Mice can be black, a coloration arising from specific genetic instructions within the species Mus musculus and related rodents. This dark coat color departs from the typical wild-type brown or agouti pattern. Black mice are found in natural populations but are more frequent in laboratory and domesticated settings. The presence of black fur is determined by the biochemical process of pigment production, governed by regulatory genes.
The Genetic Basis of Coloration
The color of a mouse’s fur is determined by the type and amount of melanin pigment deposited by specialized cells called melanocytes. There are two types of melanin: eumelanin (black and brown) and pheomelanin (yellow and red). Black fur results from the maximum production and deposition of the dark pigment, eumelanin, throughout the hair shaft.
All-black coloration is achieved through mutations in genes that regulate the pigment switch. The agouti signaling protein (ASIP) gene is central, as its protein product usually blocks eumelanin production, promoting pheomelanin. The wild-type agouti coat, which has black and yellow bands on each hair, results from ASIP cycling the production of both melanins.
Black fur, the “nonagouti” phenotype, arises from a loss-of-function mutation in the ASIP gene, represented by the recessive allele ‘a’. When a mouse inherits two copies of this allele, the agouti protein is non-functional. Melanocytes then continuously produce only the dark eumelanin pigment, resulting in a solid black coat.
Black Mice in the Wild and Labs
The rarity and origin of black mice differ significantly between wild and captive populations. In the wild, black coloration is a form of melanism, which is uncommon in the house mouse (Mus musculus). The natural agouti coat offers better camouflage in varied environments.
Melanism can be adaptive in some wild rodent populations, such as rock pocket mice living on dark lava flows, where the dark color provides protection from predators. Wild house mice may occasionally be melanistic due to spontaneous local mutations, but the coloration is not widespread because the agouti pattern is generally advantageous.
In contrast, the black coat color is ubiquitous and intentional in laboratory and domestic settings. Decades of selective breeding have fixed this black-coat genotype into numerous established strains. Laboratory mice are frequently black due to deliberate inbreeding to create genetically uniform strains for research. The black coat is a stable, easily identifiable trait that helps researchers track the genetics of a line.
Common Varieties That Exhibit Black Coats
The most famous and widely used black mouse is the C57BL/6 inbred strain, often nicknamed “Black 6.” Developed in the 1920s, this strain is characterized by its glossy, dark coat. It is the most common genetic background used for generating genetically modified mice worldwide.
The C57BL/6 mouse possesses the nonagouti allele, ensuring uniform black pigmentation. Beyond the laboratory, black mice are common in the pet trade, known as “Fancy Mice.” Breeders have stabilized the nonagouti genotype to produce deep, uniform black coats prized for their aesthetic appeal.