An individual may have two similar alleles or two different alleles. If the individual has two similar alleles, the gene is homozygote. If the individual has two different alleles, the gene is heterozygote. A homozygote individual can only give one type of allele to its offspring while a heterozygote individual could give any one of the two types of alleles to its offspring.
An example of how an offspring’s sex is determined: The male carries one X chromosome and one Y chromosome, while the female carries two X chromosomes. The female can therefore only give X to the offspring whereas the male can give either X or Y.
Despite all the varieties of colours and patterns visible on dogs, there are only two pigments – eumelanin (black) and pheomelanin (red). All coat colours are made up of these two pigments, two forms of melanin. Each of the pigments has a basic colour which may be modified by different genes.
Eumelanin is the basic type for black pigment. Eumelanin producing cells cause black areas on a dog. However, there are genes making eumelanin to other colours such as brown, blue or isabella (a moderate yellowish brown). If a dog has any of the genes that makes the black pigment change to brown, blue or isabella, the entire black coat will change. The reason is that these genes inhibit or change the production of eumelanin so that none of the cells may produce a fully black pigment. Dogs with the diluted colour are called blue or isabella. Technically the brown colour is not diluted but is caused by a change in the structure of the pigment. Eumelanin is present on other body parts of the dog such as the nose and eyes.
The other type of pigment is pheomelanin which is the red pigment. Pheomelanin is only produced in the fur and does not occur in the eyes or on the nose. In contrast to eumelanin it does not come in two colours. Pheomelanin is just one colour which varies in intensity. There are variations from a dark red in dogs as the Irish Setter to pale yellow as in a very light-coloured Labrador. Most dogs, however, have both eumelanin and pheomelanin. A black and tan Kelpie is a wonderful example of this.
Genetic research is continuously developing, and old truths are being revised all the time. In the near future we hope to clarify how the gene for the brown colour’s intensity works, or find out the exact reason why some Kelpies are “ghost tan”.
The texts about colour genetics on the Kelpiegallery pages are based on scientific research. Do note that the genetics information on these pages are written in a simplified language and addresses general home page visitors. If you are interested in delving deeper into colour genetics, I recommend looking at the list of sources.