Somatic sectors induced by ionizing radiation provide a great deal of information about cell lineage patterns in both plants and animals. Somatic sectors arise when the dominant allele of a mutation with a visible, cell‐autonomous phenotype is lost as a result of a deletion or somatic recombination. In addition to marking the fate of cells in a primordium at different stages of development and in different tissues, this technique also provides information about the distribution, orientation, rate, and duration of cell division. The technology and underlying assumptions of this method, termed clonal analysis, are described in this paper.