Rumpwhite

Left: Twice Best Marked Rumpwhite buck u/8 bred and owned by Humbug Stud Right: Quad of baby rumpwhites

The rumpwhite is a marked mouse and as such can sometimes suffer from the lack of type and size associated with marked mice. The gene that causes the rumpwhite marking is dominant, so outcrossing is easier than it is with some marked varieties (i.e. the recessives, such as brokens). Because the gene is dominant it is also possible to create new colours of rumpwhite that conform to the standard more quickly when using selfs (rather than rumpwhites of the colour desired) than it is with other marked varieties.

When breeding rumpwhites it is important that the demarcation line is even and straight all the way round the mouse, not just under or on top. Theoretically the line you see on top of the mouse and the line under its belly are of equal importance, although some judges seem to favour the top line being straight. That is, if two mice had equal faults in their lines, one under and one on top, the mouse with the better top would win. Do not ignore the line under when breeding in light of this – always aim for the best all round mouse you can.

Lines will tend to pull at the armpits of the mouse by its front legs, so beware of this. Also watch out for a ragged line rather than a crisp straight one as this could lose vital points. Some rumpwhites will show colour on their tails – try to select for those without this feature. It may not prevent them from winning a class but for higher honours there shouldn’t be any colour at all in the white portion of the mouse.

Left: Young agouti rumpwhite buck Right: Two young rumpwhite babies

Since the rumpwhite gene is homozygous lethal, any rumpwhite mouse inheriting it from both parents will die before developing into a fully formed baby and be reabsorbed by the mother. This means that rumpwhite to rumpwhite matings produce smaller litters, which are easier for the mother to handle (especially as rumpwhites are not big mice). There is no danger in breeding rumpwhite to rumpwhite, and this is usually practised to retain the markings. Breeding rumpwhite to rumpwhite will produce 50% (heterozygous/viable) rumpwhites, 25% selfs and 25% homozygous rumpwhites which will die before birth. Breeding a rumpwhite to a self will produce the same 50% (heterozygous/viable) rumpwhites, but will produce 50% selfs. In other words, the litter will be 25% larger as no babies will be reabsorbed. The extra 25% of babies are statistically likely to be selfs, and so unless you are outcrossing it is not necessary or desirable to breed rumpwhite to self.

When outcrossing, you should chose a large self in your desired colour. Usually a self buck would be used with a rumpwhite doe (or does). The babies who are genetically rumpwhite will not look as a show quality rumpwhite does, but will tend to have very little white on their rumps. For this reason pinkies should be examined carefully before being proclaimed a self at a young age (under a week). It is recommended to keep only does from the outcross, unless you produce an exceptionally large/typy buck. The does you keep should have an improved colour and type when compared to an inbred rumpwhite. The rumpwhite does that are retained should be mated back your rumpwhite stud buck to improve markings in the offspring. Outcrossing should also achieve hybrid vigour, thereby preventing your stud reaching a fertility crisis.

The line under should be as straight as possible.