Polydactyl is a condition in which a person has more than 5 fingers on a single hand or more than 5 toes on a single foot. This could happen to humans or even cats and dogs. The extra finger could be a soft tissue without any bones within and can be removed. Only in a very rare case, it could be a completely functioning finger with bones.
Genes provide the basic information for doing a specific task. When a baby is in the mother’s womb, the genes from father and mother whichever is dominant decides on the physical characteristics and the behavioral traits of the baby. This may include eye color, hair color, hair texture and many more features. So, how come children of the same parents are quite different? This is because of ‘gene mutations’. Mutations are a change in DNA in an organism that results in a new trait. They are slightly different versions of the same genes. These new versions are called alleles. Mutations change the DNA sequence, creating variations in protein coding portions of the genes. This is the reason why some infants are allergic to milk (lactose intolerance). In a single way, just a single dominant allele of a gene from parent could result in inheritance of polydactylism.
Polydactylism is associated with different mutations. For a more complex process like building a hand, many genes work together. Genes with altered DNA could be a reason for the growth of sixth finger. There are different forms of polydactylism. The common ones being pre-axial (the thumb side of the hand before axis) and post-axial (the little finger side of the hand before axis).