How RFLPs Can Help Find a Faulty Gene
Pines, Maya, ed. "Blazing a Genetic Trail." Bethesda, MD: Howard Hughes Medical Institute, 1991.
Your DNA is not quite like mine: We all have slight variations in our DNA every 500 base pairs or so, usually in regions that do not code for proteins.
The most interesting variations for geneticists are those that are recognized by certain enzymes, called restriction enzymes. These enzymes, each of which slices through DNA at a particular sequence, are monomaniacal in their missions - they may cut DNA only when they see the sequence GAATTC, for instance, and bypass the region if it has mutated to GACTTC. Thus, when a specific restriction enzyme cuts the DNA of different people, it may produce fragments of different lengths.
These DNA fragments can be separated according to size by making them move through a porous gel in an electric field. Since the smaller fragments move more rapidly than the larger ones, their sizes can be determined by examining their positions in the gel. Variations in their lengths are called restriction-fragment-length polymorphisms, or RFLPs, commonly pronounced "riflips."
RFLPs give geneticists another view of human inheritance: Not only can they track the passage of physical traits or disease through a family, but they can follow the course of genetic material itself by tracking riflips through generations - that is, if they have DNA from all the family members.
To locate a particular gene, geneticists look for RFLPs that are almost always inherited along with it. Genes on a given chromosome have one chance to become separated each generation - during the formation of egg and sperm cells, each of which has half the number of chromosomes of the parent cell. At that time, various pieces of the chromosomes recombine into the new configurations that make each of us unique.
The closer two genes are, the likelier that they will enter the egg or sperm together, as part of the same chromosome. So if a RFLP nearly always accompanies a disease in a particular family, the gene for this disease must be fairly close to it. How close it really is can be established by calculating how consistently the RFLP marker is inherited with the disease. Linkage studies of this sort depend on the mother's RFLPs being different from the father's, and on the family being sufficiently large.
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