Why Are Some People Born with Six Fingers?

Take a look at your hands.

Without thinking about it, you'll probably count five fingers on each one. It's something most of us accept as completely normal. But every year, babies around the world are born with an extra finger or toe.

This condition is known as polydactyly. Although it may seem unusual, it is one of the most common congenital differences affecting the hands and feet. Exactly how common it is varies between different populations and ethnic groups.

For example, postaxial polydactyly, where the extra digit develops on the little finger side, is significantly more common in people of African ancestry than in many other populations. This reflects differences in genetic inheritance rather than differences in health or development. 

More Than "Just an Extra Finger"

Not all extra fingers look the same.

Some are attached only by a small piece of soft tissue, while others are fully developed with bones, joints, tendons, nerves and blood vessels. In some people, the extra digit has little or no movement. In others, it functions almost like any other finger.

Doctors classify polydactyly according to where the extra digit develops:

• Preaxial polydactyly – on the thumb side of the hand or the big toe side of the foot.
• Postaxial polydactyly – on the little finger or little toe side. This is the most common form.
• Central polydactyly – where the extra digit develops between the existing fingers or toes. This type is much rarer. 

So, Why Does It Happen?

The answer begins long before a baby is born.

During the first few weeks of pregnancy, a baby's hands and feet begin as tiny paddle-shaped limb buds. As development continues, a complex network of genes carefully controls where each finger and toe should form, how long they should grow and when growth should stop.

Occasionally, small changes occur in these developmental signals. Instead of producing five digits, an additional finger or toe develops. Scientists have identified several genes involved in this process, including GLI3 and the Sonic Hedgehog (SHH) signalling pathway, both of which play a crucial role in determining the number and pattern of the fingers and toes during embryonic development. Changes affecting these pathways can result in an extra digit forming. 

Is It Inherited?

Sometimes.

Polydactyly can occur sporadically, meaning there is no previous family history and it develops for the first time in that individual. In other families, it is inherited, most commonly in an autosomal dominant pattern. This means that if one parent carries the altered gene, each child has approximately a 50% chance of inheriting it. Interestingly, the appearance of the extra finger or toe can vary considerably, even between members of the same family. 

Is Polydactyly Always an Isolated Condition?

Not always, but most of the time, it is.

In many people, polydactyly occurs on its own and is known as non-syndromic polydactyly. Apart from the extra finger or toe, the individual is otherwise healthy and develops normally.

Less commonly, polydactyly occurs as part of a wider genetic syndrome, where it is associated with other physical or developmental differences. Examples include Ellis–van Creveld syndrome and Bardet–Biedl syndrome. Because of this, when a baby is born with an extra digit, healthcare professionals carry out a thorough newborn examination. They are not simply assessing the hand or foot, they are looking for any additional clinical features that might suggest an underlying genetic condition. In the vast majority of cases, however, the extra digit is an isolated finding. 

Can People Actually Use the Extra Finger?

This is where things become fascinating.

For many years, it was assumed that an extra finger had little practical use. However, research has shown that this isn't always true.

Scientists studying people born with fully functional sixth fingers discovered that they could move the extra digit independently. Even more remarkably, brain scans showed that the brain had developed dedicated neural pathways to control it. Participants were able to perform certain one-handed tasks that people with five fingers simply couldn't, without affecting the movement of their other fingers.

In other words, the brain didn't become confused by having an extra finger, it adapted. 

Does Polydactyly Need Treatment?

Not always.

If the extra digit doesn't interfere with movement or cause discomfort, treatment may not be necessary. Many people live their entire lives without experiencing any problems.

Others may choose surgery if the extra finger or toe affects function, catches on clothing, causes discomfort or for cosmetic reasons. The decision depends on how well the digit has developed, whether it contains bones and joints, and the individual's circumstances.

A Small Difference That Teaches Us Something Big

It's easy to think of the human body as following a fixed blueprint: ten fingers and ten toes.

But biology isn't always that predictable.

 

References

• Malik, S. (2014) 'Polydactyly: phenotypes, genetics and classification', Clinical Genetics, 85(3), pp. 203–212. Available at: https://doi.org/10.1111/cge.12276
• Umair, M., Ahmad, F., Bilal, M., Ahmad, W. and Alfadhel, M. (2018) 'Clinical genetics of polydactyly: An updated review', Frontiers in Genetics, 9, Article 447. Available at: https://doi.org/10.3389/fgene.2018.00447

 

 

 

 

Disclaimer:This resource is designed for educational purposes for UK student nurses and healthcare professionals. While we strive for clinical accuracy, it does not constitute medical advice. Always refer to your specific Trust’s local policies, NICE guidelines and the NMC Code in clinical practice. Clinical scenarios can change rapidly; when in doubt, escalate to your mentor or senior clinician.⁠