THE ORIGIN OF GYNANDROMORPHS. 



31 



It is a matter of great interest to find tliat all the ten cases of somatic 

 mutation that we have recorded in Drosophila have been males. The 

 significance of this was not appreciated until the material had been 

 sorted out for other purposes. It probably means that a recessive 

 somatic mutation takes place in the sex chromosome and shows at 

 once in a male in those parts of the body whose cells contain the mutant 

 gene because the male has only one sex chromosome. Should a reces- 

 sive mutation occur in the X chromosome of a female its effect would 

 not appear in the soma because the normal allelomorph would conceal it. 

 It is interesting to apply this point of view to certain results in Lepi- 

 doptera in which mosaics or gynandromorphs have been recorded that 

 carry in parts of the body characteristics that are known to occur, 

 although rarely, in varieties of sports of the species. 



Text-figuke 11. 



Text-figure 12. 



Text-figure 13. 



Among these a number have been described with one half of the 

 body of one species and the other half of a varietal type of the same 

 species. In some cases the variety is so rare that there might seem 

 to be no question of a hybrid cross involved, since this in itself would 

 be rare, and that both this and a later mosaic condition result is beyond 

 reasonable probability. An alternative view would be that of somatic 

 mutation. If such were the explanation we should expect the indi- 

 vidual to be female and the mutation to have occurred in the single 

 Z chromosome. 



In the cases brought together by Cockayne, in which the same 

 individual is partly one species, partly a variety (1915, pp. 87-90), 

 there are about 10 such cases recorded as females, 2 as males; in 12 

 cases no sex is stated by Cockayne. If further examination of the 

 original sources shows as high a percentage of females as in the recorded 

 cases, the evidence is in favor of the interpretation suggested above. 

 The males call for another interpretation, and each such case will 

 need special examination. 



