No. 546] SHORTER ARTICLES AND DISCUSSION 371 



low by agouti. No other crosses of yellow by yellow gave agouti, 

 so that it seems in the highest degree probable that the original 

 (PJ yellows were pure for t. That being the case the F x 

 yellows must all have had the formula tY Ty. But since they 

 produced only % yellows to y s agoutis ii follows that association 

 occurs between T and Y, thus : 



Ty tY 



tYTy -y ellows - 

 (tYtY) 



TyTy—1 agouti. 



These T-bearing yellows have also been bred to chocolates and 

 blacks. If there were no association this mating should produce 

 two yellows, one agouti, and one chocolate or black. But Miss 

 Durham has obtained only 30 yellows and 37 agoutis— practi- 

 cally equal numbers, with no blacks or chocolates. On the asso- 

 ciation hypothesis this cross should produce the following result: 



Yellow— tYTy 

 Chocolate or black— tyty 



tYty—1 yellow. 

 Ty ty—1 agouti. 



Apparently the ticking factor and the factor which produces 

 yellow mice arc associated very closely. There appears to be no 

 evidence that "crossing over" ever occurs. It can not be sup- 

 posed that r is the same as t, with which it always occurs, since 

 in that case all mice not agouti, or even heterozygous for T, 

 would be yellow, and Mack and chocolate would not exist. It 

 should be noted that if one adopts Castle's mouse formulae (see 

 Castle and Little, etc.), it is still necessary to suppose that 

 association occurs but now between the restriction factor, K, 

 and the agouti factor, A. 



A. H. Sturtevant 



•Science, N. S., 30, 313, 1909. 



