Fig. 2. An example of mosaic analysis and cell autonomy. (A) In worms that are homozygous for the mutation of the blue gene mentioned in Fig. 1, four hypothetical cells develop an abnormal shape and color. (B) Worms are identified in which the four cells have green fluorescent nuclei, a consequence of inheriting the extrachromosomal array, mentioned in Fig. 1, that expresses GFP cell autonomously. The cells have therefore inherited wild-type copies of the blue gene, because they are also present on the array, and the array is known from preliminary work to complement the mutant phenotype of the blue gene in transgenic worms that are not mosaic. The cells are observed to undergo wild-type development, which involves a change in cell shape and color soon after birth. This pattern of inheritance, however, does not prove that the blue gene must function within the four cells (cell autonomously). Proper development of the four cells may instead depend on the expression of the gene in another cell or cells, which signal to the four cells to change their shape and color. This would be an example of cell non-autonomy. Mosaic worms must therefore be examined carefully for their overall patterns of mosaicism. (C) Loss of the array when the grandmother of the cells divides produces mosaicism within the four cells. The left clone has a wild-type phenotype, and the right clone is mutant. Note that the phenotype correlates with inheritance of the array, as would be expected for the cell autonomous action of the blue gene.