Ticking and Roan
Ticking is flecks or spots of colour on white areas. It can occur on any white area on a dog, so long as the white is "proper" white ( i.e. so long as it's caused by the white spotting series and not by the chinchilla gene). If a dog has the ticking allele but doesn't have any white areas, there will be no visible effect.
The gene which codes for ticking has not been found yet, but it is thought to be dominant. It has been assigned its own locus - T - and there are traditionally thought be two alleles on that locus. According to the common theory, T is the dominant ticking allele, and t is the recessive clear white allele.
However, this theory does not account for the full variation in ticking and roan in dogs. It is probable that there are at least four T-locus alleles. Recent genetic research has proven ticking and roan to be distinct (but mapped to a similar area), so my own guess is that the T-series is most likely as follows:
T - ticking
Tr - roan
Td - Dalmatian spots
t - clear white
It is not clear how these genes interact or what their order of dominance is, although ticking, roan and Dalmatian spots all appear to be dominant to their absence.
Ticking amount varies greatly between dogs, and this can be partly explained by the idea of incomplete dominance. If the ticking gene (T) displays incomplete dominance over the clear white gene (t), then a TT dog would have heavy ticking and a Tt dog would have lighter ticking. It is not clear whether this is also true for roan and Dalmatian spots.
Generally, ticking is heaviest on the legs and the muzzle. If a dog has only a small amount of ticking, it will appear in these areas before appearing anywhere else. Roan, on the other hand, is more even over the whole body.
The colour of ticking/roan corresponds to the colour that the area would have been if there wasn't any white there. For example, a black-and-tan
dog with white markings and ticking would have black ticking on its body and tan ticking on its legs, chest and muzzle, where it would be tan if it didn't
have white. The English Setter below shows this well.
Roan is a pattern which produces heavily mottled white areas. Often only a small amount of scattered white is visible.
The three dogs above are extreme examples of roan. The Australian Cattle Dog and Basset Bleu de Gascogne are genetically black-and-tan, so the roaning is black on the body and tan on the points. With the Cattle Dog, the whole dog would be white if it didn't have roaning (note there are no black patches), as it is an extreme white piebald. The overall effect on the black areas of these dogs is similar to the "salt-and-pepper" colour found on Schnauzers, except if you got a close-up look at the hairs, you'd be able to see that they're not banded like the hairs on Schnauzers are. The Basset Bleu perhaps gives a better indication that roan actually consists of lots of spots, packed very densely to cover most of the white. This has led some people to believe that roan is, in fact, just very heavy ticking. However, others believe that it is controlled by a separate gene, and recent studies appear to have confirmed this.
The German Shorthaired Pointer shows liver roan. The roan here is even more dense and difficult to distinguish from the solid liver patches on the back and head.
Black dogs with roaning often appear a greyish colour, and are commonly called "blue roans". The Cocker Spaniel above is a blue roans. Just like with "blue merles", these dogs are called "blue" but aren't actually genetically blue. The term "blue" is usually used to refer to black dogs with the dilute gene (dd), which dilutes the coat and nose to grey and the eyes to amber, but neither blue roans nor blue merles have this gene.
By contrast, these three dogs show light ticking. They are probably heterozygous for the ticking allele (Tt). As you can see, the ticking is mostly on the legs and muzzle. A dog homozygous for the ticking allele (TT) is more likely to be ticked all over.
These gundogs show heavier ticking and are mostly likely TT.
This Great Dane looks like it has ticking, but in fact the black spots are caused by the harlequin gene. It is genetically a merle, with its grey areas diluted to white. If you look carefully you can just about see a spot of actual white on its chest. This actual white is clear of ticking.
Dogs with ticking or roan are generally born white. The ticking/roaning develops as the dog grows. This can be rather dramatic, as in the Australian Cattle Dog. There is a popular myth that Australian Cattle Dogs are born white because of their Dalmatian ancestry. In fact, they are born white simply because they have the extreme white spotting pattern with roaned white areas. The roaning takes a while to develop, but the extreme white spotting is there from birth, hence the puppies are completely or almost completely white.
Dalmatian spots puzzled geneticists for a long time. They are completely unique to the breed and do not occur anywhere else in the dog world. Contrary to looks, Dalmatian spots and the harlequin pattern in Great Danes are not related.
It is now fairly certain that Dalmatian spots are in fact a modified form of ticking.
One of the main differences between standard ticking and Dalmatian spots is that the spots are often more sparse on a Dalmatian, bigger, and do not get more dense on the legs and muzzle. These are all probably to do with the modifier.
As well as the obvious similarity in looks between a dog with ticking and a Dalmatian, there are a few other things which indicate that spots are modified ticking:
- Dalmatians are born white and develop their spots later on.
- Dalmatians occasionally have patches on their head and/or body (as with the dog on the right above). These suggest that they have the piebald pattern, meaning that the spots can't be caused by any of the white genes. The high rate of deafness amongst Dalmatians also supports this (high white piebald is sometimes associated with deafness, as in white Boxers).
- Dalmatian crossbreeds are often ticked and never, as far as I am aware, spotted. This suggests that there is a recessive modifier behind Dalmatian ticking (so the Dalmatian parent passes down one copy of the modifier, but unless it's bred to another Dalmatian, there will never be another copy of the modifier, so it cannot be expressed).
- Sometimes Dalmatians are born which display some ticking or roaning (effectively "muddying" the base white). This is probably caused by a slight mutation or error in the modifier, stopping it from being expressed normally.