Rump-white is a dominant lethal mutation from which homozygotes die in utero around embryonic day 9.5. From the earliest developmental stage assessed, E7.5, smaller size is detectable in Rw/Rw embryos. Developmental arrest is found at E7.5-E8.0 likely during or near the end of gastrulation. E7.5 embryos do not have the expanded cylindrical shape normally seen at this stage. All three germ layers form, but the mesoderm is sparse and loosely organized. A rudimentary notochordal plate can be detected in some of the larger mutant embryos by E9.5. Resorption of necrotic mutant embryos appears to be underway at this point. (Searle and Truslove, 1970; Bucan et al., 1995.)

Rump-white heterozygotes have hypopigmentation in the front digits and hindquarters including white hindlegs, white tail with the tip often pigmented, and a variable degree of white in the sacral and lumbar regions with the ventral area more consistently affected than the dorsal area. At E10.5 a normal number of melanoblasts are found emigrating from the neural crest, but at E11.5 fewer are found along the neural axis particularly in the vagel and anterior trunk areas. Although fewer in number, melanoblasts are found in a normal pattern throughout the posterior axial domain until E15.5 when a reduction is seen in the hindlimbs and tail area. A rostrocaudal migration of melanoblasts to populate the vacant regions appears to occur between E15.5 and E16.5 resulting in some pigmentation in the abdomen and lumbar area, but this migration fails to extend to the hindlimbs. As with other mouse hypopigmentation mutations, Rw/+ mice have spots of hypopigmentation in the inner ear. However, Rw/+ mice do not have structural abnormalities of the cochlea and saccule that are found in many other hypopigmented mouse mutants. Rw +/+ Kit^W-19H mice, and certain other Kit mutants in repulsion with Rw, show complementation in that they have more extensive hypopigmentation than does either contributing heterozygote alone. (Searle and Truslove, 1970; Deol, 1970; Jordan and Jackson, 2000.)