With the appearance of a gene producing a black (or non-agouti) hamster in the early 1990s it became possible to create several new and very interesting colours. When I first obtained a black hamster from Joanna Roach many moons ago, I initially concentrated on combining black and rust to produce chocolate. Although I have bred quite a few chocolates over the years, I have never been able to get a good line going, and sadly my last chocolate died recently having never raised a litter. I occasionally see chocolates at shows, and although I still love the colour, I have begun to lean towards black tortoiseshell and black-yellow as my stock in trade. The mating permutations are made more interesting as the gene for Yellow (To) is found on the sex chromosomes (X and Y), and Yellow is therefore said to be sex-linked. A male hamster can only carry one Yellow gene, as he has only one X-chromosome. The female, on the other hand, has two X-chromosomes, and can have either one or two Yellow genes. The upshot of this is that a female can have the Yellow gene on one of her Xs and the normal (to) gene on the other. This produces a mosaic pattern of normal-coloured and yellow-coloured fur, the variety we call Tortoiseshell. It is the same type of gene that produces the Tortoiseshell cat.
The reason why we rarely see tortoiseshell males is that they dont have the second X-chromosome required to make a pair of Yellow genes. They have to make do with just the one, leading some women to speculate that maleness might be recessive, a view I dont share! It is, of course, possible for a female to have the Yellow gene on both her X-chromosomes, in which case she will look identical to her poor deprived male equivalent - a Yellow!
The lucky tortoiseshell females got a further boost with the appearance of the black gene. Not only could yellow patches be shown mixed with golden fur, but they could now be mixed with black fur, for even better contrast. This variety is the Black Tortoiseshell, and is a firm favourite of mine. In an ideal world, the black and yellow patches should be as separate as possible with a minimum of brindling i.e. mixing of the two coloured areas. In practice, there is a lot of variation, and it is pretty much hit-and-miss in terms of the quality of the patterning.
One other feature is worthy of note - the yellow patches on a black tort are not really yellow, as the black-tipped hairs that give the nice ticking over a yellow are now solid black hairs. The effect is of a much heavier and darker ticking than would be found on a yellow. A dramatic demonstration of this effect is the Black-Yellow hamster, generally a male, combining Black (a) and Yellow(To) to give aa To/Y. Black-Yellow females are possible, but uncommon. The yellow gene removes much of the black pigment from the normal hairs, but leaves the solid black ticking as described above. These males are your key to breeding black tortoiseshell females. If you cross one to a black female, you can expect (on average) the following results: