GENE FOR FOURTH CHROMOSOME OF DROSOFHILA 327 



genes in X, these are not normal allelomorphs (i.e., the same kind 

 as those present in the X of the wild fly) for they are recessive to 

 mutant genes in X to which the normal genes in X are dominant, 

 i.e., a male always manifests all the genes, mutant or normal, 

 recessive or dominant, that are present in its single X-chromosome. 

 In other words, if there be genes in Y allelomorphic to those in X, 

 they are abnormal allelomorphs of those in X, and are always 

 recessive to all genes in X. It is difficult to conceive why genes 

 in Y should be recessive, on the one hand, to normal genes in X, in 

 those cases where the mutant genes in X are dominant to the 

 normal, and, on the other hand, to mutant genes in X, in those 

 more frequent cases where the normal genes in X are dominant to 

 the mutant, unless the genes in Y are mere 'absences' or nearly so.^ 

 There is an a priori explanation for this lack of genes, or lack of 

 dominant genes, in the Y-chromosome, an explanation in the 

 development of which Dr. A. H. Sturtevant has cooperated with 

 me. Owing to the fact that crossing-over never occurs in the 

 male Drosophila, any mutation which originally occurred in Y 

 remained in that chromosome and was never exchanged for a 

 normal gene from X. Furthermore, these mutations in Y which 

 were recessive would not have been subject to the action of natural 

 selection for, since the normal gene in the X-chromosome will 

 dominate over them, individuals containing them will not be 

 abnormal. In the course of time, therefore, recessive changes in the 

 Y-chromosome will tend to accumulate. If, now, we assume that 

 mutations sometimes consist in losses^ of genes, a degeneration of 

 the Y-chromosome (so far as its genes are concerned) would result. 



- This might be otherwise explained on the very improbable hypothesis that all 

 of the mutations which occurred in X were in a restricted part (the end?) of the 

 chromosome, and that the Y lacked only this part of the chromosome and so ap- 

 peared to contain genes recessive to all those in X which underwent mutation, 

 although it actually contained normal genes in the other part of the chromosome, 

 which never mutated. This is not quite equivalent, although nearly so, to the 

 idea that X is attached to an autosome (see footnote 1). 



^ By losses of genes are meant not necessarily their total bodily disappearance 

 (losses of loci from the chromatin) but also any changes in them whereby they are 

 rendered permanently inactive and incapable, under any circumstances, of exert- 

 ing an influence on the organistn. 



