154 Nature of the Genetic Material 



a narrow threshold condition which, with httle variation of the effect, 

 will make for a mosaic of female and male spots, as explained above. 

 In the same way the position effect in Drosophila and maize may be 

 all or none throughout (e.g., normal or mutant type throughout), or 

 it may be weakened and fluctuate around a threshold of action, 

 resulting in mosaic Drosophila eyes or maize kernels. We cannot 

 breed from mosaic spots in insects. But we can in maize, where the 

 all-or-none type of weakening the position effect ( the apparent return 

 mutation) leaves the unchanged locus to act normally. This requires 

 that the control of the threshold action in the cell is also of the all-or- 

 none type. What this means in genetic terms is difficult to state. 



It seems that such an all-or-none setup is present in many of the 

 maize variegations, and especially those in which entrance into the 

 germ line was described. If we can explain the constancy in the germ 

 line, this does not necessarily mean that the same explanation applies 

 to the somatic spots. It is quite possible that these are based solely 

 upon the variability of the position effect and that only some cells may 

 have changed so that they would reproduce their kind in the germ 

 line. A first type of interpretation would not require this difference; a 

 second type, the more probable one, would have to assume it. The 

 first possible explanation is this: the genetic conditions which weaken 

 the position effect to zero — we may call them modifiers, whatever they 

 are — may be homozygous and thus counter the position effect for 

 good. The second one is that in some of the completely colored cells 

 a new rearrangement may have occurred — after the manner experi- 

 mentally produced in Drosophila (see I 3 C c cc) — which counteracts 

 the position effect completely by removing the break to another 

 position. We have already mentioned that this is probably the ex- 

 planation for so-called return mutations in Drosophila, and now we 

 must draw the same conclusion for maize. This conclusion requires 

 that sometimes germ-line cells from a colored sector reproduce this 

 condition, but sometimes they breed true to the recessive mutant type. 



At this point the danger of being trapped by semantics appears, 

 which in the interest of clear thinking should be emphasized. If we 

 call the new rearrangement a mutation ( though not a return mutation, 

 which it simulates), we may call the locus a mutating one (though 

 the locus remains unchanged) as far as this one event is concerned, 

 but it is not a mutating locus so far as threshold conditions alone 

 produce the effect, as in the standard variegation effect. If the inter- 

 pretation of the entire group of phenomena is correct, it is better to 



