Typically the mutated gene is involved in developmental patterning, and a syndrome of congenital anomalies results, of which polydactyly is one feature or two.Polydactyly has been linked to the prenatal environment, however, with a recent study showing a relationship to maternal pollution exposure in China.Types include:

97 genetic syndromes have been associated with different kinds of polydactyly. Ulnar polydactyly
Ulnar polydactyly is often bilateral and associated with syndactyly and polydactyly of the feet. This can be a simple or complex polydactyly. Ulnar polydactyly occurs as an isolated congenital condition, but can also be part of a syndrome. The syndromes which occur with ulnar polydactyly are: Trisomy 13, Greig cephalopolysyndactyly syndrome, Meckel syndrome, Ellis–van Creveld syndrome, McKusick–Kaufman syndrome, Down syndrome, Bardet–Biedl syndrome, Smith–Lemli–Opitz syndrome

Radial polydactyly
Type VII of radial polydactyly is associated with several syndromes:
Holt–Oram syndrome, Fanconi anemia (aplastic anemia by the age of 6), Townes–Brocks syndrome, and Greig cephalopolysyndactyly (also known to occur with ulnar polydactyly). Central polydactyly
The syndromes associated with central polydactyly are:
Bardet–Biedl syndrome, Meckel syndrome, Pallister–Hall syndrome, Legius syndrome, Holt–Oram syndrome. Also, central polydactyly can be associated with syndactyly and cleft hand. Other syndromes including polydactyly include acrocallosal syndrome, basal cell nevus syndrome, Biemond syndrome, ectrodactyly-ectodermal dysplasias-cleft lip/palate syndrome, mirror hand deformity, Mohr syndrome, oral-facial-digital syndrome, Rubinstein-Taybi syndrome, short rib polydactyly, and VATER association. It can also occur with a triphalangeal thumb. Diagnosis
Classification is performed by using x-ray imaging to see the bone structures. Ulnar polydactyly
The classification of ulnar polydactyly exists of either two or three types.
In most cases, a longitudinal and sagittal osteotomy is needed to centralize the bony parts of the ulnar thumb. While the soft-tissue of the radial thumb was preserved, it is now attached to the radial side of the ulnar thumb together with the periosteal sleeve. The APB and FPB of the ablated radial thumb are attached to the distal phalanx for more stability. If necessary, the extensor pollicis longus and the flexor pollicis longus are reattached to centralize their course.In Wassel type 5 and 6 the opponens pollicis muscle must be transferred to the ulnar metacarpal. Soft tissue with collateral ligament reconstruction is used to avoid any angular deformity in the preserved thumb. Tendon centralization is also often used for correction. Still, cases with osseous deformities may happen. To provide alignment, osteotomies are necessary to be done. This operation may need bone grafting, which is obtained from the amputated thumb. On top plasty procedure
This type is indicated when one thumb is larger proximally and the other thumb has a larger distal component. (The procedure is initially described as a way to lengthen amputated digits.) The goal is to create a functional thumb by combining less-hypoplastic components. On top plasty procedure is rarely employed in the treatment of congenital thumb duplication. It might be necessary for Wassel types 4, 5, 6.At the level of the mid-proximal phalanx or mid-metacarpal, the distal component is transferred to the proximal component. The tendons of the distal component are preserved as the rest of the distal component is amputated.