Mitotic crossing-over in a leaf of Nicotiana tabacum. A double heterozygous mutant (Su+/+cl) was chosen (+ = wild type allele). The two gene loci Su and cl are coupled. The leaves look yellow-greenish, since Su is dominant (the wild type allele is recessive!) and suppresses a complete green. The cl allele effects the phenotype only in a homozygous state. Without the corresponding wild type allele, no chlorophyll can be produced, cl/cl plants colourless are consequently. A crossing-over event that happened during the ontogenesis of the leaf led to the combinations ++ and Su cl that were inherited by the daughter cells. Both daughter cells formed a cell clone during the further development of the leaf. The clones are neighbouring and can be recognized as a twin spot. They are of nearly the same size. The size and shape of such patterns allow conclusions concerning the processes (for example the sequence of cell divisions) during leaf development. The dark clone (++), for example, extends over one of the veins meaning that the cells of the vascular tissue have also developed from the cell with the crossing-over. (P. S. CARLSON, 1974).