BASES OF CHANGE 



tctra5omic diploid is more deranged than a trisomic, or, of course, 

 than a monosomic or tetrasoniic polyploid in a group of organisms 

 working with the same chromosome series. 



Intergcuic and Intragenic Change: Presence and Absence 



There is one other genetic effect of structural change. This is the 

 physiological effect of change of position itself. Changes of order 

 might be expected to produce no effect on the phcnotypc. Indeed 

 in maize, where some two hundred have been produced by breeding 

 with X-rayed pollen, it is doubtful whether they ever do so. In 

 Drosophila and Oenothera, however, an effect often has to be inferred 

 as the result of the mere change in order itself, a position effect. 



The position effect is now known to be responsible for many 

 changes which were described as gene-differences in breeding experi- 

 ments with Drosophila before the chromosomes could be directly 

 examined in polytenc nuclei (that is, before 1933). Allelomorphs 

 of the gene scute affect the number of bristles on the thorax and 

 scutellum, and several of them are due to inversions of various 

 lengths, following breakage by X-rays. The Pale gene likewise goes 

 with a spontaneous translocation from the second to the third 

 chromosome and is probably just due to the change of position. 

 The scute mutations are simple recessives having no special effect 

 on the hcterozygote ; the Pale gene is lethal when homozygous and 

 varies in its effect on the heterozygote. 



Another side of the picture is shown by the fact that a large 

 proportion of inversions, which have arisen spontaneously as well 

 as by X-raying in laboratory cultures of Drosophila, are lethal in 

 the homozygous state, while some of them are invariable concomi- 

 tants of visible mutations like Curly and Plum. It thus seems that 

 the action of one gene may be modified, directly or indirectly, by 

 its neighbours. And, when it changes its neighbours, it does not 

 necessarily work as it did before. Either the shapes of the chromo- 

 some constituents themselves, or at least of their products inside the 

 nucleus depend on their linear arrangement. To this extent whole 

 segments, or even whole chromosomes, must be considered as units 

 of action in physiology just as they are units of structure, single 

 giant moleailes, in chemistry. 



106 



