Xo. 042] VABIATIOK IX INDIVIDUAL GENE 41 



bent and eyeless in the fourth chromosome of Drosophila 

 it has even been proved, by Bridges, that the effects are 

 of exactly the same kind, although of lesser intensity, than 

 those produced by the entire loss of the chromosome in 

 which they lie, for flies having bent or eyeless in one 

 chromosome and lacking the homologous chromosome are 

 even more bent, or more eyeless, than those having a 

 homologous chromosome that also contains the gene in 

 question. The fact that mutations are usually recessive 

 might be taken as pointing in the same direction, since 

 it has been found in several cases that the loss of genes — 

 as evidenced by the absence of an entire chromosome of 

 one pair — ^tends to be much more nearly recessive than 

 dominant in its effect. 



The effect of mutations in causing a loss in the char- 

 acters of the organism should, however, be sharply distin- 

 guished from the question of whether the gene has 

 undergone any loss. It is generally true that mutations 

 are iiiueli more apt to cause an apparent loss in character 

 than a gain, but the obvious explanation for that is, not 

 because the gene tends to lose something, but because 

 most characters require for pro])ci- (U'vel()])ineiit a iiie<4y 

 adjusted train of processes, and >(> aii\- diaimc in ilie 

 genes — no matter whether loss, gain, snh>litutitni or rear- 

 rangement — is more likely to throw the developmental 

 ]neclianisni ont of gear, and give a " weaker " result, 

 than to intensify it. For this reason, too, the most fre- 

 (|nent kind of mutation of all is the lethal, which leads 

 to the Ids- of the entire organism, but we do not conclude 

 from this tliat all tlu^ geiu's liad been lost at the time of 

 the nuita-tion. The explanation for tliis tendency for most 

 chanuvs to hr drgciu'ratixv. an.l also for the fa.-t tliat 

 certain othrr knuU ..f rhanuv^ ^ like that from hmI to 

 pink in nrnsn/>hil>, - aro more frcpient lhan .^tlu^r.^ ^ 

 such a> red t.^ brown ..r giv.-Ti ryv - lie> rath.-r in dcxvlop- 

 nn'ntal nicrhani.- tlia!i in uviu'ti^-s. It is luvanse the 

 drvehipnu-ntal pro.-e^xv^ nro in..i-e nn^tahlc in <.ne dirrc- 

 tion than anothei-, and easier to ].nsh downhill " than 

 up. and so any mntation> that oc.-in--no matter what the 

 gene eliange is like— ai'e more apt to luuv th.-e rffrrfs 



