GENE FOR FOURTH CHROMOSOME OF DROSOPHILA 335 



the fourth chromosome is probably the small one, and so any 

 genes in it must lie near together. The close parallel existing 

 between the number and relative sizes of the groups of genes and of 

 the chromosomes leaves little doubt that it must be the pair of small 

 chromosomes with which the factor for bent wings is connected, 

 and that mutations occur more frequently in larger groups of 

 genes, which are connected with larger chromosomes, than in 

 smaller groups; mutation therefore would happen pretty much at 

 random, in that it would usually take place about as often in one 

 group as in another of equal size.^ This, too, makes it probable that 

 the mutations in the larger chromosomes have occurred at various 

 points scattered throughout their whole length, and are not con- 

 fined, as a group, to a particular region or regions. The excep- 

 tional case of no mutations having been observed in the Y-chro- 

 mosome, as we have seen, does not really form an argument against 

 this view, which other facts support. That more mutations have 

 been found in the X-chromosome than in either of the two long 

 autosomes, which are nevertheless about the same size as X, is also to 

 be expected, because a larger proportion of the mutations occur- 

 ring in X would be noticed, since the male flies manifest all genes 

 present in their single X, whereas in the case of other chromo- 

 somes, any mutant gene that is recessive to normal cannot mani- 

 fest itself unless it be present in duplex. Granting, then, the cor- 

 respondence between size and number of chromosomes and of 

 groups of genes, it is difficult to see why larger groups of genes 

 should follow the distribution of the larger chromosomes unless we 

 conceive the connection between the genes and the chromosomes 

 to be that the genes are material particles actually lying in and 

 forming a part of the chromosomes with which they go. In any 

 case, we must admit that the occurrence of a mutation in a 

 fourth independent group of genes in Drosophila forms a further 

 argument, if any more still be needed, in favor of the chromosome 

 theory of heredity. 



Columbia University. 



^ This should not be taken to mean that any particular gene mutates as often 

 as any other; it is defiinitely known that, both in Drosophila and in other forms 

 (corn, Marabilis, etc.), some genes are more likely to mutate than others. 



