GENES, MOLECULES AND PROCESSES 



recognizable lieterochromatin. In the onion Allium cepa, for instance, 

 some varieties have visible hcterochromatin and others not. And in 

 species with blocks of hcterochromatin, these blocks are always 

 more variable in size and number among individuals than are the 

 parts of the euchromatin. 



What Major Genes Do 



The mode of action of polygenes can, as yet, only be inferred 

 from the activity of hcterochromatin in producing small proteins 

 and ribose nucleic acid. The modes of action of the major genes, 

 on the other hand, are recognizable by other means. The most 

 important of these is the dosage method of MuUer, By X-raying 

 sperm, Muller produced flies with broken chromosomes and, 

 combining these with normal chromosomes, he was able to obtain 

 particular genes or their mutant allelomorphs present in one, or 

 three, or four doses instead of the proper two. He also obtained 

 combinations of wild type and mutant in different doses. What he 

 then found was that most mutant genes, whether themselves 

 spontaneous or induced by X-rays, had the same action as their 

 normal allelomorphs, only to a lesser degree. 



Similarity of action of the normal and mutant gene could be 

 shown in the following way. The mutant gene scute-i normally 

 removes, or appears to remove, certain of the wild-type bristles. But, 

 when an extra scute-i gene is added, the number of bristles is almost 

 normal. With two extra scute-i genes the fly actually has more 

 bristles than the normal. Thus, far from removing bristles, scute-i 

 helps to produce them. It does the same job as the wild type, but 

 less than half as effectively. Such a gene Muller calls a hypomorph or, 

 in the extreme case, when it does nothing, an amorph. All 

 deficiencies will, of course, appear as amorphs. 



Other types of action are also shown by this method. The 

 hypermorph is more efficient than the wild-type gene. The antimorph 

 opposes the action of the wild-type gene. The neomorph does some- 

 thing new. Clearly these terms are comparative. The wild-type 

 gene is hypomorphic to its hypermorphic mutant and amorphic to 

 its neomorphic mutant. Antimorphs and neomorphs frequently, if 

 not always, involve structural changes, though these may be only 

 small duplications as in the neomorph Bar. It may well be, therefore, 



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