Gene mapping is a method of identifying and locating genes on the chromosomes within cells. Genes carry the chemical instructions that determine an organism’s traits. These instructions are stored in DNA (deoxyribonucleic acid), the chief substance of each chromosome. By studying the inheritance of a trait within a family, scientists can map the gene that controls the trait to a position on a particular chromosome.
Modern techniques of gene mapping focus on DNA, unlike previous methods, which involved breeding and crossbreeding plants and animals. Today, scientists construct most chromosome maps by cleaving (cutting) a chromosome’s DNA into gene-sized fragments. Cleaving is done by means of restriction enzymes. Each restriction enzyme cleaves a particular segment of the DNA molecule at a point called the cleavage site. By using combinations of restriction enzymes, researchers can isolate a fragment that contains a single gene. The cleavage sites can be used to indicate the location of this fragment in the chromosome, and thus the gene’s position on the chromosome map. See Genetic engineering (How genes are reintroduced into cells).
Each human body cell contains 23 pairs of chromosomes and an estimated 20,000 to 30,000 genes. Scientists have identified the genes associated with certain hereditary illnesses, including cystic fibrosis and sickle cell anemia. Researchers hope to map the entire human genome—that is, all the genes in a human cell. To reach this goal, scientists worldwide share the results of their research in large computerized databases. Part of this cooperative research effort, known as the Human Genome Project, aimed to determine the sequence of the bases in the human genome. Bases are four chemical compounds found in DNA. The National Human Genome Research Institute (part of the National Institutes of Health) and the Department of Energy coordinated and funded research on the project in the United States. During the early 2000’s, the Human Genome Project and Celera Genomics Corporation, a private company, completed the sequencing of essentially all the human genome’s bases. Discovering this sequence should prove a vital step in mapping the human genome.
See also Genetics; Genomics; Heredity (Hereditary disorders); Human Genome Project.
