Most commonly, the extra finger is rudimentary, consisting of an end phalanx with a nail, and connected to the hand with a small skin pedicle.

[20] In the specific case of preaxial polydactyly (Hemingway mutant), a cis-acting mutation approximately 1Mb upstream of SHH gene has been implicated.

[21] Normally SHH is expressed in an organiser region, called the zone of polarizing activity (ZPA) on the posterior limb side.

Typically the mutated gene is involved in developmental patterning, and a syndrome of congenital anomalies results, of which polydactyly is one feature or two.

[citation needed] Polydactyly has been linked to the prenatal environment in a 2020 study showing a relationship to maternal PM10 pollution exposure in China.

[17] In 1961, Frantz and O’Rahilly proposed that congenital anomalies of the limb could be classified in seven categories, based on the embryonic failure causing the clinical presentation.

[12] As of 2009, research has shown that the majority of congenital anomalies occur during the 4-week embryologic period of rapid limb development.

[26] The treatment of Type A ulnar polydactyly is complex as its goal is to remove the accessory digit while maintaining a stable, functional small finger.

When the duplicated proximal phalanx articulates with a common, broad metacarpal head, the ulnar collateral ligament must be considered.

In those cases with a common articulation or with a sixth metacarpal the muscle executing the abduction of the little finger (abductor digiti minimi) must be preserved.

At last the collateral ligament and abductor digiti minimi are reinserted at the base of the preserved proximal phalanx and a wire is then placed across the reconstructed joint.

In patients with a duplicated metacarpal, the accessory digit is amputated in a standard ray fashion with transfer of the abductor digiti minimi to the retained small finger.

The surgical technique is analogous to radial polydactyly, in which the level of duplication and anatomical components should guide operative treatment.

[12] The pedicled ulnar extra digit can be removed by suture ligation to devise the skin bridge of the newborn child.

[12][11] Ligation occludes the vascular supply to the duplicated digit, resulting in dry gangrene and subsequent autoamputation.

][citation needed] A 2011 study opined that excision of an extra digit in the neonatal nursery was a safe and simple procedure with a good clinical and cosmetic outcome.

[39] As of 2022, for infants with ulnar type B polydactyly the recommended treatment is surgical excision or suture ligation, when no bony structures exist.

Wide articular surfaces should be narrowed and phalangeal wedge osteotomies may be required to provide an axial alignment.

This is mainly caused by the fact that there is a generally normal function of these patients’ hands following ablation with collateral ligament reconstruction.

[12] In a study on 27 patients undergoing surgical excision for Type A ulnar polydactyly, only one complication was noted in the form of an infection.

[12] In a study on 21 patients with Type B ulnar polydactyly treated with suture ligation it was found that the duplicated digit was typically amputated at an average of 10 days and no complications of infection or bleeding were reported.

[47] In general, suture ligation is safe and effective when applied to appropriate cases of Type B polydactyly in which no substantial ligamentous or osseous structures are present within the pedicle.

Parents should be educated as to the progression of necrosis, and that revision of residual tissue or scar may be necessary when the child is six months of age or older.

Tada and colleagues note that satisfactory surgical correction of central polydactyly is difficult to achieve and that outcomes are generally poor.

All patients required secondary surgical procedures to address flexion contractures and angular deviation at the IP joint level.

Therefore, treatment is wholly dependent on the anatomic components present, the degree of syndactyly, and the function of the duplicated finger.

The chicken breeds known for being polydactyl are the Dorking, Faverolle, Houdan, Lincolnshire Buff, Meusienne, Sultan, and non-bearded Silkie Bantams.

[citation needed] Polydactyly also occurs in dogs, cats, and small mammals such as guinea pigs[78] and mice.

[75] Polydactyly is believed to have been common in early tetrapods, the extinct amphibians that represented the earliest landliving vertebrates.

[83] Polydactyly was a non-pathological, reacquired condition in extinct marine reptiles such as ichthyosaurs and hupehsuchians, some of which containing upwards of ten digits within their flippers.