Polydactyl persons have extra "digits', usually meaning fingers. This is a trait that is inscripted into their deoxyribonucleic acid. Or DNA. It is carried on their parent..chromosomes where all DNA comes from. So, its inherited. MRNA, messenger RNA requires it to put into order the set of characteristics to be encoded in a cells individual traits. MRNA 'reads' as it were what is coded by the DNA into the nucleus of the cell, then carries it out to the "body" or cytoplasmic portion of the cell..where repeating patterns are required to 'create' another cell. This group of cells created can make exactly what the DNA says to make, and it could be anything from osteocytes, (bone cells) to myocytes, (muscle cells)..then it continues to create an entire organ. In this case polydactylism is very dominantly a feature of extra fingers and even toes in some animal life forms as well as people.