370 



THE AMERICAN NATURALIST 



[Vol. XLVI 



together they produce two yellows to one black or chocolate, 

 no T being present. Her total figures are 451 yellows to 241 

 blacks and chocolates, to which should be added Little's 7 record 

 of 31 yellows to 24 blacks and chocolates. If ordinary yellows 

 be mated to chocolates or blacks the result is an equal number 

 of yellows and of blacks or chocolates. Miss Durham's figures 

 are 282 yellows and 319 blacks and chocolates. Both these 

 crosses are cases of monohybridism, T never being present, and 

 Y alone being involved. 



If ordinary yellows be bred to pure agoutis, the result should 

 be equal numbers of yellows and agoutis, as shown above. The 

 actual results obtained are as follows: 



53 %9 Durham^. 



16 14 Morgan. 



69 53 



If the P 2 agouti used in this cross were heterozygous in T, then 

 the expectation would be two yellows, one agouti, and one black 

 (or chocolate). Apparently Morgan is the only one who has 

 reported such a cross. He obtained 4 yellows, 5 agoutis, and 

 1 black. 



The results of the four crosses above are explicable without 

 the assumption of association between T and Y, since in no case 

 was an animal bred from which was heterozygous in both, and 

 only in such cases does association ever become apparent. But 

 such heterozygous mice should be obtained in the cross between 

 agouti and yellow. If these be mated together the ordinary 

 expectation, as shown by the diagram above, would be 8 yel- 

 lows, 3 agoutis, and 1 black or chocolate. The actual offspring 

 recorded from yellow by yellow giving agouti are : 



108 A 62 tl Black and Chocolate Vuihm. 



15 9 Morgan. 



125 71 0~ 



This is approximately two yellows to one agouti. It is the result 

 which would be expected if one of the parent yellows were pure 

 for T (TY Ty). But, with the possible exception of 20 of Miss 

 Durham's, 8 all the above 194 mice were from yellows out of yel- 

 T Science, N. S., 33, 896, 1911. 



8 It would appear from the context that these 20 also belong to the 



