Figure 1.

Mating scheme of the mouse SLT, and origin of the visible and masked mosaics. m designates any one of the seven marker alleles contributed by the T stock (a, b, p, c^ch, d, se, or s). m* designates a new spontaneous mutation at any one of the loci. + designates the corresponding wild-type allele. ///separates the two component genotypes of a mosaic. “s-s perigametic event” designates a single-strand mutation in the interval between the last premeiotic mitosis and the first postmeiotic one, an event that would produce a conceptus that is a 50:50 mosaic (see text). If the event occurs in an H mouse (in G₁), it produces a scored-generation (G₂) mosaic that is visible. An analogous event occurring in a 101 or C3H mouse (in G₀) produces a G₁ mosaic that is masked (because m* is covered by a dominant +) but reveals itself by producing clusters of mutants (m/m*) in the G₂, the scored generation. (Note that mutant clusters can also be produced by rare H mice that are heterozygous for m*; for simplicity, these are omitted from the figure.) We provide evidence (see text) that most, if not all mosaics detected in the SLT are 50:50 mosaics.