Whether you adore cats or you’re allergic to them, you might be surprised to learn that you can better understand genetics by studying calico cats. More specifically, you can learn about epigenetics, or the study of changes in organisms due to the modification of gene expression, not the alteration of DNA. Known for their distinctive patches of black, orange, and white fur, calico cats illustrate a genetic process known as “lyonization.”
Understanding the Genetics of Calico Cats
A Brief Overview of Sex Chromosomes
Most humans get one sex chromosome from each parent:
- Biologically female humans (XX) receive one X chromosome from their mother and one X chromosome from their father.
- Biologically male humans (XY) receive an X chromosome from their mother and a Y chromosome from their father.
The X chromosome is large and carries hundreds of genes, some of which are essential for survival. The Y chromosome, on the other hand, doesn’t carry many genes, and none of its genes are necessary for the cell to survive. This is why all humans have an X chromosome, but only some have a Y chromosome.
Sex chromosomes work the same way in cats. Cats receive an X chromosome from their mother and either an X or Y chromosome from their father. A typical male cat with one of each sex chromosome (XY) cannot have a calico fur pattern, which means that almost all calico cats are female. (There are exceptions. In 2018, a rare male calico cat was barred from competing in cat shows.) Strange, right? Not if you know how those distinctive orange, black, and white splotches are produced . . .
Lyonization: The Inactivation of X Chromosomes
Each cell of a biological male cat receives instructions from two chromosomes, one X and one Y. For female felines, the process works a little differently because cells cannot receive instructions from two different X chromosomes. During embryonic development, one of the X chromosomes in each cell becomes inactive by coiling up into a tight structure known as a Barr body. Because the chromosome is curled up, its genes cannot be expressed. Only the genes on the uncoiled X chromosome are expressed, which means that all physical traits (such as fur color and eye color) are produced by the genetic information of the active X chromosomes.
This random and irreversible process is known as Lyonization. It ensures that only one active X chromosome exists in the cell of a female embryo. Lyonization occurs during the embryonic development of every female mammal, including female cats, and it’s impossible to predict which of the two X chromosomes will be inactivated.
How Lyonization Impacts Cats’ Coloring
In cats, the X chromosome carries a gene that determines fur color, and there are two different versions (or alleles) of this gene in calico cats. One allele produces orange fur, and the other produces black fur. A cat must receive both of these alleles, one from each parent, in order to have fur with orange and black patches. Then, during lyonization, if the X chromosome carrying a black fur allele is inactivated, the cell will produce orange fur. If the X chromosome carrying an orange fur allele is inactivated, the cell will produce black fur. Because lyonization occurs at random, the pattern produced on each calico cat is distinct.
So that explains the multicolored fur, but what about the patches? How are black and orange patches of fur produced, as opposed to individual hairs? The answer lies in the timeline of embryonic development.
Lyonization occurs early, when there are about eight cells in the embryo. After one X chromosome in each cell inactivates, those eight cells divide over and over again, producing the millions of cells required to build a whole cat. All of the cells originating from one of the eight original cells will have the same X chromosome inactivated as the original cell. This means that millions of cells with the same type of active allele (producing either black or orange fur) will exist in the same location, creating a solid patch of fur color.
What About the White Fur?
We’ve explained how the genetics of calico cats produce orange and black patches of fur, but what about the white fur? Calico cats also have a genetic condition known as piebalding, which causes unpigmented (white) spots of fur. Many different types of mammals can have this condition, including cats, dogs, horses, deer, cattle, pigs, and even snakes. If a cat has alleles producing orange and black fur but does not have piebalding, it is called a tortoiseshell cat.
So, the next time you see a calico or tortoiseshell cat, take a moment to appreciate the complex genetics that led to the cat’s distinct multicolored pattern.
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