisms, especially with the development of new types of genetic mapping and whole genome sequencing.