THE GENE AS A UNIT OF CHANGE 



The common type of wild mouse has entirely agouti fur; each 

 hair is black with a yellow band near the tip. Some wild mice are 

 agouti with light bellies, and in fancy mice there are corresponding 

 types, a plain black and black with light belly (black and tan). 

 Finally, there is an all-yellow type in the fancy. All these types 

 behave as though governed by a multiple allelomorph series. Yellow 

 stands by itself: the homozygote is never born alive, so that all 

 yellow mice are heterozygous. It is like the Notch effect in DrosopJiila 

 and, like it too, may well be due to a deficiency. The four other 

 types evidently consist of combinations of light or dark belly with 

 agouti or non-agouti (black). We might, therefore, again say that 

 they were due to the combination of two gene differences 

 completely linked in all known experiments. 



The evidence for the two-gene view is better in the mouse than 

 in Primula, for all four combinations are found. Furthermore, how- 

 ever they are combined, light belly is always dominant to dark 

 belly, and agouti hair to non-agouti hair. Thus the mouse hetero- 

 zygous for agouti and black-and-tan looks just the same as a 

 light-bellied agouti. Finally, there is another gene which has the 

 effect of reducing the dominance of agouti over non-agouti and 

 this gene has no effect on the dominance of light over dark belly. 

 Hence the dominance relations accord with the two-gene view. 



A similar two-gene explanation may be given for the dumpy 

 multiple allelomorph series affecting the development of wings and 

 hairs in DrosopJiila. But here the two possibly independent com- 

 ponents are afiected in the same way by the same dominance 

 modifiers. 



How then are we to resolve the question of one gene or two ? 

 The answer is that it often cannot be resolved. What behave as two 

 units of recombination under one set of conditions may, it seems, 

 behave as one under others. Suppression or even reduction of the 

 frequency of crossing-over, either general or local, may have this 

 effect. Such a structural change as the inversion of a couple of genes, 

 will, as we shall see, prevent crossing-over, or at least effective 

 recombination, between them except when the two genes are 

 homozygous and the recombination is, therefore, ineffective with 

 respect to these two genes. The absence of crossing-over in the 

 agouti group might be due merely to an inversion which constantly 



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