An introductory guide to the genetics of Huntington’s disease.
What is a gene? Which parts of the genetic code correspond to genes?
The material behind genetic inheritance is a surprisingly simple chemical substance called deoxyribonucleic acid (DNA). Each molecule of DNA is a long, continuous chain of smaller molecules called “bases” that are strung one after the other. The four different bases (abbreviated A, T, C, and G) can be arranged in many different ways. It is the sequential order of these bases that provides the chemical information or “instructions” for inheritance. The DNA ladder can be very, very long, twisted and coiled again around proteins into the shape of chromosomes that sit inside each cell’s nucleus. A chromosome is, in fact, one very long “super coiled” molecule of DNA. (For more on the chemical information in DNA, click here.)
Some regions of DNA contain instructions for making a specific functional product, such as a protein. These regions of functional DNA are called genes. Every long molecule of DNA has some segments that are genes and some that are not. Together, they make up the structures called chromosomes that are found in cell nuclei. (See Figure C-1.) The non-gene segments are sometimes half-jokingly called “junk in the genome” - we still have little idea of what, if anything, these sequences do! Fortunately, though, the genes are always found in the same place on a particular chromosome. The gene responsible for causing HD, for example, is always located on chromosome 4. (Humans have 23 pairs of chromosomes, each of which has been assigned a conventional number. Click here for a look at all 23 pairs.) When scientists say that they have “located” a gene for a disease, they usually mean that they have found a region on one of the chromosomes that codes for a protein that somehow contributes to causing the disease.
Last Modified: 9-13-02
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Some regions of DNA contain instructions for making a specific functional product, such as a 
