GENE MAPPING AND ANALYSIS
In many cases where a person may be heterozygous for a disease-causing recessive gene or a carrier of a late-acting dominant gene, it is not possible to determine the presence or absence of the mutated gene directly. Instead, an easily identifiable gene located nearby can act as a "marker" for the presence of the disease-causing gene. The genetic marker permits identification of a pattern of inheritance of the mutated gene in a family, a process known as linkage analysis.
The task of gene mapping has been greatly aided by the use of restriction enzymes, which cut DNA at certain specified places on the various chromosomes. Different family lines vary in the pattern of DNA fragments produced by a restriction enzyme, and analysis of an extended pedigree can reveal linkage of a particular DNA fragment with a disease-causing gene. Using this type of linkage analysis, the genes for Huntington disease, Duchenne muscular dystrophy, cystic fibrosis, and neurofibrosis have been mapped to specific locations on their chromosomes.
The Human Genome Project is a coordinated effort by designated laboratories worldwide to provide the complete nucleotide sequence of all 22 autosomes plus X and Y sex chromosomes (see genome). With this sequence it should be possible to test directly for the presence or absence of specific disease-causing genes in any human being.