This is a gross oversimplification, but – black eyesare black because the rat has lots of melanin (black pigment) in theeyes. If you reduce this black pigment, the eyes become ruby, then red, then dark pink, then finally light pink if there’s no melanin at all.
Markings (white bits) on rats are where there’s no melanin too – there’s no melanin, so there’s no colour, so the fur becomes white. When a white patch is near the eye, there is a chance that the eye will be involved in this and lose some or all of the melanin, and so become paler coloured – and if this only happens on one eye, it will be odd eyed.
Certain genes makes odd eye more likely to happen. The colour gene red eye dilute (that gives buff/topaz) makes it more likely for odd eye to express. The Essex gene – especially when combined with RED – often gives odd eyes. The white spotting gene that gives chinchilla type rats is more likely to produce odd eyes. You can select towards the likelyhood of it happening but as there’s not a single gene that creates it, you can’t make any guarantees as to it happening.
I come at this from the point of view of someone who wants to breed rats that are good pet animals but also good show animals. Some varieties are easier than others and there are different reasons for it.
Some varieties have health issues, which may or may not make them less suitable for owning and selling as pets. For example – British blues based varieties are prone to having poor immune systems, topaz/buff are more likely to have bleeding/clotting disorders, chinchilla based rats if not carefully bred can produce kittens with megacolon. Most of these can be minimised through careful breeding, but it’s another selection pressure, and the more plates that need kept spinning then the more likely one of them will fall down (or that they’ll all stay up there, but wobbly and ready to collapse with little prior warning).
Most marked varieties are not particularly difficult to breed, but difficult to produce an excellent example for show (I would consider a show rat to be one that wins stars and will be a contender for the supreme challenge in a show, not one that just wins a class rosette and nothing else), and lots of the rats you keep for breeding will not be show suitable. If you’re only intent on breeding a very small number of litters then having a variety that may only give you a quality show rat every couple of years isn’t very encouraging. Some marked are easier than others – Essex tend to produce well marked animals fairly easily, and roans (although they don’t show well for long) often are nicely marked and will win as young rats at least.
Most of the AOV varieties are easy enough to breed good examples of. Things like silver fawn, agouti, cinnamon – agouti based rats tend to have a good long show life, there’s a lot of them about to find crosses of, and so they tend to do well in shows. There is a lot of competition in those classes, so you need a decent one to win, but finding the right starter rats to get to that point isn’t ever going to be difficult either.
Every variety has their own challenges – different shades to get right, nuances of how that particular colour or marking works, what you need to breed to. Realistically, it’s not rocket science, and although some are more difficult than others, the most important part is that you are breeding something you love. It’s much easier to put your heart into it if it’s something you want to do to start with, and much easier to get past the inevitable obstacles if you have more motivation to climb over them.
The pink eye dilute gene (which gives champagne) turns a rat into a much paler version of itself. If you have a black rat, then add PED, you get a diluted black. Black is a cold colour, you add PED, you get a paler but cold colour. Chocolate is a warm colour, and a bit paler than black. If you add PED to chocolate, you get a paler and warmer colour than a version based on black. It’s not that black is making it darker – it’s that the chocolate gene is making it paler.
I’ve seen people advise adding black to a Siamese line to darken up the points, but it doesn’t make any sense and it doesn’t work. Adding black can be useful because you can see that blacks have dark feet, so you can use it to help to eliminate white feet from the line. But it won’t make the points any darker, it will just help you to breed white toes out of it.
The depth of colour of the Siamese shading is dependent on hundreds of other little modifier genes. You want to breed in as many of those as possible to your Siamese line to get good dark points. If you have a black outcross from a dark Siamese line he might carry lots of these modifiers so be useful for darkening the Siamese, but his Siamese brother would be just as useful