Polydactyly or polydactylism (from Ancient Greek πολύς (polus) "many" + δάκτυλος (daktulos) "finger"), also known as hyperdactyly, is a congenital physical anomaly in humans, dogs, and cats having supernumerary fingers or toes.^[1]
The extra digit is usually a small piece of soft tissue that can be removed. Occasionally it contains bone without joints; rarely it may be a complete, functioning digit. The extra digit is most common on the ulnar (little finger) side of the hand, less common on the radial (thumb) side, and very rarely within the middle three digits. These are respectively known as postaxial (little finger), preaxial (thumb), and central (ring, middle, index fingers) polydactyly. The extra digit is most commonly an abnormal fork in an existing digit, or it may rarely originate at the wrist as a normal digit does.^[2]. The world record holder for highest number of digits is Akshat Saxena from Uttar Pradesh, India. He was born in 2010 with 7 digits on each hand and 10 digits on each foot, for a total of 34 digits.^[3]

 Genetics

Polydactyly can occur by itself, or more commonly, as one feature of a syndrome of congenital anomalies. When it occurs by itself, it is associated with autosomal dominant mutations in single genes, i.e. it is not a multifactorial trait.^[4] But mutation in a variety of genes can give rise to polydactyly. Typically the mutated gene is involved in developmental patterning, and a syndrome of congenital anomalies results, of which polydactyly is one feature or two.
Types include:

OMIM	Type	Locus
174200	Postaxial A1	GLI3 at 7p13
602085	Postaxial A2	13q21-q32
607324	Postaxial A3	19p13.2-p13.1
608562	Postaxial A4	7q22
174400	Preaxial I	?
174500	Preaxial II	SHH at 7q36
174600	Preaxial III	?
174700	Preaxial IV	GLI3 at 7p13

Syndromes including polydactyly include Acrocallosal syndrome, Basal cell nevus syndrome, Bardet-Biedl syndrome, Biemond syndrome, Ectrodactyly-ectodermal dysplasias-cleft lip/palate syndrome, Ellis van Creveld syndrome, Meckel Gruber syndrome, McKusick-Kaufman syndrome, Mirror hand deformity, Mohr syndrome, Oral-facial-digital syndrome, Pallister-Hall syndrome, Rubinstein-Taybi syndrome, Short rib polydactyly, and VATER association.^[5] It can also occur with a triphalangeal thumb.

Epidemiology

Left hand with postaxial polydactyly.

The condition has an incidence of 1 in every 500 live births.^[6] Postaxial hand polydactyly is a common isolated disorder in African black and African American children, and autosomal dominant transmission is suspected. Postaxial polydactyly is more frequent in blacks than in whites and is more frequent in male children.^[7] In contrast, postaxial polydactyly seen in white children is usually syndromic and associated with an autosomal recessive transmission. One study by Finley et al. combined data from Jefferson County, Alabama, United States and Uppsala County, Sweden. This study showed incidence of all types of polydactyly to be 2.3 per 1000 in white males, 0.6 per 1000 in white females, 13.5 per 1000 in black males, and 11.1 per 1000 in black females.^[8]

 Society and culture

 People with polydactyly

• Gemma Arterton, an actress who has starred in Prince of Persia and Quantum of Solace, was born with an extra finger on each hand. The vestigial digits were removed when she was a child.^[9][10]
• Anne Boleyn, former Queen of England, was rumoured to have six fingers on one hand.
• Antonio Alfonseca, professional baseball player.
• Hampton Hawes, jazz pianist, was born with six fingers on each hand (surgically removed shortly after birth).^[11]
• Kamani Hubbard, a boy born with a rare case of polydactyly with both 12 fully functioning fingers and 12 toes.^[12]
• Varalakshmi V, a girl from Bangalore with eight fingers in each hand and about four to five extra toes in each foot.^[13]
• Hrithik Roshan, a Bollywood actor born with a supernumerary thumb on his right hand.^[14]
• Garfield Sobers, West Indian cricketer, had an extra finger on each hand which he removed himself during childhood "with the aid of catgut and a sharp knife".^[15]
• Yoandri "24" Hernandez Garrido, Cuba with six fingers on each hand and six toes on each foot [1].