An extremely rare cat was up for adoption at the Humane Society Silicon Valley — but his time as a rescue kitty didn't last long. He's just been adopted.

The reason for the excitement is that Sherman is a calico — a common feline color pattern — but he's also a boy, and only one out of every 3,000 calico cats is male.

"It's like a unicorn," said Dr. Andrea Berger, an HSSV veterinarian. "I've been involved with shelters for 20 years, and I have never seen one."

What Makes Male Calicos so Uncommon?

It has to do with genetics.

Coat color in cats is typically a sex-linked trait, meaning the physical characteristic is related to the cat's gender.

Both male and female cats can be orange or black because the gene that controls orange fur color is on the X chromosome. However, in males, an orange color is usually expressed in a striped or tabby pattern.

Female cats, on the other hand, can be tabby, calico or tortoiseshell. Calico and tortoiseshell are similar, but calicos have patches of white, orange and brown or black while torties' coats are only orange and black.

Because the X chromosome is responsible for both orange and black fur, female cats can display both colors because they have two X chromosomes. But males, having only one X chromosome, can be either orange or black.

Chromosomal Abnormalities in Male Calicos

For a male cat to have a calico pattern, the feline has to have three sex chromosomes: two Xs and a Y.

This XXY combination can occur when there's an incomplete division of the male's XY chromosome pair at the time of fertilization — and it doesn't just happen in cats. It can occur in people too, resulting in a genetic disorder known as Klinefelter syndrome.

While you might assume that a cat like Sherman is valuable because of his rarity, male calicos are of little interest to breeders because they're typically sterile.

In fact, it's estimated that only one out of 10,000 male calicos is fertile.