Syndromology: an updated conceptual overview. VI. Molecular and biochemical aspects of dysmorphology.

The role of chance using a stochastic single gene model has been shown to generate a continuous liability curve resembling that obtained from a multifactorial threshold model. Segregation of some malformations may be explained by a single defective gene that predisposes to, but does not necessarily result in, the malformation. Low penetrance and remarkably variable expressivity that characterize a number of presumed autosomal dominant malformation syndromes are possibly reflections of specific stochastic influences that are intrinsic to the embryonic process itself. Gene analysis is discussed and illustrated. Using polymorphic DNA probes to study cleft palate and ankyloglossia in males and ankyloglossia only in females in a large Icelandic family, the responsible gene was found to be located on the long arm of the X chromosome in the Xq21.1 region. In addition to gene analysis, some of the implications of transgenic analysis using mice are discussed. Among disorders of collagen metabolism, both the osteogenesis imperfectas and the Ehlers-Danlos syndromes are shown to represent genetically heterogeneous groups of connective tissue disorders. The days of thinking about osteogenesis imperfecta as one disorder and the Ehlers-Danlos syndrome as another are a thing of the past; persistence of such thinking is erroneous and misleading. Of the many disorders affecting bone mineral, the complexities of hypophosphatasia and pseudohypoparathyroidism are singled out for discussion. For lysosomal storage disorders, an overview of the mucopolysaccharidoses is provided. Finally, the recently delineated peroxisomal disorders--hyperpipecolic acidemia, rhizomelic chondrodysplasia, neonatal adrenoleukodystrophy, Zellweger syndrome, and infantile Refsum disease--are known to share a distinctive biochemical phenotype, although fibroblast complementation analysis suggests that some of these disorders are etiologically distinct.