The fascinating science of Calico cats

By | August 9, 2014

Just like humans, all animals have DNA, the information-code of life. Almost all the DNA between any two individuals is identical. However, in the case of male vs. female, there are some larger differences. DNA is organized into chromosomes, and females have two X chromosomes whereas males have one X chromosome and one Y chromosome. The X chromosome is very large and the Y chromosome is very small. To achieve dosage compensation, every cell in a female has one copy of the X chromosome inactivated.

All calico cats are female. Each of their cells has one X chromosome that came from their mother, and one X chromosome that came from their father. The code for orange fur color is located on the X chromosome from their mother, and the code for the black fur is located on the X chromosome from their father, or vice versa. The cells that give rise to fur color, then, lead to either black or orange, depending on which X chromosome is inactivated.

X chromosome inactivation actually occurs early on in life, when the organism is just 200-300 cells large and is called a blastocyst. At this point, the different cells give rise to different parts of the body, and the X chromosome that is inactivated remains inactivated in all the daughter cells of a particular cell line. For instance, if 1 cell in the blastocyst will give rise to the all the cells that form the fur color on the left side of the head, and if that 1 cell has the black-color X chromosome inactivated, then the left side of the head will be orange.

So where does white come from? The truth is that the above model is a simplified version of what actually happens. Genetics is normally much more complicated than just simple genes. The interaction of several genes often gives rise to physical traits. For example, there is not a single "tall" gene in humans. The areas of color on a calico cat are cells called melanocytes, specific cells that contain color pigments. Sometimes though, melanocytes do not reach all parts of the body. In early development, the melanocytes are derived from the dorsal aspect of the organism, and have to migrate to the other locations. This is why bellies are often white on animals. When the melanocyte does not reach a particular location, that fur becomes white.

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