Some Aspects of Position Pseudoallelism

Contrasting interpretations of position pseudoallelism are discussed, and the types of position effect which characterize this phenomenon are illustrated, with special reference to the case of the bithorax series of five pseudoallelic loci in Drosophila melanogaster. Most of the results in this latter case can be simply interpreted on the basis of a chain of gene-controlled reactions in which each intermediate substance is postulated to act in a dual capacity: as substrate for the succeeding reaction and as a determiner of a specific physiological process. The model also gives a consistent picture of the way in which different levels of developmental determination may be controlled by the different genes of the series.