A central question of biology is the nature of the regulation of gene expression that allows a fertilized ovum to develop into a mature organism. Genes differ in their expression both in time and in space. The early stages of development are concerned with the establishment and differentiation of the morphological structures of the body. Despite its clinical relevance and the extensive descriptive information available about the morphology of the human embryo (Moore, 1988), knowledge of the genes that control the process is still sparse. For many practical and ethical reasons, research on human embryos is limited, and almost all our information about these processes has come from studies in other organisms, most often the fruit fly Drosophila. Many of the genetic manipulations used with Drosophila have now become possible with the mouse, raising the prospect of rapid progress and new insights in the genetic analysis of mammalian development.
Not infrequently, the process of human development goes wrong. The most abnormal embryos and fetuses, of course, are not born alive but are aborted spontaneously at some stage between fertilization and viability. Still, a significant proportion of pregnancies end with the birth of a child with some congenital anomaly or are selectively terminated because an anomaly has been identified in the fetus. The identification and delineation of syndromes of congenital abnormalities constitutes the field of dysmorphology, one of the most rapidly advancing specialties within clinical genetics.