polydactyly

The medical term for having six fingers. It is a male only genetic defect, females are immune. A lot of males in the royal family had polydactyly (Queen Elizabeth's husband had it) . Fortunately for the current living royal family their fathers, grandfathers etc. did not have the defect.

Polydactyl literally means many-fingered; this doesn't limit the mutants to six fingers. Indeed, since five fingers is the 'norm' (the pentadactyl limb concept) for vertebrates any more than this is considered 'polydactylous' - six, seven, eight and so on. However, this is somewhat confusing if you consider horses and whales (both vertebrates) which don't seem to fit the pattern. The standard explanation is embryological - the foetus has all five fingers, but they are lost or fused together during development.

However in an essay called "Eight Little Piggies" (the title essay of a compilation), Stephen Jay Gould makes the case for the pentadactyl limb as a 'frozen accident'. In other words, there is nothing especially unusual about limbs that have more or less than five digits. Some evolutionary experimentation with other numbers may have taken place. This makes sense if you consider the potential for extra fingers that polydactylous mutations show. Also, frogs have four fingers on their front feet - although the panda's apparent sixth digit (the false thumb) is not a true digit.

A congenital birth defect in which a person is born with more than 10 fingers or toes.

Doctors aren't 100% sure about what causes this but it is known to be an error on the Trisomy 13 chromosome of DNA. The cause is meiotic nondisjunction.

The odds of this happening are 1 in 20,000 live births.

While it's better to have too many than too few fingers (The doctor can remove them later), A polydactal baby may also have other problems with this chromosome, such as:

Severe retardation
Apneic spell in infancy
Cryptorchidism
Polydactyly
Low-set ears
Flexion
deformity of fingers
Cleft lip/palate
Scalp defects

Polydactyly is a polymorphism in which an individual is born with an extra digit on the hand or foot. This is commonly observed in cats, especially in those cats occupying Ernest Hemingway's house in Key West. Polydactyl is both an adjective and a noun meaing polydactyly.

When a human is polydactyl, it is classified as a specific case within a genus. Polydactyl is the most common congenital hand disability.

There are a number of famous cases of polydactyl people, among them the Biblical giant Goliath, Antonio Alfonesca, and Anne Boleyn. In the story of the The Princess Bride, Inigo Montoya vows revenge against the six-fingered man who murdered his father. 

All pandas appear to be polydactyl but as The Alchemist states above, their extra digit does not count.

The condition may appear in one or both of the upper or lower phalanges. The extra digit may jut off from one of the outer digits or from within the center. In the case of the latter there is a greater chance of functionality. If the digit is deemed non-functional it may be removed by a plastic or orthopedic surgeon, most easily upon birth.

Polydactyly can occur even if there is no family history of the condition. It may skip a generation and it may not.