Polydactyly is a congenital limb difference where a person is born with one or more extra fingers or toes. This condition is one of the most common congenital abnormalities, occurring in an estimated 1 in 500 to 1,000 live births globally. The word comes from the Greek meaning “many digits” and manifests in a wide spectrum of forms. The extra digit can range from a small, soft-tissue projection, sometimes called a nubbin, to a fully formed and functional digit complete with bones and joints.
Understanding the Types of Polydactyly
Polydactyly is categorized based on the location of the extra digit on the hand or foot, a distinction that often correlates with the underlying cause.
Postaxial Polydactyly
This is the most common type, where the extra digit appears on the side of the little finger or little toe (the ulnar side of the hand). This form is particularly frequent in certain populations, like those of African descent.
Preaxial and Central Polydactyly
Preaxial polydactyly involves an extra digit on the side of the thumb or the big toe (the radial side of the hand). The third and rarest type is Central polydactyly, where the duplication occurs within the middle three digits. Understanding these classifications is important because the prevalence and specific inheritance patterns differ between the types.
The Primary Inheritance Mechanism
The most common form of polydactyly, Postaxial polydactyly, is inherited through an Autosomal Dominant pattern. This means a person only needs to inherit one copy of the altered gene from either parent to display the physical trait. If one parent carries the gene, there is a 50% chance of passing it on to each child.
This dominant inheritance pattern is often associated with mutations in genes involved in the limb development pathway, such as the GLI3 gene. Because the gene is located on a non-sex chromosome, it is termed “autosomal” and affects both males and females equally. While dominant inheritance is most frequent, some less common types can be inherited in a recessive manner, requiring two copies of the altered gene to manifest the trait.
In dominant forms, the altered gene disrupts the normal signaling pathways that pattern the developing limb bud during early gestation. Defects in the anterior-posterior patterning of the limb lead to the formation of the extra digit. The majority of non-syndromic cases follow these straightforward Mendelian rules.
Why the Trait Varies: Penetrance and Expressivity
The inheritance of polydactyly often appears inconsistent in families, which is explained by the concepts of incomplete penetrance and variable expressivity.
Incomplete Penetrance
Incomplete penetrance describes a situation where an individual carries the gene for polydactyly but does not physically display the extra digit. For example, a parent may carry the dominant gene and pass it to a child who has polydactyly, even though the parent has a normal number of digits.
Variable Expressivity
Variable expressivity refers to the wide range of ways the trait can appear among people who do show the condition. One person might have a small, non-functional skin tag, while a relative with the same gene mutation may have a fully formed, functional extra finger. The variation in the expression of the trait can even be seen in the same person, with one hand having a small projection and the other having a developed digit. This variability is believed to be influenced by other genes, known as modifier genes, and environmental factors that subtly alter the gene’s function during development.
Distinguishing Isolated from Syndromic Polydactyly
Polydactyly is classified based on whether it occurs alone or as part of a larger medical condition.
Isolated Polydactyly
Isolated, or non-syndromic, polydactyly is when the extra digit is the only physical difference present in an otherwise healthy individual. These cases typically follow the simple autosomal dominant inheritance pattern with variable penetrance.
Syndromic Polydactyly
Syndromic polydactyly occurs when the extra digit is one of several symptoms associated with a complex, multi-system genetic disorder. For instance, polydactyly is a feature of Bardet-Biedl syndrome, which also includes vision loss, obesity, and kidney abnormalities. Syndromic cases often involve more complex inheritance patterns, including autosomal recessive inheritance. The difference between an isolated finding and a syndromic one is crucial for understanding the overall health implications.