Fig. 5. Correlation of phenotype with mosaic cell lineages. The phenotypes of certain progeny of a worm of genotype unc-36(–); Ex100[unc-36(+) sur-5::gfp] are indicated below diagrams of the early divisions of the invariant cell lineage of C. elegans. Ex100 is an extrachromosomal array that has wild-type (+) copies of the unc-36 gene, which rescue a loss-of-function mutation in the endogenous copies of unc-36. The array also expresses GFP from a cell-autonomous marker gene. Based on which cells are green, the marker allows one to deduce the cell division at which the array was lost in a mosaic animal. (A) Inheritance of the array by all cells, indicated in green in the diagram, produces a non-mosaic worm whose movement is completely coordinated. (B) Failure to inherit the array (indicated in black), owing to meiotic segregation in the mother, results in an uncoordinated (Unc) animal. (C) Loss of the array (indicated in black) in P₁ produces a mosaic worm that has normal coordination. The focus of action of the unc-36 gene is therefore not in P₁ or its descendants. (D) By contrast, loss of the array in AB, the sister of P₁, produces a mosaic worm that is fully uncoordinated. (E) Loss of the array in ABp also produces the uncoordinated phenotype. (F) Loss of the array in ABa and P₁ – an example of consecutive losses of the array – gives a coordinated worm. The focus of action of unc-36 is therefore among the descendants of ABp (Kenyon, 1986).