Genie and Non-genic Parts of the Chromosome 81 



and its variegation, so that the effect decreases with the distance 

 from the break. This apphes as much to variegation as such (i.e., 

 penetrance) as it does to the degree of variegation (i.e., expressivity, 

 meaning, e.g., for the white locus more white facets if the break is 

 nearer to white). This can only mean that something emanates from 

 the block heterochromatin with a diminishing gradient which affects 

 the threshold of the position-effect action in development. We see 

 two things at work: first, the position effect of the break, making 

 a locus nearby act as if it were mutated; and second, an action 

 of heterochromatin influencing the penetrance and expressivity of 

 the action of the break during development with narrow threshold 

 conditions, which by the inherent variability of action above and 

 below the threshold produce the mosaic effect. We also find the 

 breakage effect acting with a gradient, decreasing with distance. 

 Not all loci clearly show this distance relation and, further, differ- 

 ences in the effect have been observed, when the heterochromatin ad- 

 jacent to the break was derived from different blocks of chromocentral 

 heterochromatin, which might be caused by qualitative or only quan- 

 titative differences. 



In maize, conditions are known which affect the variegation 

 ( McClintock's Ac and Rhoades' Dt). The latter is described as a 

 dominant locus within a heterochromatic block. We do not know 

 what this means, but the action parallels that of extra heterochromatin 

 in Drosophila. It is quite possible that in plants heterochromatic 

 conditions exist — symbolized as Dt — which, morphologically, are dif- 

 ferent from those in Drosophila and do not necessarily have to be 

 additions or subtractions of substance. (More details on maize varie- 

 gation in a later chapter.) 



The facts discussed thus far relate to euchromatic loci as influ- 

 enced by adjacent heterochromatin. The last example refers to 

 euchromatic loci within heterochromatic blocks. The experiment with 

 variegation should give information. On the side of the break in the 

 euchromatin, euchromatic loci are located. If there are euchromatic 

 loci within the heterochromatic block, they would be put into the 

 neighborhood of euchromatin via the heterochromatic break (each 

 rearrangement requiring two breaks ) . A heterochromatic locus should 

 also show variegation, provided that variegation, generally, is the 

 result of abnormal union of eu- and heterochromatin on both sides 

 of a break. Actually, many such cases have been described (Sutton, 

 1941) for loci in different chromosomes, like bobbed, light, pink, 

 radius incompletus, all of which have been localized in heterochro- 



