The Calico cat helps us better understand human genetics

When someone says “cats,” our minds typically conjure up images of their cute, furry, and playful natures, or we might recall funny viral cat videos that made us laugh. However, we rarely associate cats with genetics classes. Surprisingly, cats, or more precisely female cats, can teach us an important lesson about a genetic phenomenon that also occurs in humans, known as X chromosome inactivation.

Chromosomes act as structures that house our genetic material, DNA, packaged inside the cell nucleus. Human DNA is divided into 23 pairs of chromosomes. Each of these pairs is made up of a chromosome inherited from our mother and a chromosome inherited from our father. In other words, each of us inherits 23 chromosomes from our mother and 23 chromosomes from our father, for a total of 46 chromosomes in most of the nuclei in our cells.

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In all mammals, females have one pair of X chromosomes, while males have one X and one Y chromosome. Microscopic image of male chromosome pairs, with the sex chromosomes located in the lower right In all mammals, females have one pair of X chromosomes, while males have one X and one Y chromosome. Microscopic image of male chromosome pairs, with the sex chromosomes located in the lower right

In all mammals, females have one pair of X chromosomes, while males have one X and one Y chromosome. Microscopic image of male chromosome pairs, with the sex chromosomes located in the lower right

(Photo: NCRI, Scientific Photo Library)

Of the 23 pairs of chromosomes in humans, 22 pairs consist of two chromosomes of similar size and contain similar types of genes, in both males (boys and men) and females (girls and women). The rest are referred to as the sex chromosomes. In all mammals, females have two similar sex chromosomes, called the X chromosomes. In contrast, males have only one X chromosome and a much shorter chromosome called the Y chromosome. In rare cases, individuals can be born with other chromosome combinations, such as XXY, XYY, or just a single X without a Y chromosome.

Having two X chromosomes is not essential to living a normal life, as evident from the composition of the sex chromosomes. In fact, the presence of an extra X chromosome in females can create problematic situations. The X chromosome contains numerous genes associated with the immune system, and having two X chromosomes increases a woman’s susceptibility to developing autoimmune diseases such as lupus.

Therefore, a mechanism is needed to balance the expression of genes originating from the X chromosome. Such a mechanism must ensure that the total amount of proteins produced according to the instructions encoded in those genes remains consistent in both males and females. To achieve this balance, females have a mechanism that allows only one gene on the X chromosome to be active and produce proteins.

In 1961, researcher Mary Lyon proposed the existence of a mechanism that randomly silences one of the X chromosomes in female cells. This mechanism prevents genes on the silenced X chromosome from producing proteins, effectively addressing the problem of producing a large amount of protein. Subsequent studies conducted over the years have provided growing evidence for the existence of this mechanism. It is now known as X-chromosome inactivation or lyonization, named after Mary Lyon in recognition of her contributions.

Now, let’s dive into how this mechanism works.

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In Calico cats, one of the genes for fur color is silenced in each cell, while the other color is expressed.  A Calico cat puts its colors on display for all to see In Calico cats, one of the genes for fur color is silenced in each cell, while the other color is expressed.  A Calico cat puts its colors on display for all to see

In calico cats, one of the genes for fur color is silenced in each cell, while the other color is expressed. A calico cat shows off its colo

(Photo: Shutterstock)

For reasons not yet fully understood by biologists, one of the two X chromosomes in every cell in the female body starts randomly producing RNA molecules called Xist. This process occurs very early in embryonic development in female placental mammals, including elephants, wolves, humans, and cats. The Xist RNA then proceeds to wrap and loop around the chromosome it originated from, eventually covering most of the surface area of ​​the chromosomes.

Once Xist builds up, the chromosome undergoes structural changes that prevent the genes within it from being expressed and making proteins. Furthermore, proteins are recruited to the chromosome, serving as an additional barrier to gene expression. The chromosome that is silenced in this specific way within a specific cell remains silent throughout the lifespan of the cell. This process implies that each cell produces proteins based solely on genes on an X chromosome inherited from either the father or the mother, but not both. If the genes on the father’s X chromosome differ from those on the mother’s X chromosome, each cell will produce slightly different proteins, potentially resulting in distinct traits. This phenomenon, where distinct cells within an organism either generate proteins based on different segments of the same genetic code or possess an entirely different genetic code (which can occasionally occur due to various factors), is referred to as cellular mosaicism.

So let’s revisit the topic of cats. If you happen to come across a cat with patches of ginger, black and white fur, it is likely that you are witnessing the inactivation of the X chromosome in a female female. These cats are commonly known as calico cats. The reason for their colorful appearance is that the gene responsible for fur pigment (color) is located on the X chromosome. There are two different genes for fur color in the X chromosomes of these cats: black and ginger. In specific cells, one of the X chromosomes is silenced, while in other cells, the other chromosome is silenced.

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If you come across a cat with ginger patches, black and white fur, chances are you are witnessing X chromosome inactivation

(Photo: Taliya Levav)

As a result, patches of fur in both colors emerge. The variations in the size of the white, devoid of pigment fur can be attributed to a mutation in a gene not found on the X chromosome. Cats without this mutation display only ginger and black fur and are commonly referred to as tortoiseshell or tortoiseshell cats. “torties”.

But why did we say that “almost” all three-colored cats are female? This is because there are rare cases where males also possess two X chromosomes alongside a Y chromosome. In these rare cases, one of the X chromosomes is randomly silenced in each cell, allowing them to exhibit calico-like coloration. However, the likelihood of a calico cat being male is estimated to be less than one in a thousand.

The same phenomenon of cellular mosaicism occurs in women’s bodies, where the activity of the X chromosome is silenced. However, unlike cats, humans do not possess a visible trait whose gene is located on the X chromosome, so this mosaicism does not manifest itself externally.

So the next time you come across a cat with colored patches on its fur, you can both appreciate its beauty and reflect on the fascinating genetic phenomenon you’ve witnessed, which reminds us that every living creature has something to teach us about ourselves.


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