Figure 4.—

Phenotypic reversion, event 2. (A) The crossing of two ew/S73A-Δneo compound heterozygotes (not shown) generated a mouse (#1662), which was fully pigmented. This mouse carried a novel Mitf allele, S73A²-Δneo, that could be bred to homozygosity using the indicated breeding scheme. The presence of S73A²-Δneo renders mice fully pigmented even if their second Mitf allele is ew, a strong Mitf allele. (B) Genomic PCR of the indicated mice, using PCR primers as in Figure 2. The analysis indicates that #1857 contains only the targeted band of 578 bp, while the other mice all contain both the wild-type 504-bp band and the targeted 578-bp band, although at different relative intensities. A hypothetical gene rearrangement and the expected ratios of the two bands, consistent with the PCR results, are shown on the right. The solid triangles represent the loxP site.