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1.7: Model Organisms Facilitate Genetic Advances

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  • 1.7.1 Model organisms

    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 sought the best organisms for their experiments. Today, a small number of species are widely used as model organisms in genetics (Fig 1.17). All of these species have specific characteristics that make large number of them easy to grow and analyze in laboratories: (1) they are small, (2) fast growing with a short generation time, (3) produce lots of progeny from matings that can be easily controlled, (4) have small genomes (small C-value), and (5) are diploid (i.e. chromosomes are present in pairs).

    The most commonly used model organism are:

    • The prokaryote bacterium, Escherichia coli, is the simplest genetic model organism and is often used to clone DNA sequences from other model species.
    • 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.
    • The mouse (Mus musculus) is the model organism most closely related to humans, however there are some practical difficulties working with mice, such as cost, slow reproductive time, and ethical considerations.
    • The zebrafish (Danio rerio) has more recently been developed by researchers as a genetic model for vertebrates.Unlike mice, zebrafish embryos develop quickly and externally to their mothers, and are transparent, making it easier to study the development of internal structures and organs.
    • Finally, a small weed, Arabidopsis thaliana, is the most widely studied plant genetic model organism. This provides knowledge that can be applied to other plant species, such as wheat, rice, and corn.

    Figure 1.17:
    Some of the most important genetic model organisms in use today. Clockwise from top left: yeast, fruit fly, arabidopsis, mouse, roundworm, zebrafish.
    (Original/Flickr/Wikipedia – Deyholos/M.Westby/D.Joly/Z.F.Altun/Masur/Azul – CC:AS/ANS/AN &GFDL )

    1.7.2 Society benefits from model organism research

    The study of genetic model organisms has greatly increased our knowledge of genetics, and biology in general. Knowledge from model organisms has also provided important implications in medical research, agriculture, and biotechnology. By using these species genetic researchers can discover more knowledge, faster and cheaper than using humans, farm animals or crop plants directly. For example, at least 75% of the approximately 1,000 genes that have been associated with specific human diseases have similar genes in D. melanogaster. Information about how these genes function in model organisms can usually be applied to other species, including humans. From research conducted thus far, we have learned that the main features of many biochemical, cellular, and developmental pathways tend to be common among all species. What is genetically and biochemically true in yeast, worms, flies and mice tends to be true in humans, too.

    However, it is sometimes necessary to study important biological processes in non-model organisms. In humans, for example, there are some diseases or other traits for which no clear analog exists in model organisms. In these cases the tools of genetic analysis developed in model organisms can be applied to these other, non-model species. Examples include the development of new types of gene discovery techniques, genetic mapping of desired traits, and whole genome sequencing.