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Mapping Genes

Gene mapping is a very important technique that is necessary for the advancement of genetics. A gene map shows the location of genes on chromosomes, just like a road map shows the location of roads in a region. There are two basic types of gene maps, genetic maps and physical maps.

Genetic maps, which are constructed by studying the behavior of linked genes, contain estimates of the distance between gene loci. As we discussed in the Incomplete Linkage, Crossing Over portion of this web site, linked genes do not assort independently during gamete formation. Rather, they tend to be inherited together unless recombination separates them during meiosis. The frequency with which this separation occurs can be used to estimate the distance between two genes. Basically, if two genes recombine 1% of the time, they are considered to be one map unit (also known as a centi-Morgan) apart.

Physical maps represent the actual physical location of genes on a chromosome and can be constructed using a variety of techniques. DNA sequencing is the most accurate of these methods, as it determines exactly how many bases exist between two genes. However, DNA sequencing can be quite time consuming, so other techniques are frequently used.

In situ hybridization, which is shown in this animation, takes advantage of the complementary nature of the DNA double helix. In this technique, all of the chromosomes in a nucleus are spread on a microscope slide and treated to make their DNA single-stranded. Next, a single-stranded piece of a gene, called a probe, is allowed to bind to its complimentary gene on the chromosome spread. Probes are generally labeled with some sort of molecule that allows us to see them. In this case, our probe has a fluorescent dye that will glow bright red if we look at it using a fluorescent microscope. As you can see, there are two copies of our gene in this chromosome spread, one on each homolog.

Once a gene map has been generated for a species, it can be used to study genetic diseases, evolution, and much more.





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