Syndactyly is one of the most common congenital hand deformities with an incidence of 1 in 2000 live births and is associated with Poland's syndrome (chest wall deformity with limb hypoplasia), Apert syndrome (autosomal dominant inheritance with mental retardation, craniosynostosis), Holt-Oram syndrome (also known as "heart-hand syndrome" with abnormalities of the heart and upper limbs), and Carpenter syndrome (acrocephaly, peculiar facies). There is no association of syndactyly with Tay-Sach's disease.
Ireland and colleagues reviewed 43 consecutive cases of Poland's syndrome and found that clinical features were variable but always included congenital aplasia and syndactyly which is usually incomplete and simple. It can involve all fingers and not uncommonly the thumb as well.
Galland and Bora wrote a review on congenital disorders of the upper extremity, and describe that syndactyly may occur as part of a syndrome including Poland's syndrome (chest wall anomalies and syndactyly) or Apert's syndrome (severe syndactyly with craniosynostosis, mental retardation, ankylosed IP joints, flattened faces and hypotelorism). There is no known increased incidence in Tay-Sach's disease, however.
Ireland DC, Takayama N, Flatt AE. Poland's syndrome. J Bone Joint Surg Am. 1976 Jan;58(1):52-8.
PMID:175070 (Link to Abstract)
Gallant GG, Bora FW Jr. Congenital Deformities of the Upper Extremity. J Am Acad Orthop Surg. 1996 May;4(3):162-171.
PMID:10795051 (Link to Abstract)