ABSTRACT

One of the prominent applications of the various molecular markers described in this book is the establishment of genetic linkage maps. The process of genetic mapping can be defined as the determination of the linear order of molecular markers or genes (generally, loci) along a stretch of DNA (e.g., a bacterial artificial chromosome [BAC] clone, a nuclear chromosome, or an organellar genome). The result is a genetic map, which may be described as a graph depicting the relative positions of markers along so-called linkage groups (LGs), based on their frequency of crossovers or recombinations during meiosis. The distance between markers on a genetic map is given as Morgan (M) or centimorgan (cM), where one cM is the distance that separates two markers (or genes), between which a 1% chance of recombination exists (corresponding to one recombination event in 100 meioses). The average extent of recombination is dependent on the genome (e.g.,

Arabidopsis thaliana

,

1 cM = 139 kb; human, 1 cM = 1108 kb). Linkage maps with different marker densities (depths) are now commonplace with

most of the more important crop plants (see Chapter 7.1.4), and can be constructed with relative ease and speed. The following steps are prerequisites for a successful genetic mapping of a target genome. First, a careful selection of parent plants precedes their mating to produce a suitable mapping population, then the progeny is individually tested for marker profiles, and pairwise recombination frequencies are calculated, LGs are established, and map distances are estimated, using powerful computer programs such as MapMaker.