Through the years, genetic studies have been conducted on thousands of different species, including almost all major groups of bacteria, fungi, protists, plants, and animals. Nevertheless, a few species have emerged as model genetic organisms: organisms with characteristics that make them particularly useful for genetic analysis and about which a tremendous amount of genetic information has accumulated. Six model organisms that have been the subject of intensive genetic study are Drosophila melanogaster, a fruit fly; Escherichia coli, a bacterium present in the gut of humans and other mammals; Caenorhabditis elegans, a nematode (also called a roundworm); Arabidopsis thaliana, the thale cress plant; Mus musculus, the house mouse; and Saccharomyces cerevisiae, baker’s yeast (Figure 1.7). These species are the organisms of choice for many genetic researchers, and their genomes were sequenced as a part of the Human Genome Project (described in Chapter 15).

At first glance, this group of lowly and sometimes unappreciated creatures might seem to be unlikely candidates for model organisms. However, all possess traits that make them particularly suitable for genetic study, including a short generation time, large but manageable numbers of progeny, adaptability to a laboratory environment, and the ability to be housed and propagated inexpensively. The life cycles, genomic characteristics, and features that make these model organisms useful for genetic studies are included in special illustrations in later chapters for five of the six species. Other species that are frequently the subject of genetic research and considered model genetic organisms include Neurospora crassa (bread mold), Zea mays (corn), Danio rerio (zebrafish), and Xenopus laevis (clawed frog). Although not generally considered a model genetic organism, humans have also been subjected to intensive genetic scrutiny.

The value of model genetic organisms is illustrated by the use of zebrafish to identify genes that affect skin pigmentation in humans. For many years, geneticists have recognized that differences in pigmentation among human ethnic groups (Figure 1.8a) are genetic, but the genes causing these differences were largely unknown. The zebrafish has become an important model in genetic studies because it is a small vertebrate that produces many offspring and is easy to rear in the laboratory. The mutant zebrafish called golden has light pigmentation due to the presence of fewer, smaller, and less dense pigment-containing structures called melanosomes in its cells (Figure 1.8b). Light skin in humans is similarly due to fewer and less dense melanosomes in pigment-containing cells.

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Keith Cheng and his colleagues hypothesized that light skin in humans might result from a mutation that is similar to the golden mutation in zebrafish. Taking advantage of the ease with which zebrafish can be manipulated in the laboratory, they isolated and sequenced the gene responsible for the golden mutation and found that it encodes a protein that takes part in calcium uptake by melanosomes. They then searched a database of all known human genes and found a similar gene called SLC24A5, which encodes the same function in human cells. When they examined human populations, they found that light-skinned Europeans typically possess one form of this gene, whereas darker-skinned Africans, East Asians, and Native Americans usually possess a different form of the gene. Many other genes also affect pigmentation in humans, as illustrated by mutations in the OCA2 gene that produce albinism among the Hopis (discussed in the introduction to this chapter). Nevertheless, SLC24A5 appears to be responsible for 24% to 38% of the differences in pigmentation between Africans and Europeans. This example illustrates the power of model organisms in genetic research. However, we should not forget that all organisms possess unique characteristics and that sometimes the genetics of models do not accurately reflect the genetic systems of other organisms.