Syndactyly is a term used to describe webbed or conjoined digits (fingers or toes). It may occur as an isolated finding or may be a symptom of a genetic syndrome. There are over 300 genetic syndromes that involve syndactyly, such as Apert syndrome and Saethre-Chotzen syndrome. There are many different ways to classify or group non-syndromic (isolated) syndactyly. Presently, researchers classify the different types of syndactyly based on how severe the syndactyly is, which digits are involved, how the trait appears to run through families, and whether an underlying gene mutation has been identified. Using this system, nine different non-syndromic types of syndactyly have been defined. 

Syndactyly is caused by a mixture of genetic and environmental factors. In about 10-40% of individuals with syndactyly, there is a family history. Syndactyly can run through families in many different ways, depending on the underlying cause. When isolated, it can be inherited in an autosomal dominant, autosomal recessive, or X-linked recessive manner. Treatment usually involves surgery to separate the digits.

During normal embryonic (fetal) development, the hand initially forms in the shape of a paddle, and then eventually splits into separate fingers. Syndactyly results when one or more fingers fail to separate during this time.

Research continues into further understanding why this happens. Many cases seem to occur without an apparent cause, while some may occur due to a genetic (inherited) defect, environmental exposure during pregnancy, or a combination or both factors. Syndactyly may also occur as a part of an underlying genetic syndrome and can be found in over 300 different genetic syndromes.