NATURE OF THE GENETIC EFFECTS 381 



removed by means of a structural change from the genes normally on 

 one side of it and placed near to others, its effect is often changed some- 

 what, much as if it had undergone a mutation (see Sect. 5). Most often 

 it undergoes more or less inactivation, as happens in many mutations, 

 and in that case the organism with one changed gene and one normal one 

 usually shows the effect chiefly of the normal, the latter then being said 

 to be dominant and the altered gene recessive. Occasionally, however, 

 the genes changed by position effect are more or less dominant, and in 

 these cases their effect can sometimes be shown to be neomorphic, i.e., 

 qualitatively different from that of the normal allele (see Sect. 13). 

 That the change is entirely due to the influence of the neighboring genes 

 upon the given gene is shown, for example, by cases in which the gene in 

 question is returned to its original position, for it is then found to revert 

 to its original mode of functioning (Panshin, 1935; Dubinin and Sidorov, 

 1935). 



Although the spatial range of the effect is minute, it has been shown 

 that not only directly adjacent genes but also those removed by a distance 

 of one or several genes can exert such an influence. For this reason 

 several of the genes located near any point of structural change are likely 

 to have become affected in this way. Since a considerable proportion 

 of all the genes perform some necessary function in the complicated web 

 of biochemical and morphogenetic reactions upon which survival and/or 

 reproduction depend, it is not surprising that, in Drosophila, the great 

 majority of structural changes (when received from both parents, and 

 thus allowed to express their recessive effects) result in death prior to 

 maturity or in sterilit}^, and that many of the remainder cause visible 

 abnormalities. This is another reason why, in this organism, relatively 

 few of the structural changes which arise can persist indefinitely in a 

 population. 



When genes from a euchromatic region are by means of a structural 

 change placed in the vicinity of a heterochromatic region, or vice versa, 

 the position effects, for some unknown reason, are usually more pro- 

 nounced, extend over a larger distance (i.e., over a larger number of 

 intervening genes), and express themselves in a peculiar, variegated or 

 mosaic manner (Muller, 1935c, 1938). At the same time the euchro- 

 matin comes somewhat to simulate in its cytological appearance and 

 behavior the heterochromatin which has been placed next to it, and the 

 heterochromatin in the same neighborhood becomes more like euchro- 

 matin. Under these circumstances, the addition of more heterochro- 

 matin to the genetic composition (as when an extra Y chromosome has 

 been inherited) results in both the eu- and heterochromatin in the regions 

 near the point of structural change becoming more like euchromatin 

 cytologically. Along with this goes a lessening of the abnormality of 

 functioning of the genes located near the break in the displaced euchro- 



