A plan showing the relative positions of genes along the length of the chromosomes of an organism. It is constructed by making crosses and observing whether certain characteristics tend to be inherited together. The closer together two allele pairs are situated on homologous chromosomes, the less often will they be separated and rearranged as the reproductive cells are formed (see chiasma; crossing over). The proportion of offspring that show recombination of the alleles concerned thus reflects their spacing and is used as a unit of length in mapping chromosomes (see map unit). The information obtained from such a classical linkage map can be combined with a restriction map, which is a linkage map of sites cleaved by restriction enzymes (see restriction mapping), providing a huge number of potential marker sites for genes of interest. Linkage maps provide valuable frameworks for constructing detailed physical maps giving the base sequence of the chromosomal DNA.