Why do some people have an extra finger or toe - is it genetic?

Polydactyly - having extra fingers or toes - is one of the more common minor congenital differences and it is usually genetic.

The most common form is autosomal dominant, meaning a person with the affected gene variant has the trait, and each of their children has a 50% chance of inheriting it. Several genes can cause it, the best-studied being GLI3.

In some families, polydactyly has been passed down for many generations and is treated as a normal family trait. In other cases, it appears as part of a broader syndrome that has additional features. The non-syndromic version (just extra digits, nothing else) is the more common one and is medically uncomplicated - the extra digit is often surgically removed in infancy, more for cosmetic and practical reasons than medical ones.

A few interesting facts. Polydactyly is more common in some populations than others - it occurs at higher rates in some West African and African-American populations than in European-descended populations. The hand is more commonly affected than the foot. The extra digit is usually on the pinky side, not the thumb side.

It is one of the visible reminders that the genetic instructions for limb development are intricate and that small variations in those instructions show up as small variations in our bodies.