As you have observed, puppies or kittens with very different coats can be born in the same litter. Some are plain, some are spotted or mottled, while others may be bi- or tricoloured. But where do these differences come from and how is the coat colour determined ?
To answer these questions, we need to understand how hair is made and how it grows in dogs and cats.
Hair is produced inside the hair follicle, which looks like a small sac of epidermis invaginated in the dermis.
Surrounding the hair follicle are structures important for hair growth and maintenance, such as skin glands and hair follicle muscles, as well as an area called the dermal papilla, which is highly vascularised and responsible for providing the nutrients and oxygen needed to produce new cells.
The lower part of the hair follicle bulges into a bulb that contains the matrix. The matrix is composed of cells that are constantly renewing themselves. Among them are mainly the keratinocytes, which produce keratin. As the hair grows, the keratinocytes are gradually pulled upwards. They stop multiplying, but continue to produce keratin. When the hair emerges from the surface of the skin, it is composed mainly of “dead” keratinocytes filled with keratin.
The cells that produce pigment (the melanocytes) are also in the matrix. But the melanocytes are not transported along the shaft. The pigments they produce are transferred to the keratinocytes, which carry them along with them as the hair grows.
The two main pigments are pheomelanin, which colours the hair from yellow to red, and melanin, which gives it black or brown. In the absence of any pigment, the hair remains white.
During fertilisation, the genes of the father and mother are combined. Many physical characteristics are thus inherited from both parents, including hair colour, patterns, and the distribution of different coloured spots.
There are also genes that control the density and length of the coat, which can influence colour perception. So an animal with a dense coat will appear darker than one with a sparse coat.
The environment and deficiencies in essential nutrients can influence the quality of the coat and change its colour, as can autoimmune diseases or certain hormonal disorders. Age is also a factor: older dogs often have a whiter coat because melanin production gradually decreases, especially in the muzzle area.
Finally, seasonal changes, levels of sun exposure, diet quality, and living conditions can all affect coat colour and texture.
In summary, variations in pattern and colour of the coat of animals are the result of a complex combination of genetic, hormonal, and environmental factors, and can lead to large differences within members of the same species and litter.
Cummings school of veterinary medicine Tufts University: Why did my pet’s black hair coat turn red?
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