150 PROCEEDINGS OF THE AMERICAN ACADEMY. 



with one having the chocolate character, the former produced by the 

 albino, the latter by the golden-agouti. The black offspring, on this 

 hypothesis, may contain the chocolate in a recessive, or at least in a 

 segregable condition. Fortunately the two black young were a male 

 and a female, and they were therefore mated. In three litters, they 

 produced 6 black, 5 chocolate, and 1 albino offspring. On Mendeliau 

 principles, 25 per cent or 3 of these young are expected to be chocolate, 

 as regards pigment characters. There are actually 2 more chocolates 

 than expected, disregarding the albino, whose character was not deter- 

 mined. It therefore seems jiossible that these two black mice did arise 

 in the manner suggested, and produced gametes, some of which were 

 black and some chocolate, as regards pigment characters. This same 

 pair has been previously mentioned in discussing the relation of choco- 

 late to black. 



The gray heterozygotes resulting from the cross of golden-agoutis 

 with albinos may have originated either (1) by the dominance of the 

 gray character which came from the house mouse through the albinos, 

 or (2) by the union of the yellow-chocolate character furnished by the 

 golden-agouti parent with the black character, which, as well as gray, the 

 albinos transmitted, though to a lesser degree. If, now, at the formation 

 of gametes the color characters undergo segregation to the extent only 

 of undoing the original (heterozygous) combination of gray and 

 chocolate-yellow, or black and chocolate-yellow, it is plain that genera- 

 tion Fg will consist of offspring exhibiting three types of color, as seen 

 in the following diagram of the possible unions : 



gr. -1- yell.-ch. = gametes of gray hybrid of first sort, 

 gr. -|- yell.-ch. = gametes of gray hybrid of first sort, 



3 gr. -f- 1 gold-ag. = offspring. 



bl.-ch. -f yell.-ch. = gametes of gray hybrid of second sort, 

 bl.-ch. -|- yell.-ch. = gametes of gray hybrid of second sort, 



1 bl. -|- 2 gr. + 1 gold.-ag. = offspring. 



gr. -f yell.-ch. = gametes of gray liybrid of first sort, 



bl.-ch -I- yell.-ch. = gametes of gray hybrid of second sort, 



3 gr, 4- 1 gold.-ag. =: offspring. 



The chances in favor of the third combination are twice as many as 

 for either of the other two, hence generation Fo should consist of young 



