[Chap. XXXIX MUTATIONS 479 



account for tliese different species and genera. The chromosomes must 

 also be different. Species of other genera differ by multiples of other 

 basic numbers, such as 6, 8, 9, and several others. The different species 

 of chrysanthemum have 9, 18, 27, 36, and 45 pairs of chromosomes. 



Apparently these species originated in part by a doubling of chromo- 

 somes; but there have been certain changes within the chromosomes 

 themselves, such as fragmentation or even changes in the genes. If a 

 gene consists of a group of a few (perhaps a dozen or less) molecules, 

 then changes in a gene might occur by ( 1 ) a gain or a loss in molecules, 

 or (2) a rearrangement, a gain or a loss of atoms within the molecules. 

 On the other hand, if a gene consists of only one molecule, then altera- 

 tions within it would be limited to rearrangement, gain or loss of atoms 

 or groups of atoms. 



What are mutations? Any of the irregularities in the composition and 

 arrangement of chromosomes and genes and in the consequent changes 

 of development that cannot be ascribed to the direct effects of cross- 

 fertilization may be called mutations. The term mutation was first used 

 to refer to the occurrence of observable differences in hereditary char- 

 acters that were considered not to be the direct result of cross-fertiliza- 

 tion. It was later discovered that these recognized mutations were the 

 results of differences in the composition, behavior, and arrangement of 

 both chromosomes and genes. Some authors use the expressions "chromo- 

 somal aberrations" and "gene mutation," preferring to limit the term 

 mutation to changes that occur only in the genes, whenever such a dis- 

 tinction can be made. For convenience we shall use the term mutation in 

 these chapters to include an aberrant condition in either chromosomes 

 or genes, or any hereditary variation that is not the direct result of 

 cross-fertilization. 



Mutations during reduction division. We have seen that the pairing 

 and segregation of chromosomes during reduction division are very regu- 

 lar processes, and that the constancy of pure lines, as well as the 

 Mendelian ratios of hybrids, are the consequences of this regularity. 

 But here again, with a little scientific imagination we may visualize the 

 possible occurrence of certain irregularities. Reduction division might 

 fail to occur, with the result that the spores and gametes formed would 

 have the diploid ( 2n ) number of chromosomes, just like the cells of the 

 vegetative body of tlie plant. The synaptic mates, instead of separating 

 and moving to opposite poles of the cell, might continue to adhere to 

 each other and move to the same pole of the cell. This might happen in 



