Linkage studies use data from very large families with a history of HD. The principle behind linkage studies is that if nearly every person with HD in a single family shares the same version of a particular “marker trait,” such as the same color eyes or the same blood type, then the genes that code for that marker trait must be located close to the Huntington gene on the same chromosome. The marker’s gene and the Huntington gene are then said to be “linked” to one another. The logic behind such studies is straightforward—genes that lie close together on the same chromosome will tend to be inherited together over the generations. (Genes lying farther apart on the same chromosome are often not inherited together due to a complex process called recombination.)
In reality, eye color and blood type were not themselves useful as markers in HD studies, for their genes lie on chromosomes different from that of the Huntington gene. However, other markers were a tremendous help in locating the Huntington gene, including markers in a region of the Huntington-bearing chromosome called the “non-coding region.” The DNA in these regions does not code for proteins, but it still consists of a linear sequence of the chemical components called nucleotides. Using laboratory techniques, researchers found a few regions or “loci” of this DNA where family members with HD all had the very same nucleotide sequence. Since the researchers knew the locations of these particular loci, they were able to determine that the Huntington gene lies close by on the same chromosome. For example, a locus called “D4S90” had the same nucleotide sequence in every person with HD in a particular family. Since D4S90 was known to reside on chromosome #4, researchers concluded that the Huntington gene must lie along chromosome #4, very near D4S90. Data from other loci have confirmed this fact.
Once the location of the Huntington gene was identified through linkage studies, further genetic research (including linkage studies and other types of genetic investigation) revealed more details about the Huntington gene. With the cutting-edge technologies available today, it is likely that genetic research will continue to tell us a great deal about HD.
Last Modified: 1-28-04
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