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 have had many pet cats throughout my life, but I currently have two 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 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, also known as 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 XO) and black (referred to as Xo) coloring in fur. This gene has two different kinds of alleles present. An allele is an alternative form of a gene, and is usually represented with letters. 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 instead codominance. Codominance is whenever both alleles are displayed equally in a trait. Therefore, whenever both alleles are present in the gene, they will both be expressed. In the calico cat, the alleles O and o are heterozygous, meaning that there is one of each allele, making the fur color gene Oo.
Calico cats are almost always female cats. This is due to this 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, and becomes a small condensed structure referred to as a Barr body. This X chromosome inactivation happens randomly in cells. This means that the one cell with the X chromosome containing the O allele could be inactivated, leading to the o allele being displayed and showing black color. Likewise, one cell with the X chromosome containing the o allele could be inactivated, leading to the O allele being displayed, demonstrating orange color.
Some calicos are tri-colored and display some white color on their fur as well, like my cat Patchy. 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 normal male will not have an extra X chromosome and will not display the calico color. 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 unfortunately cannot produce any offspring.