Polydactyly is a congenital condition where an individual is born with extra fingers or toes, known as digits. This anatomical variation means “many digits,” reflecting the presence of more than the typical five digits on a hand or foot. It is a common congenital anomaly observed globally.
Understanding Polydactyly
Polydactyly presents as an extra digit on the hand or foot. The extra digit might be a small, raised bump of skin and soft tissue, or a more developed structure containing bone, with or without a joint. It can even be a fully formed, functional digit. This condition can affect one hand or foot, or both.
The location of the extra digit classifies polydactyly into three main types. Postaxial polydactyly, the most frequent type, involves an extra digit on the side of the little finger or toe. Preaxial polydactyly occurs on the thumb side of the hand or the big toe side of the foot. Central polydactyly, the least common type, involves an extra digit located between the other fingers or toes.
Common Inheritance Patterns
For many individuals, polydactyly is inherited, most frequently following an autosomal dominant pattern. In autosomal dominant inheritance, only one copy of an altered gene is needed for the condition to manifest. If one parent has polydactyly due to an autosomal dominant gene, there is a 50% chance that each child will also inherit the condition.
This inheritance pattern means the gene responsible for polydactyly is located on one of the non-sex chromosomes, and its effect is dominant over the normal gene. For example, if a parent carries one copy of the gene for postaxial polydactyly, their child has a significant probability of inheriting that gene and developing the extra digit. This is the most common genetic cause for isolated polydactyly, where the extra digit is the only physical difference present.
While less common for isolated polydactyly, autosomal recessive inheritance can also play a role. In this pattern, an individual must inherit two copies of the altered gene, one from each parent, to develop the condition. If both parents carry one copy of the recessive gene, they typically do not show symptoms themselves but have a 25% chance with each pregnancy of having a child with polydactyly. Autosomal recessive forms of polydactyly are often associated with broader genetic syndromes rather than occurring as an isolated trait.
Variations in Genetic Expression
The way polydactyly appears can vary greatly, even among individuals within the same family who share the same genetic alteration. This phenomenon is termed “variable expressivity,” meaning that while a gene may be present, the extent to which it manifests can differ. For example, one family member might have a fully formed extra finger, while another carrying the identical gene might only present with a small skin tag.
Another factor influencing how polydactyly manifests is “incomplete penetrance.” This occurs when an individual inherits the genetic alteration associated with polydactyly but does not display any physical signs of the condition. They carry the gene and can pass it on to their children, who may then exhibit polydactyly, even though the parent did not. This explains why the condition might seem to skip a generation in some family pedigrees. These variations make genetic counseling and prediction of the exact outcome complex, even when the genetic cause is known.
When Polydactyly is Not Inherited
While often inherited, not all instances of polydactyly are passed down through family lines. Some cases are considered sporadic, meaning they occur without a clear family history of the condition. These sporadic cases may arise from a new genetic alteration that occurs for the first time in the affected individual, rather than being inherited from a parent. Such new mutations happen spontaneously during early development and are not present in the parents’ DNA.
Environmental factors during pregnancy can also sometimes contribute to the development of polydactyly, though genetic factors are more commonly implicated. When polydactyly is not inherited, it often signifies an isolated occurrence, meaning the individual is otherwise healthy without other associated conditions. However, polydactyly can also be a feature of certain genetic syndromes, where it appears alongside a collection of other physical and developmental differences.