A gene is a set of instructions for making a molecule given through a set of nucleotides in a molecule of DNA or RNA, with most of the DNA existing on the 23 pairs of chromosomes in the nucleus of each human cell. The bases of the nucleotides—which are adenine, cytosine, guanine, and thymine—define the information in the gene and the molecular product, often a protein.
Gene mapping refers to one of two different ways of definitively locating the gene on a chromosome. The first type of gene mapping is also called genetic mapping. Genetic mapping refers to the use of linkage analysis to determine how two genes on a chromosome relate in their positions. Physical mapping, the other type of gene mapping, locates genes by their absolute positions on a chromosome using any technique available. Once a gene is located, it can be cloned, its DNA sequence determined, and its molecular product studied.
The first report of mapping a gene to a human autosome was published in 1968 by Roger Donahue and associates. Using a linkage analysis, he was able to estimate the genetic distance of 2.5 map units between two loci or gene locations on chromosome 1. In 1971, chromosome banding techniques were developed, which opened the way for researchers to be able to identify more types of alterations, included insertions, deletions, and translocations, as well as mapping to position. In connection with this, restriction fragment length polymorphism (RFLP) analysis was developed and led by the early 1990s to the identification of a number of genes associated with disease in humans. A complementary technique, fluorescence in situ hybridization, developed about the same time, also contributed to the mapping efforts.
An example of this process in application is the work done with the gene for cystic fibrosis. The cystic fibrosis gene was mapped by linkage analysis in 1985. This paved the way for its cloning in 1989 by Francis Collins and his associates. This led to a better understanding of the cause of the disease.
The foundation of gene mapping also laid the foundation for the Human Genome Project. The idea of sequencing the entire human genome was explored in the 1980s, but was not universally thought to be feasible. Impetus from the U.S. Department of Energy along with the National Institutes of Health (NIH) helped foster the 1990 launch of the project. The technical achievements mentioned above contributed to the project’s momentum. The project was completed in 2003.
