Genes and Mutations 391 



sometimes fuse end to end. In the Japanese violet large 

 chromosomes seem to have been formed by the fusion of 

 smaller ones, which are still separate in related species. 



Irregularities in the behavior of chromosomes may oc- 

 cur earlier in the history of the germ-cell lineage, resulting 

 in the production of numerous aberrant eggs or sperms, and 

 leading to several mutations from the same parentage. If 

 the chromosomes fail to divide in the usual manner in the 

 body cells, the result would not show itself in the progeny, 

 but, if it occurred early during the development of the in- 

 dividual, it might show itself in some abnormality that is 

 not transmitted to the offspring at all. 



Genes and Mutations 



More striking and more directly useful to the student 

 of experimental genetics and evolution are changes in out- 

 ward appearance and behavior that can be traced to changes 

 in the genes, rather than to changes in whole chromosomes. 

 A modification of a single gene may be expected to pro- 

 duce slight changes in all parts of the developing body, 

 with a more distinct departure from the parental type in 

 some particular organ or character. This marked varia- 

 tion would therefore attract the attention of the student and 

 serve also as a means of recognition in further studies. It is 

 from the studies of gene mutations that we have learned 

 most about the principles of heredity, and about the actual 

 origin of species by mutation. ^ 



In the cells of a plant or animal body, the chromosomes, 

 as we have seen, are paired. One member of each pair is 

 derived from the mother through the egg, and the other 

 from the father, through the sperm cell. If a change takes 

 place in a gene, it would be in one of a pair corresponding 

 to certain combinations of characters — for example, one 

 making for longer hair or for shorter, for straighter hair 

 or for kinkier, for pigmentation or for whiteness, and so on. 

 Now, this change in the gene might come about at the time 



