Abstract
It has been known for well over a century that the corresponding ossification centres of the hand tend to appear before those of the foot (Mall 1906), although even now the range of variability remains poorly defined. Presumably a similar asynchrony also obtains for chondrification, although precise timing is more difficult here than for ossification. Accordingly, it is tempting with respect to this syndrome to relate fusions restricted to the phalanges of the hand and to the tarsal bones to the action of a gene which during a very limited period of development exerts an effect on those small bones of the hand and foot which are in a very specific stage of development. But since there are other inherited abnormalities of the skeleton, such as brachydactyly, where homologous bones are involved, it is clear that at the descriptive level gene action can either appear to be "stage-specific" or "homologue-specific." There are also mutations affecting the axial skeleton such as the type of polydactyly of the foot described by Neel and Rusk (1963) which appear to be neither "stage" nor "homologue" specific, involving only the foot. Finally then, there are syndromes, such as Lawrence-Moon-Biedl-Bardot, in which involvement of the hands and feet (polydactyly) is associated with such other apparently completely unrelated defects as retinitis pigmentosa and hypogonadism. While it is tempting to try to construct theoretical patterns and systems of developmental processes on the basis of results such as these, it is clear that the ultimate understanding of how genes can appear to act in so many ways almost certainly depends on the identification of a genetically controlled, qualitative or quantitative biochemical lesion. With the current parallel developments in biochemical and developmental genetics, new insights into these enigmas will surely be forthcoming in the relatively near future.
