CHAPTER XIX 



GENERAL CONCLUSIONS 



THE preceding chapters have dealt with two main 

 topics : with the effects following a change in the 

 number of the chromosomes ; and with the effects 

 following a change within a chromosome (a point muta- 

 tion). The theory of the gene is broad enough to cover 

 both these kinds of changes, although its main concern is 

 with the gene itself. The term mutation also has come, 

 through usage, to include the effects produced in both 

 these ways. 



These kinds of changes have important bearings on 

 current genetic theories. 



The Effects Produced by a Change in Chromosome 

 Number and by a Change in a Gene. 



When the number of the chromosomes is doubled, tre- 

 bled, or multiplied any number of times, the individual 

 has the same kinds of genes as before, and they stand in 

 the same numerical ratio to one another. There is no 

 a priori expectation that this kind of change would affect 

 the character of the individual, were it not that the vol- 

 ume of the cytoplasm may not increase to correspond 

 with the increase in the number of the genes. Just what a 

 failure to attain a corresponding increase of cytoplasmic 

 volume means is not clear at present. At any rate, the 

 results show that triploid, tetraploids, octoploids, etc., 

 do not differ markedly in any special characters (except 

 size) from the original diploid type. In other words, the 

 changes produced may be very numerous, but not strik- 

 ingly different from the original ones. 



