Tuesday, October 19, 2010

Of Mousers and Men

This is my cat, Chairman Mao (RIP). Mao (猫) means cat in Chinese. Funny, right?
What else is funny about this cat? 1) He's in a bag, isn't that cute? and 2) He has thumbs! Most cats have four toes on their front paws. Chairman had five toes and at least six claws (one between toes). Many people are surprised to learn that cats with extra digits are fairly common. They are known variously as polydactyl cats, mitten cats, and sometimes Hemingway cats. Apparently Ernest Hemingway was a big fan. Many Hemingway cats still reside at the Hemingway estate in Key West. They are especially common in port towns because sailors thought they were good luck, or at least very good mousers. They can come in a few varieties:

Most polydactylism is associated with an autosomal dominant gene, meaning that 1) it doesn't matter whether the cat is male or female (unlike, say, male pattern baldness in humans) and 2) that if a cat has the gene, then he will have extra toes. It also means that chances are good that the cat's kittens will also be mitten cats. Here is Fang, Chairman Mao's mother:

She is also polydactyl. I'm not sure who Chairman's father was, but regardless of how many toes he had, Chairman always had very high chances of having extra. Here's why:

You probably remember these from high school biology class, but in case you need a refresher...

The capital letter 'A' here represents the dominant allele of a gene, while the lowercase 'a' represents the recessive allele. Cats (and humans) have two alleles for each gene. 'AA', 'Aa', and 'aa' is the genotype (what genes it has) of the cat. In the picture above the colors represent the phenotype (what it looks like), where blue=mitten cat and green=boring cat. Because the 'A' allele is dominant, anytime this allele is present, the cat will be polydactyl. I don't know whether Fang is 'AA' or 'Aa', but either way Chairman was very likely to turn out with thumbs--14 out of 16 (87.5%) of the squares above are blue.

Humans and cats share a lot of the stages of embryological development, so it's not surprising that humans can also be polydactyl. Here is a picture of Devendra Harne, who has 12 fingers and 13 toes.
Devendra holds the Guiness world record for polydactyly. Congratulations dude!

There are differing accounts of the incidence of polydactyly in humans, but it is generally estimated at 1-2 per 1,000. The genetic basis is not super well understood, but it is often assumed to also be an autosomal dominant trait, just like in cats. Extra digits are often removed from babies at birth, so you won't necessarily know if a potential mate carries a gene for polydactyly. But before you freak out about genetic screening for potential partners, I'd like to remind you of the 12-fingered virtuoso in GATTACA.

Sorry Ethan Hawke, but sometimes it DOES matter how many fingers you have.

*Post by guest author, Elena Spitzer.  Elena is a graduate student in philosophy of science at University of Wisconsin, Madison.