**2. Linkage and molecular markers**

MAS is based on the concept of genetic linkage between loci. This describes the tendency of loci located closely together on the same chromosome being more likely to be inherited together in a recombination event during meiosis. Thus, two alleles located very close on the chromosome will almost always be inherited together.

Molecular markers are used in MAS to highlight a place on the chromosome close to or in a specific gene of interest. The technique is based on detecting different alleles (polymorphisms) between several individuals. Due to genetic linkage, the molecular marker will reveal if the linked allele is present or not in a line. Several types of molecular markers exist, depending on the type of polymorphism. In today's MAS, markers detecting single nucleotide differences are usually employed.
