IN THE SECOND CHROMOSOME. 320 



Other data known. But the other data for Ci Cur show con- 

 clusively that b and p, give very little crossinK-ovor (0.2 per cent) 

 while either with v„ c, or Sp, gives about 1.1 per cent; and v and c 

 give only 0.1 or 0.2 per cent with s,. That is, v, and c are 'on the 

 same side of b and p,. And the extensive data for b p, c show that 

 the sequence is b p, c. Therefore the one individual that suggested 

 the sequence p, b Vg must have been a double cross-over. 



(2) In the case of CVr only three cross-overs between c and «, 

 were obtained. Of these, two were also cross-overs between 6 and c, 

 while one was not. These data alone would indicate the sequence 

 as bSpC, instead of the usual b c s^. No great significance can be 

 attached to the difference between 2 flies and 1 fly among a total of 

 1,615. In any case, the data suggest a ver>' high coincidence. Mcjre 

 data of the same sort will be necessary before this exceptional case 

 can appear significant.^ 



C C 



(3) In the case of ^ ', only 3 cross-overs were obsen-ed 



between b and p,. All of these were also cross-overs between p, and 

 Sp.^ If the coincidence in this case is 100, approximately the value 

 usual for b Pr c, then nearly half of the b p, cross-overs should be also 

 Pr Sp cross-overs. Therefore the fact that all 3 were such doubles 

 need not cause surprise; even though, taken alone, it would indicate 

 the sequence a^ p^ b Sp. 



The three exceptional cases are, then, of no great significance, ex- 

 cept as indicating rather high coincidence. There are a large numlxr 

 of cases in which the evidence is much clearer and in which the sequence 

 is certainly the same as that usually found. 



HOW DO Cm AND Cur PRODUCE THEIR EFFECTS? 



The question of the mechanism whereby the cross-over genes pro- 

 duce their effects is not yet satisfactorih' answered. Cytological 

 examination might conceivably furnish the solution, but has not yet 

 been seriously attempted. A study of coincidence might give a clue, 

 but is difficult to make, because of the very small percentages tliat are 

 concerned. 



In the case of Cur and Cm it is to be noted that two like chromo- 

 somes cross over freely, while two unlike ones do not.' \Miile this is 

 only a restatement of the facts, it at least offers an attractive o|XMiing 

 for speculation as to the nature of the case. 



*In a culture derived from Cm, u experiments discussed above, a female that wan apparently 

 of the constitution r (without Cju, n) has recently been toatcd. One », daushter waa 



O Pf C Sp 



produced. If this record represents what it appears to, the count l^ccomes 2 double croM-OTera 

 against 2 single cross-overs. 



^Two of them were also recorded as cross-overs between 5' and 6; but thia u prolwibly iDcorrect. 

 as was pointed out above (p. — ). Cm 



'So far as the evidence goes, this is also true for Cn |, but t; is unknown. 



