226 ' CHARLES ZELENY 



is twice as great as in the male, since oogonia have two X chro- 

 mosomes and the spermatogonia only one. This is true not only 

 in case the mutation of single chromosomes is independent of 

 their location, but also in case mutations are due to enviromental 

 forces acting equally in male and female. In the latter case 

 there would be simultaneous mutation of both chromosomes in the 

 female. For the sake of simplicity in exposition, it may be as- 

 sumed that the mutations occur during the growth period preced- 

 ing maturation, though the result would be the same if the change 

 occurred at any time prior to this. If one of the two sex chromo- 

 somes of the primary oocyte contains a changed gene, the chance 

 is one in two that the mature ovum will obtain it. If the single 

 sex chromosome of the primary spermatocyte contains a changed 

 gene, one-half of the four resulting spermatozoa will contain it. 

 It follows on the hypothesis of equality in rate of mutability of 

 individual bar genes that there should be twice as large a propor- 

 tion of eggs with a changed bar factor as of changed spermatozoa. 

 This result, as stated above, is independent of the period during 

 the gonial divisions during which the mutations appear. 



Of the eleven mutants from bar to full observed by May ( '17) 

 six were full males and five heterozygous females. Assuming the 

 whole number of males and females examined to be equal, this 

 approach to equality in the number of mutants seemed to favor 

 the view that the mutations are confined to the females. This 

 conclusion led May further to consider the possibility that the 

 changes from full to bar and of bar to full may be cases of partial 

 nondisjunction. This latter hypothesis is improbable on other 

 grounds which need not be discussed here. 



Taking all the valid cases of mutation from bar to full in the 

 present report, there are thirty-one heterozygous daughters and 

 twenty full sons among the mutants. In these observations 

 there was no chance of overlooking full sons, but heterozygous 

 daughters derived from low individuals in unselected stocks may 

 have been overlooked because of their closeness to high-bar 

 homozygotes. Heterozygous females may likewise have been 

 overlooked in May's observations. This larger number of cases 

 therefore supports the view that the mutations may occur in 



