Polydactyly. Close-up of a baby's hand showing an extra finger. This condition is called polydactylism, and it is a fairly common congenital abnormality. The extra digits may be fully formed, or as is often the case, they are rudimentary (undersized) and are usually removed by surgery shortly after birth. The condition is genetic, and therefore hereditary, and if both parents had an extra digit, there would be a one in two chance of their offspring showing polydactyly as well. In normal births, the occurrence is around one in 2000.