Everyone has used a map at one time or another. Maps are indispensable for finding a new friend’s house, the way to an unfamiliar city in your state, or the location of a country. Each of these examples requires a map with a different scale. To find a friend’s house, you would probably use a city street map; to find your way to an unknown city, you might pick up a state highway map; to find a country such as Kazakhstan, you would need a world atlas. Similarly, navigating a genome requires maps of different types and scales.

Genetic maps (also called linkage maps) provide a rough approximation of the locations of genes relative to the locations of other known genes (Figure 15.1). These maps are based on the genetic process of recombination (hence the name genetic map). The basic principles of constructing genetic maps are discussed in detail in Chapter 5. In short, individual organisms of known genotype are crossed, and the frequency of recombination between loci is determined by examining the progeny. For linked genes, the rate of recombination is proportional to the physical distance between the loci. Distances on genetic maps are measured in recombination frequencies (centiMorgans, cM), or map units.