Genetic aspects of polydactyly.

The early limb development follows the similar pattern in all vertebrates since different species develop using the same regulatory genes in the formation of the body plan. Some of these genes remained well conserved during evolution and can be traced back as far as Drosophila--while some others changed their structure or developed new functions. This is why the limbs from different animals still look different from one another. However, all existing tetrapods have, like Homo Sapiens, limbs with five, or fewer digits. It has been argued that the interplay of the factors controlling the patterning and differentiation during the embryonal limb development can provide five distinct "genotypes" allowing development of only five different digits. This would imply that the Greek definition of polydactyly, namely "duplication of the finger or a part of it", is correct, not only in morphological but also in a genetical sense. Genes involved in the determination of the outline of the limb are candidates for disorders like polydactyly and syndactyly. Recently, we have localised the gene for triphalangeal thumb (TPT) on chromosome 7q. As almost 50% of our patient population also had rudimentary postaxial polydactyly and/or syndactyly, the interesting question arose whether the TPT gene also was responsible for isolated post-axial polydactyly. Our preliminary evidence suggests, however, that different gene(s) are involved in the pathomorphogenesis of postaxial polydactyly. Studies of human congenital hand malformations--combined with genetic studies in lower vertebrates--will help us to understand not only the molecular basis of these disorders, but also to get insight into the fascinating mechanisms involved in the normal development of the human hand.