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A diagrammatic representation of the order of genes on a chromosome. Linkage maps can be constructed by crossing appropriate strains carrying different alleles on homologous chromosomes. Results show that the arrangement of genes is linear, although the distance between the genes is expressed not in units of length but in crossover units or Morgan units (named after Τ. Η. Morgan who discovered linkage). Infrequent crossing over is interpreted to mean that genes are near each other (closely linked), and is expressed quantitatively by a small crossover value (% crossing over). Conversely, genes are regarded as being further apart if the frequency of crossing over is higher, as indicated by a large crossover value.
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