Single vs. double coat in dogs

There are two types of coats in dogs – single and double. A double coat consists of two layers: longer, coarser outer hairs and a soft, dense undercoat. The undercoat acts as thermal insulation, protecting the dog from both cold and heat. Dogs with double coats usually shed significantly, especially during seasonal changes. This type of coat is the original and is found in wolves and many Nordic, herding, and mountain breeds. Single coats, on the other hand, do not have an undercoat. They consist only of guard hairs, which tend to be finer, silkier, or curly. Dogs with a single coat usually shed less but are more sensitive to cold and overheating. This type of coat is typical for many companion and water breeds.

The development of undercoat is significantly influenced by two mutations (CFA28:24,860,187 C>T and CFA28:24,870,184 G>A) located in the regulatory region of DNA near the ADRB1 gene. The presence of one or both variants increases the likelihood that a dog will have a single coat without undercoat. These mutations are most common in breeds such as Poodles, Bichons, Maltese, Yorkshire Terrier, Portuguese Water Dog, and Irish Water Spaniel, but are very rare in Nordic, Alpine, or herding breeds, where they may explain the unusually sparse undercoat or its complete absence, which does not correspond to the breed standard. Dogs without these mutations usually have a double coat. However, coat type is a complex trait, so the final appearance may also be influenced by other genes and, to a lesser extent, by the environment.

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Reference:

Whitaker, D.T., Ostrander, E.A. : Hair of the dog: Identification of a cis-regulatory module predicted to influence canine coat composition. Genes (Basel) 10:323, 2019. Pubmed reference: 31035530