Genome Mapping: A Guide To The Genetic Highway We Call The Human Genome
Imagine you're in a car driving down the highway to visit an old friend who has just moved to Los Angeles. Your favorite tunes are playing on the radio, and you haven't a care in the world. You stop to check your maps and realize that all you have are interstate highway maps--not a single street map of the area. How will you ever find your friend's house? It's going to be difficult, but eventually, you may stumble across the right house. This scenario is similar to the situation facing scientists searching for a specific gene somewhere within the vast human genome. They have available to them two broad categories of maps: genetic maps and physical maps. Both genetic and physical maps provide the likely order of items along a chromosome.
However, a genetic map, like an interstate highway map, provides an indirect estimate of the distance between two items and is limited to ordering certain items. One could say that genetic maps serve to guide a scientist toward a gene, just like an interstate map guides a driver from city to city. On the other hand, physical maps mark an estimate of the true distance, in measurements called base pairs, between items of interest. To continue our analogy, physical maps would then be similar to street maps, where the distance between two sites of interest may be defined more precisely in terms of city blocks or street addresses. Physical maps, therefore, allow a scientist to more easily home in on the location of a gene. An appreciation of how each of these maps is constructed may be helpful in understanding how scientists use these maps to traverse that genetic highway commonly referred to as the 'human genome'.
Just like interstate maps have cities and towns that serve as landmarks, genetic maps have landmarks known as genetic markers, or 'markers' for short. A marker may be used as one landmark on a map if, in most cases, that stretch of DNA is inherited from parent to child according to the standard rules of inheritance. Markers can be within genes that code for a noticeable physical characteristic such as eye color, or a not so noticeable trait such as a disease. DNA-based reagents can also serve as markers. These types of markers are found within the non-coding regions of genes and are used to detect unique regions on a chromosome. DNA markers are especially useful for generating genetic maps when there are occasional, predictable mutations that occur during meiosis--the formation of gametes such as egg and sperm--that, over many generations, lead to a high degree of variability in the DNA content of the marker from individual to individual.