The question of whether black fur is dominant in cats has a simple answer that quickly becomes complex, as coat color is determined by the interaction of several different genes. A gene is a segment of DNA that determines a specific trait, and an allele is one of two or more forms of that gene. A dominant allele requires only a single copy to express its trait, while a recessive allele requires two copies to be visible. The deep black color is governed by an allele that is dominant over the alleles for brown or cinnamon, but this dominance is only the first step in a multi-layered genetic process.
The Gene That Determines Black Color
The foundation for black fur rests on the gene responsible for creating the black pigment, known as eumelanin. This gene, often called the B locus, dictates the type of pigment molecule a cat can produce. The dominant allele at this locus, represented as “B,” directs the production of full, dense eumelanin, which results in a black coat color. A cat only needs to inherit one copy of the “B” allele from either parent to be genetically black.
The “B” allele is dominant over all other color variants at this locus, specifically the alleles for chocolate and cinnamon. The chocolate allele is recessive to black, and the cinnamon allele is recessive to both black and chocolate. These alternative colors only appear when a cat inherits two recessive alleles.
The cat’s genetic makeup at this locus can be either homozygous dominant (BB) or heterozygous (Bb), and in both cases, the cat will appear black. Only when the cat is homozygous recessive for one of the alternative colors, such as (bb) for chocolate, will the black pigment itself be altered.
How Patterns Affect Black Fur Expression
While the B locus determines the color of the pigment, a different gene controls where that pigment is placed along the hair shaft. This second gene, the Agouti gene, determines whether a cat will display a patterned coat, such as a tabby, or a solid, self-colored coat. The dominant Agouti allele, represented as “A,” allows for the expression of a banded hair shaft, where alternating bands of pigment create the characteristic tabby pattern.
The dominant “A” allele is responsible for the visible striped, spotted, or ticked patterns seen in many cats. In this case, the dominant trait leads to a patterned coat, which means that a genetically black cat (B-) will look like a black tabby. To achieve a solid, unpatterned black coat, the Agouti gene must be completely inactivated.
The non-Agouti allele, represented as “a,” is recessive, meaning a cat must inherit two copies (aa) to suppress the tabby pattern. This recessive combination causes the black pigment to be deposited uniformly along the entire length of the hair shaft, resulting in a solid black appearance. A cat must possess the dominant black pigment allele (B) and the recessive non-Agouti pattern allele (aa) to truly look solid black.
Changing Black: The Dilution Effect
A third major gene modifies the intensity of the black color, determining whether the pigment remains dense or is “diluted.” This is controlled by the Dilution gene, which impacts the distribution of the pigment granules within the hair shaft. The dominant allele at this locus, represented as “D,” ensures that the eumelanin pigment is densely packed and evenly distributed, resulting in a full, rich black color.
The recessive allele, “d,” causes the pigment granules to clump irregularly instead of spreading evenly throughout the hair. This clumping effect changes how light reflects off the fur, visually lightening the coat color. A cat must inherit two copies of this recessive allele (dd) for the dilution effect to be visible.
When this dilution gene acts on a genetically black cat, the black pigment is visually diluted to a shade of grey, commonly referred to as “blue” in cat fancy. This means a cat that is genetically black (B-) can appear blue if it is also homozygous recessive for the dilution gene (dd).
Understanding Solid Black and Genetic Myths
To achieve the striking appearance of a jet-black cat, three specific genetic conditions must align. The cat must possess at least one dominant black pigment allele (B-), two copies of the recessive non-Agouti allele (aa) to eliminate patterning, and at least one dominant dense color allele (D-) to prevent dilution. This precise combination of dominant and recessive genes explains why black fur is common, but a truly solid black coat requires specific genetic alignment.
A common observation is that a black cat’s fur can develop a reddish or brownish tinge, often called “rusting,” with frequent sun exposure. This is a physical change, as the eumelanin pigment that creates the black color can break down when exposed to ultraviolet light. Another visual variation, the “black smoke” pattern, is often mistaken for a true solid black cat.