Most mammals have one pair of sex chromosomes in each cell: females are characterized as having two X chromosomes per cell, while males are characterized as having one X chromosome and one Y chromosome per cell. The X chromosome is much larger than the Y chromosome and contains thousands of genes responsible for producing many products that are essential for cell activity. The amount of gene products encoded by the X chromosome is relatively equal in males and females. How can this be if female cells contain double the number of X chromosomes? The answer is referred to as dosage compensation, whereby the expression of X chromosome – encoded gene products is restricted. In female mammals, dosage compensation is achieved through the inactivation of one of the two X chromosomes. Essentially, in each cell, a female mammal only has one functioning X chromosome. This regulatory mechanism is called X inactivation and has interesting implications when considering coat color in cats.

Tortoiseshell cats are not a specific breed of cat; instead they are cats that share a common coat coloring, characterized by brindled or mottled patterns of orange, black, chocolate or cinnamon. Tortoiseshell cats with white patches can either be tortoiseshell and white cats or calico cats, depending on the amount of white present and the uniformity of the other patches of color (calico cats have larger patches of white with well defined patches of color).

Figure 1: Kuma is our tortoiseshell colored cat.

Kuma

Coat coloration in cats is complex and controlled by many genes. The O gene is responsible for determining if orange coat color will be present and is “sex-liked” because it is found on the X chromosome. The O gene’s dominant allele results in orange fur, while the recessive allele results in black, chocolate or cinnamon fur. Tortoiseshell coloration results from the inheritance of both the recessive and dominant alleles of the O gene (one on each X chromosome) and the subsequent inactivation of one X chromosome in each cell.

Male cats only have one copy of the X chromosome and are therefore generally not able to express tortoiseshell coloration, except under rare circumstances of chromosomal abnormalities such as genotype XXY (which results in sterile males), chimericism (an individual results from the fusing of two embryos), or chromosomal mosaicism (only some cells have genotype XXY and may result in fertile males). Only 1 in 3,000 male births results in a calico cat; as a result, both tortoiseshell and calico cats are almost always female.

In 1961, Mary Lyon proposed the following set of hypotheses to account for tortoiseshell coloration resulting from X inactivation:

As the embryo develops, pigment cells form along the nueral crest (which will eventually become the cat’s spine) and migrate over the body during skin formation. The number of different pigment cells in one given area of forming skin is presumed to be related to the degree of color brindling in that area. In areas where only one pigment cell is present, the cell has the opportunity to cover the whole area, resulting in a large patch of solid color.

The genes that control the presence of white fur act by inhibiting the migration of pigment cells. When pigment cells arrive after the skin has formed, no color will be incorporated into skin, resulting in white fur. Pigment cells take longest to reach areas furthest from the cat’s spine; therefore, white patches are most commonly observed on paws, the chest and the tip of the tail.

Tortoiseshell and calico cats have long been prized for their unique coloring and in many cultures are thought to bring luck. Japanese sailors took calico cats to sea with them for protection during long voyages and many homes believed that good fortune would come from owning a calico cat. Even if tortoiseshell cats are not particularly beautiful to your eyes, you can certainly admire the complexity of the genetic and developmental pathways involved in producing this coat color.

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This work is licensed by Andrea Liévana-MacTavish under a Creative Commons Attribution License (CC-BY 3.0), and is an Open Educational Resource.

Last edited by Andrea Liévana-MacTavish on Aug 31, 2011 1:54 pm -0500.