No. 599] 



THE GENETIC BEHAVIOR OF MICE 



671 



at the possible difference between these forms and ordi- 

 nary yellows. It is time now to inquire as to the real 

 genetic nature of these mice, and to attempt a preliminary 

 explanation of their differences from yellow. By far the 

 largest number of mice have been bred and are being bred 

 toward this end. 



Let us consider first the black-and-tan variation. By 

 its behavior it evidently forms two sorts of gametes, black- 

 and-tan (yellow) and black. Each of these has an added 

 something which makes the zygote into whose composition 

 it enters darker than ordinary yellow or black. We may 

 call this something "darkener"— be it singular or plural 

 —and indicate the gametes by YD and BD. Eed, similarly, 

 forms gametes red (yellow) and brown; and these also 

 show an addition which we may call ' ' intensifier. " The 

 gametes of red are then YI and bl. The sables produced 

 by red X black-and-tan can only be referable to a union of 

 YD and bl, or YI and BD since YDYI is non-viable, and 

 since YDbl and YIBD unions have been demonstrated in 

 the brown X black-and-tan and red X black crosses, re- 

 spectively, and have produced in both of these latter cases 

 similar sables. The presence of the darkener and the in- 

 tensifier in the same zygote weakens both and demon- 

 strates their physiological and genetic independence. 



The next point to be noted is that both darkener and in- 

 tensifier are variable. All gametes formed by zygotes 

 containing D or I are not equivalent in their D or I con- 

 tent. It is possible to demonstrate this for the darkener; 

 the variable intensification from crosses with red cannot 

 yet be as satisfactorily shown on account of the difficulties 

 of grading. For light on the action of the darkener we 

 may turn to the agouti crosses. 



The ordinary wild house-mouse when bred pure, shows 

 the agouti pattern and gray color with great uniformity. 

 It possesses the black and yellow pigments of the black- 

 and-tan mouse as well as brown pigment, but contains no 

 factor to dilute or darken these pigments. These facts 

 make it an ideal race with which to test for a suspected 



