PROBLEM ^ Hoxv Can New Hereditary Characters 



Appear? 



Entirely new characters may appear. In the 



last problem vou learned that with the 

 reshuffling of genes in every mating new 

 combinations are formed and thus off- 

 spring differ from their parents. From 

 time to time, however, parents produce 

 offspring with a really new character; 

 something that is not the result of the 

 normal recombination of genes. Such a 

 new character appeared in 1910 in a 

 fruit fly culture in a laboratory. A fruit 

 flv was found that had white eyes, not 

 the normal red ones. This was the first 

 white-eyed fruit fly that anyone had re- 

 corded. When the fly was crossed \\'\x\\ 

 a normal red-eved fly the F^ were red- 

 eyed; but in the Fo some flies had \\'hite 

 eyes. Thus, it was learned that this new 

 character could be inherited. Evidently 

 a change had occurred in the chromo- 

 somes of the germ plasm. This discovery 

 led to many others. 



It has been estimated that in about one 

 out of 50,000 flies there occurs some 

 change in the chromosomes which can 

 be detected as a new character. In the 

 many millions of fruit flies that have 

 been studied, more than 500 changes in 

 characters have been noted. In Figure 

 429 a very few of these are shown. Such 

 changes have been observed somewhat 

 more often in fruit flies than in other or- 

 ganisms, but changes of this kind have 



been known to occur in all organisms 

 studied. 



Changes in genes. The changes that 

 occur in the chromosomes of the germ 

 plasm are of different kinds. The simplest 

 possible change is a change in a single 

 gene. Just \\'hat this change is no one 

 knows for no one yet knows the chemical 

 make-up of the gene. There is a theory, 

 however, that a gene is a single large pro- 

 tein molecule. Every molecule has its 

 atoms arranged according to a definite 

 pattern. According to this theory, it is 

 believed that the atoms may rearrange 

 themselves, thus making a new definite 

 pattern. But, however it may happen, the 

 change in the gene is sudden and com- 

 plete. Such a change in a gene is now 

 called a ?mitatio7i (mew-tay'shun). The 

 mutation (gene change) may produce a 

 large or small change in the organism. 

 That is, the new character may be readily 

 noticeable or scarcely visible. However 

 small the difference, if the new character 

 can be inherited and follows the law of 

 segregation it is clear that it was pro- 

 duced by a gene not present in the organ- 

 ism before. 



When a single gene mutates, the other 

 member of the pair is not changed. The 

 organism is then a hybrid for this pair of 

 genes. Most new genes are recessive to 

 the unchanged gene; some are dominant. 



