THE MUTATED GENE 43 



half thorax are exceptionally frequent. (Occasionally they occur 

 also in long-winged stock.) Such animals are at the limit of 

 viability; if one side only is affected, they may survive; both sides 

 affected would be fatal. 



Here, then, we are facing the point where a gene effect begins 

 to be lethal, and we see why: The action of the gene comes into 

 effect so early in development that a group of cells is affected 

 that still has a rather unrestricted prospective potency, which 

 would be restricted or segregated only in later development. 

 The effect of the gene-controlled process therefore influences a 

 whole group of processes of differentiation instead of a single one 

 in case of an effect at a later stage at which the material is already 

 subdivided in regard to determination. Therefore these early 

 effects tend to be lethal. In the v^-series, this is especially clear: 

 the highest alleles of the No- wing type are lethal in the homo- 

 zygous condition. Probably comparable cases are more frequent 

 than is at present known. 



The same facts are found in the cases of mammalian abnormali- 

 ties which were described on page 24. Here it was possible, at 

 least in one case, to compare homozygous and heterozygous 

 action. It was clear that the lethality of the homozygous effect 

 was a consequence of the early action of the gene in destroying 

 Anlagen of a considerable prospective potency, involving whole 

 organ systems. 



The facts here described are of considerable general impor- 

 tance; they show that the action of mutant genes may be con- 

 fined to definite periods of development; this would not imply 

 necessarily that these genes are inactive otherwise. It would 

 probably mean that the tissues were in a condition to react only 

 at definite moments. And it seems, furthermore, that these 

 moments of susceptibility are those in which processes of deter- 

 mination take place, i.e., of subdivision or segregation of potencies. 

 If further work should prove this to be a rule, an important link 

 between genetics and development would have been established. 



We do not mean to express the opinion that all lethal actions 

 are of this type. Certainly lethal gene actions may also occur 

 if some physiological process of vital importance is damaged or 

 completely inhibited. The best examples of this are the chlorotic 

 mutants of plants, which cannot survive for lack of chlorophyll. 

 But shoots of this constitution may grow upon a green stalk. 



