82 GERT BONNIER 



their maturation all possible combinations ot the sex-chromosomes 

 occur with the consequence that also these females will produce ex- 

 ceptional daughters and sons. This is the secondary non-disjunction, 

 and it follows that the exceptions arise from ovocytes with hetero- 

 synapsis (synapsis between .Y and Y) since it is possible only after 

 heterosynapsis to obtain two-A' or non-A'-containing eggs. After such 

 a heterosynapsis there are furthermore two different possibilities: the 

 free X may go with the synapsed A or it may go with the Y, and it is 

 from the first of these two cases that the exceptions result. Now, under 

 the condition that it is purely a matter of chance which of these two 

 possibilities it is that occurs Bridges derives the formula giving the 

 relation between the percentage of heterosynapsis {x) and the percen- 

 tage of exceptions {y): y= mux (^^^^^^^ \%l^ p. 17). And if there 



is a random distribution of the sex-chromosomes during synapsis, 

 which means that there are twice as many possibilities for AF-synapsis 

 as for XX-synapsis or that x is 66 % , this gives the value _i/ = 20 %. 

 But as a matter of fact Bridges found as a rule not 20 % of exceptions 

 but only 4,3 % (corresponding to a; = 16,5 %). 



One may therefore ask the reason why the XX-synapses are so 

 much commoner than the A'^Y-synapses. This, of course, is a question 

 of the cell-physiology, but if we could find different lines of non- 

 disjunction giving different percentages of exceptions we could at least 

 analyze the genetical basis of such a difference. Bridges himself found 

 a few cultures with exceptionally high percentages of exceptions and 

 he suggested that it was due to a mutation in the Y-chromosome. But 

 as his data are very few, they seem not to be conclusive. In a footnote 

 (p. 142) he states, however, that he has made extensive tests, which 

 prove this view and which show that the mean percentage of excep- 

 tions in this high non-disjunctional line is about 20. 



Now I possess a stock of non-disjunctional flies always giving a 

 high percentage of exceptions, viz. the so-called eosin line of high non- 

 disjunction, i. e. a line where the AAy-females carry eosin in both 

 the Z:s and forked in one of them and where these females in every 

 generation are mated to Bar males. This culture was sent to me by 

 Dr. MoHR in Christiania who brought it with him from Dr. Bridges in 

 New York, but I do not know anything about its origin. 



In order to examine if the high percentage of exceptions also in 

 this stock was due to something in the Y-chromosome I crossed at a 

 venture in masscultures exceptional females to various males. Thus 



