I have two main loves in my life: science and cats. For the sake of this blog, I thought that it would be fun if I combined the two. I currently have two cats at home named Thomas and Patchy. Thomas is a sixteen year old white American short hair male cat and Patchy is a fourteen year old female calico cat. As with any other cat parent, I think my cats are unique and have their own distinct personality. Thomas is basically an old geezer, who likes to nap a lot, but who wouldn’t if they were a cat with no responsibilities? He is also extremely intelligent and responds to you when you say his name. With this intelligence, he can also be very stubborn. My other cat, Patchy, is a very lovable and chubby cat. She has hilarious antics of licking herself uncontrollably when you scratch her lower back. She also routinely performs the downward facing dog pose with her lovely mitten paws outstretched in front of her. Since Patchy is a calico cat, she has many beautiful colors in her fur. What makes the fur of these calico cats so unique and beautiful? The answer to that, my friends, is genetics.

To begin with, cats have nineteen pairs of chromosomes, with thirty-eight chromosomes in total. Thirty-six of these chromosomes are referred to as autosomes (non-sex chromosomes). The two remaining chromosomes are the sex chromosomes X and/or Y. Similar to humans, the distribution of the X and Y chromosomes determines if the cat is male or female. If a cat inherits one X and one Y chromosome, it will be male. If it inherits two X chromosomes, it will be female. On these sex chromosomes, there are many genes present. One of the genes found in the X chromosome on cats is responsible for orange (referred to as X^O) and black (referred to as X^o) coloring in fur. This gene has two different kinds of alleles present (which is an alternative form of a gene). For this gene, the uppercase O represents the orange allele, and the lowercase o represents the non-orange allele. These alleles do not display dominant and recessive traits to each other, but codominance and are displayed equally in a trait. Therefore, whenever both alleles are present in the gene, they will both be expressed. The O and o alleles are both present (heterozygous expression) in the calico cat, making the fur color gene Oo.

Calico cats are almost always female cats. This is due to the heterozygous gene present in their X chromosomes. However, females do not need these two copies of the X chromosome, because the information contained in these chromosomes is the same. Therefore, one X chromosome is inactivated randomly, becoming a Barr body (a small condensed structure).  This means that the one cell with the X chromosome containing the O allele could be inactivated, leading to the o allele being expressed and demonstrating the black fur color. Likewise, one cell with the X chromosome containing the o allele could be inactivated, leading to the O allele being expressed, demonstrating orange color.

Some calicos are tri-colored (like Patchy) and also display some white color on their fur. This white coloration is due to the spotting gene, which delays melanocytes from reaching the skin’s surface. At this time, the location of the spotting gene is not exactly known. This gene consists of the S allele, which yields white fur, and the s allele, which yields all other colors. These alleles display dominant and recessive traits. For these alleles, the uppercase S is dominant over the lowercase s allele. Whenever a cat is heterozygous, Ss, for this gene, there is white color in the stomach area, and paws. This may give some cats, such as Patchy, a gloved appearance around their paws.

As I mentioned earlier, calico cats are often female. Typically, male cats have one X chromosome and one Y chromosome. Therefore, a genetically-typical male will not have an extra X chromosome and will not display the calico coloring. However, in rare cases, a male cat may have two X chromosomes and one Y chromosome. This genetic irregularity occurs in humans as well, and is referred to as Klinefelter’s Syndrome. By having two X chromosomes, one can be inactivated randomly in each cell and can produce the different fur colors. However, due to this chromosomal imbalance, these male cats are sterile and cannot produce any offspring.