The following article appeared in the April 1984 issue of Mouse Fancy Review and may be of interest to Siamese/Himalayan breeders, especially those new to the fancy. It should be noted that in 1984 the Himalayan was shown in the marked section. Ed 

Three years ago I embarked on a programme of test matings of Seal Point and Blue Point Siamese. The object was threefold: to help a fancier friend who was having difficulties with them, to attempt to clarify current confusion in the Fancy concerning their make-up, and to add to my own knowledge of their problems as they were among the very few varieties I had never previously bred. In addition to the test matings I produced ten generations of Siamese, and seven generations of Himalayans derived from them, in direct lines concluding with a notable improvement on the original stock, thanks not merely to selective breeding but more significantly to the elimination of unwanted recessives. The “end product” stock has now been returned to the fancier who provided the originals, and these notes are published for her benefit and that of other breeders of these varieties.
The first and most essential point to be understood is that both Siamese and Himalayans inherit a gene known in genetical literature (confusing to fanciers) as Himalayan. Siamese, as standardised in the Fancy, inherit this gene from both parents. The Himalayans of the Fancy inherit the gene from only one parent, plus the albino gene (true PEW) from the other parent.
It should be clearly understood that the Himalayan gene (whether inherited from both parents to provide Siamese, or from one parent only, plus albino from the other to give Himalayans) is essentially a dilution gene, having localised effects: that is to say, it dilutes the main body pigmentation first of all, and then the extremities. When the albino gene is added, the dilution effect is increased, with equal effect on the main body and the extremities. This seems to be a good reason for not breeding both varieties together, since the objectives are in conflict: in the case of Siamese to conserve enough body pigmentation to secure the shadings required by the standard, and in the case of Himalayans to reduce it to as near white as possible.
It should be noted that the objective of “as near to white as possible” is not “white” as known in other marked varieties; and that the whiter the main body area of a Himalayan the paler the points. For this reason alone there is a strong reason for Himalayans to be transferred from the Marked Section to the AOV Section. They were originally classified as “Marked” by comparison with the equivalent rabbit and cavy breeds, but the comparison is by no means exact in its mechanism, not is its effect, in the case of the Himalayan mouse.
The one reliable way to produce better Himalayans (within the limits of capability of the present gene) is to use albinos carrying Self Black or Self Blue. This means either using PEWs derived from outstanding Himalayans (which are at present few and far between) or making them by a procedure of test-mating, which can be time consuming. As a preliminary stage it would be useful to eliminate chocolate which I must emphasise is a disastrous ingredient (whether in Himalayans or Siamese). Since Seal Points resemble Chocolate, and most Himalayans are more or less of this colour, if not Blue, it has long been assumed that the outcross to dark Self Chocolate would be beneficial; but this is not so – it merely adds to the dilution effect of the gene. I am personally responsible for some of the damage that has been done in this respect, since as the breeder of Self Chocolates I supplied stock for crossing to several Himalayan and Siamese breeders before I realised the harmful effect. Another practical problem is that if Blue is carried by the Himalayans or Siamese (or indeed if they are Blue-pointed) there will inevitably be Lilac-pointed in due course. These are not only unstandardised but visually intermediate in colour, and very pale in shade, and once they occur the number of each litter from which to select is correspondingly reduced.
Siamese x Siamese (if true-breeding on both sides to begin with) produce only Siamese offspring; Himalayan x Himalayan (in Fancy terms) produce an expectation of 50% Himalayans, 25% Siamese, and 25% albinos; Siamese (if true-breeding) x Himalayans should give 50% Siamese and 50% Himalayans, and Siamese (if true-breeding) x albino gives all Himalayans in the first generation, all of them carrying albino.
The difference between Seal Point and Blue Point Siamese (and Himalayans) is simply a matter of whether the “background” make-up is Self Black or Self Blue. In practice there is no disadvantage of breeding them together, so long as it is realised that although black may carry and pass on blue, blue x blue eliminates black. Any other “background” make-up than Self Black or Self Blue has proved disastrous to both Siamese and Himalayans. Some of the original stock test-mated proved to carry Self Chocolate, and it was confirmed that this had been introduced with the object of darkening the points. Not so: the points are already reduced in strength by the action of the essential gene from black to seal or from dark blue to paler blue. The effect of introducing chocolate (however dark) is to reduce strength further to paler seal or, in the case of blue, to lilac.
Since the difference between Siamese and Himalayan is merely the inheritance of albino from one parent, in the latter case it might be presumed that the two varieties could conveniently be bred in conjunction. This has not proved to be the case. In practice, in terms of Fancy requirements, the two standards are incompatible, and it is evident that mixing them in the past has been to the disadvantage of both. Siamese derived from Himalayans are deficient in shading, and Himalayans bred with Siamese are impure in body white.
Similarly, it might be thought that since Himalayans (as standardised) must inherit albino from one parent they could be improved in size and type by crossing any good PEW with a Siamese. This is again a false move which can have disastrous consequences (and probably often has to date). The reason is that PEWs can carry other genes than black or blue, and more often than not do (indeed some in the Fancy at present are not albinos but PE dilutes of various kinds). The only PEWs suitable for Himalayan breeding are those true albinos that carry only Self Black or Self Blue, and nothing else. Anyone wanting to improve Himalayans needs either to test-mate whites from the best Himalayans available to check that they carry only black and/or blue, or to create PEWs to ensure this make-up.
Most Siamese exhibited at present are more or less defective in shadings on saddle and hindquarters in relation to the standard requirements. This deficiency is unlikely to be corrected by selective breeding without an equivalent improvement of strength of the points – if the points are weak, so are the shadings.
Breeders and exhibitors of Himalayans live in hope that selective breeding will ultimately produce markings as densely pigmented and sharply defined as in the equivalent rabbits and cavies. This seems to me to be unrealistic, since the nature and mechanism of the respective genes involved is quite different, even though the effects are superficially alike. We may ultimately achieve Himalayan mice comparable to the rabbits and cavies, but not with the present genetic make-up. There are on record, however, at least two other similar mutations in the mouse (both now apparently extinct) which suggests the possibility of recurrence, since mutations often strike again with similar (but not necessarily identical) effects.