X-Chromosome inactivation leads to the formation of mosaic cell populations in the somatic cells of mammalian females. Cells have either a maternal or paternal X-chromosome active. If an individual is heterozygous for a cell autonomous X-linked trait, then a set of built-in cellular markers is provided for the investigation of various developmental phenomena, including selection. In the absence of somatic cell selection the tissues of an X-linked heterozygote should all be mosaic and should have a mosaic composition approaching 1:1. If somatic cell selection is occurring, it should be detectable by a significance shift from the random expected 1:1 mosaicism. The system is effective at detecting selection acting on X-linked loci and on newly arisen somatic autosomal variants, and several examples of somatic cell selection are described. However, it is concluded that somatic cell selection, as described above, is not a normal aspect of ontogeny.