374 PRINCIPLES OF EMBRYOLOGY 



conclusions about the gene which are precise enough to illuminate 

 the nature of gene-activity (Reviews: Lea 1946, Mullet 1947, Catcheside 

 1948, Auerbach 1952). 



The induction of high rates of mutation has, however, provided us with 

 an enormous mass of genetic variations which have thrown light on our 

 problem from other angles. Irradiation by x-rays, for instance, frequently 

 causes chromosomes to break and rejoin in abnormal ways. From the 

 study of such chromosome rearrangements, the fundamentally important 

 point has emerged that the behaviour of a gene may in some cases be 

 influenced by its position in the chromosome. This is the so-called position 

 effect. Several theories have been proposed to account for it (cf. Lewis 

 1950, Serra 1949). There are two main hypotheses, which seem at first 

 sight to be of rather radically different nature. The first accepts the con- 

 ventional idea of the gene as a distinct and individual particle and supposes 

 either that neighbouring genes produce substances which can react to- 

 gether (Offerman 1935, Stern et al. 1946) or that they interfere with one 

 another by competing for the same substrates (Waddington I939<3); if the 

 diffusion of these substances is slow, it becomes important whether the 

 genes are close together or far apart. The second involves a more profound 

 change in previous ideas. It suggests that gene-activity is not to be attri- 

 buted to circumscribed particles, which could be considered as separate 

 'beads along the chromosome thread', but that the basic elements are short 

 stretches of chromosome which are not sharply bounded off against each 

 other, but rather shade into or overlap one another. A change in the order 

 of the chromosome thread will in that case alter the character of the 

 fundamental reactions carried out by it (cf. Goldschmidt 1938, 1946). 



It is not easy at present to decide fmally between these two theories; 

 it is significant, for instance, that Pontecorvo (1950) was led by a variety 

 of the first theory to postulate that genes which take part in a series of 

 reactions in which only a few molecules are involved in each cell will 

 tend to lie close together, like successive machine tools on a production 

 line; a special attempt was made to fmd such genes in a mould, Aspergillus; 

 several genes controlling the production of biotin were duly discovered, 

 all located very close to one another; but by that time Pontecorvo was 

 feeling tempted to interpret the phenomenon by the second theory rather 

 than by the first, wliich had led him to predict it (Pontecorvo 1952^, b). 



The study of multiple allelomorphs has recently produced evidence 

 which seems to reinforce the theory which postulates less definitely 

 defmed genes. It has long been known that there may be many different 

 mutant forms of the same 'gene', if one may use the old terminology for 

 the moment without begging the question. At one time it was thought 



