Positional cloning is a technique that is used in genetic screening to identify specific areas of interest in the genome, and then determine what they do. This type of screening is sometimes referred to as reverse genetics, because researchers start by figuring out where a gene is, and then they determine what it does, in contrast with methods which start by determining the function of a gene and then finding it in the genome. Genes related to conditions such as Huntington's Disease and cystic fibrosis have been identified with this technique.
In this process, researchers find an area of interest on the genome by looking for genetic markers. They often take advantage of databases that collect information about people with medical conditions so that they can identify common traits that can be used to narrow down an area of the genome that might reveal useful information. Once markers have been found, a researcher can clone and investigate the area of interest to determine what it does. Positional cloning can also be used to screen specific individuals for genetic issues.
This technique depends on a very extensive and well equipped lab that a researcher can use to investigate the genome and experiment with mutations. It also relies on a genome database that researchers can use to compare their results with the genomes of normal individuals, along with individuals who have various mutations and medical conditions. In organisms like plants and fruit flies, a researcher can actually stimulate the creation of mutations with genetic manipulation to learn more about how an area of the genome functions. Such experimentation in humans is not ethical, forcing researchers to compare the information they find with genetic samples, looking for signs of mutations and the way in which mutations express themselves.
Using procedures like this, researchers can map out the genome and slowly but steadily find out what each area of it does. The more information researchers find, the easier their work is, as they can start to find connected markers and traits that interact with each other. Analysis of the genome will also allow researchers to come up with tests that can be used to look for specific inherited conditions and mutations. These tests can be used to screen fetuses for potential birth defects, and to test children and adults for underlying genetic conditions that could manifest later in life.
Like many types of research connected to the genome, positional cloning can sometimes attract controversy. By finding the sites of specific genetic conditions, researchers lay the groundwork for potential treatments and cures, but they also raise questions about genetic testing and how it can be used. For example, with a genetic test to look for the genes that are related to Huntington's Disease, parents might opt to test a fetus during pregnancy and abort it if the test is positive, a choice that could raise uncomfortable ethical questions.