Can cats have Down syndrome? Veterinarians hear this question more often than you might think. People typically ask it when their cat seems to look and behave abnormally, in a manner that resembles Down syndrome.

Cats with unusual facial features and certain behavioral abnormalities have become popular in social media circles. Claiming they have a "Down syndrome cat," some pet parents created social media accounts for them, thereby advancing the belief that Down syndrome can occur in cats.

Can Cats Have Down Syndrome?

Internet buzz notwithstanding, cats don't develop Down syndrome. In fact, they can't.

First, a bit about Down syndrome: It's a disorder that affects one in 700 human babies born in the U.S. each year. It occurs when the developing fetus's genetic material is copied incorrectly, resulting in an extra chromosome 21 (or a partial chromosome 21). This condition is also called trisomy 21.

Essentially, chromosomes organize the DNA in each cell into bundles, helping cells to pass on this genetic material when they divide. An extra chromosome 21 (or partial chromosome 21) causes a variety of birth defects that give people with Down syndrome their shared physical traits.

According to the National Down Syndrome Society, people with Down syndrome tend to share some or all of the following traits:

• Low muscle tone
• Small stature
• An upward slant to the eyes
• A single, deep crease across the center of the palm

It's important to note that people with Down syndrome do not all look the same.

Why Down Syndrome Cats Don't Exist

Humans have 23 chromosomes. Cats have 19. As such, having an extra chromosome 21  is clearly impossible for cats. But that doesn't mean cats can't occasionally have extra chromosomes.

In fact, a 1975 paper published in the American Journal of Veterinary Research identified a rare chromosomal abnormality in male cats that allows for one extra chromosome, resulting in a condition similar to Klinefelter syndrome in humans. These cats are especially noteworthy because the extra chromosome carries genetic material that affects their coloration. This condition causes these male cats to be tricolored ("calico" or "tortoise-shell"), a color pattern normally only seen in female cats.

Abnormalities That May Resemble Those of Down Syndrome

There have been some particularly noteworthy cats on Instagram that became internet sensations after their parents claimed that their cats owe their unusual appearances to extra chromosomes. It's unclear, however, whether these claims of chromosomal disease have ever been verified through genetic testing.

Despite the questionable claims and biological realities, "feline Down syndrome" has become a popular term. It is important to note, however, that the veterinary community does not recognize feline Down syndrome as a veterinary condition and, furthermore, does not advocate the transference of human conditions to animals on the basis of physical appearance or behavior. To do so may be construed as disrespectful to people who live with these conditions.

Nonetheless, there are some physical and behavioral traits that may lead well-meaning people to mistakenly assign human conditions to cats. A so-called "Down syndrome cat" typically manifests some distinctive characteristics, including:

• Broad noses
• Upturned eyes (which may be set widely apart)
• Small or unusual ears
• Low muscle tone
• Difficulty walking
• Difficulty with elimination (urination or defecation)
• Hearing or vision loss
• Heart problems