Colour
Cesky Terriers are found in two basic colours:
Black (later turning grey) and dark brown (later light coffee brown)
Puppies are either born uniformly black or brown (brown b is recessive to black B) or bi-coloured black (brown) with tan markings.
The uniform colour (given the genetic nomenclature As) is dominant to the bi-colour (denoted by at). Occasionally individuals are born that show a saddle (asa) pattern
The Cesky Terrier also has a gene for lightening the pigment (G) that has been inherited from the Sealyham Terrier. If the recessive (g) is present instead, the pigmentation stays dark. In bi-coloured terriers, the additional presence of the gene cch (also inherited from the Sealyham) will cause the lightening of the tan markings, which may then appear visually almost white.
Because the breed was developed from a combination between the fully-pigmented Scottish Terrier (SS) and the mainly white Sealyham (swsw) the Breed Standard allows for up to 20% white markings, although the breeder's aim should be for minimal white.
Genetically there are three levels of colour found in both black and brown Cesky terriers - dark, mid and light.
Mating together two dark coloured dogs will give 75% dark:25% mid:0%light
Mating together two light coloured dogs will give 0% dark:0%mid:100% light
What does all this mean to the breeder?
In practice there are two factors that should be born in mind:
1) Mating together two individuals which are both bi-colour will never give a puppy that is uniformly grey.
2) Mating together two individuals which are both light grey will only give light grey offspring.
Thus, in order to retain the full range of colour pattern within the breed it is essential to utilise both uniformly-coloured individuals and bi-colours, as well as the full range of shading from dark through to light.
Continually mating together very dark Cesky Terriers will eventually produce dogs that are visually black, rather than dark grey, whilst crossing together light-coloured dogs, without occasional recourse to a darker individual will lead to the breeding of Cesky Terriers that are optically white.