IN THE SECOND CHROMOSOME. 327 



OTHER CASES OF LINKAGE VARIATIONS. 



The cases reported in this paper are not the only ones in which 

 linkage variations are known. As has been pointed out above, there 

 is a gene in the third chromosome that affects the percentage of cross- 

 ing-over in that chromosome. It has been shown (Morgan, 1912; 

 Sturtevant, 1913a; Morgan, Sturtevant, Muller, and Bridges, 1915; 

 etc.) that there is no crossing-over in the male of Drosophila, even 

 between loci that give almost 50 per cent of crossing-over in females. 

 The reverse relation crossing-over in males but not in females 

 has been shown by Tanaka (1914) to hold for at least two loci in the 

 silkworm moth. Bridges (1915) has shown that the percentage may 

 change with age, and Plough (1917) has shown that it may be changed 

 by temperature. Genetic factors (other than sex) influencing the 

 process are suggested by the results of Baur (1912) with Antirrhinum, 

 of Punnett (1913, 1917) with sweet peas, of Tanaka (1913, 1914) 

 with silkworm moths, and of Chambers (1914) with Drosophila. 

 In none of these cases is the evidence yet clear enough to warrant 

 detailed discussion. 



BEARING OF METHOD ON CHROMOSOME VIEW. 



The work reported in this paper deals with the effects on crossing - 

 over produced by certain definite genes. These genes do not, so far 

 as I have been able to discover, produce any visible somatic effects; 

 and their presence can not be detected, except in females, and in 

 females that are heterozygous for other genes in definite regions of 

 the chromosomes, i. e., that are capable of being tested for linkage 

 in those regions. In the case of other females, or of any males, such 

 tests can not be made directly, but only by producing female de- 

 scendants heterozygous for the necessary genes. The fact that it 

 has been possible to work out in great detail the inheritance of these 

 "invisible' ' genes and the effects produced by them is a striking illus- 

 tration of the possibilities of the chromosome view of inheritance and 

 of the advantages of using a rapidly breeding form like Drosophila. 



The chromosome view itself is perhaps not necessary for the handling 

 of such a case; but the conception of genes that form independent 

 groups that behave as units, the members of which are only separable 

 according to definite rules, is necessary. And such a conception, I 

 think, presupposes some material basis for the independent groups. 

 The great body of evidence that points to the chromosomes as forming 

 such a material basis is too familiar to need discussion here. 



