No. 599] THE GENETIC BEHAVIOR OF MICE 667 



classes of young. (1) One of these may be described as a 

 black-and-tan in which the black pigmentation is lessened 

 in amount and intensity, this decrease being attended by 

 an increased development of yellow pigmentation. This 

 class closely resembles the variety known as sable. (2) 

 The other class of young consisted of blacks, which also 

 were less intensely pigmented than the recessives pro- 

 duced by pure-bred black-and-tans mated inter se. It 

 was found convenient in classifying the young of later 

 generations to recognize six arbitrary grades of black- 

 ness of which yellow (showing no black pigment) forms 

 grade 1, and black-and-tan grade 6. On this scale the 

 mean of the F 2 " sable" young was close to 3.5, the inter- 

 mediate point between yellow and black-and-tan. The 

 distribution can be plotted by translating the descriptive 

 terms in Mr. Whittier's notes into terms of numerical 

 grades, as follows: 



These descriptive notes were made before the grading 

 scale had been adopted, and it is quite probable that no 

 real discontinuity in the variation occurred as would be 

 suggested by entire absence of animals of grade 4. No 

 such discontinuity is found in the work done since the 

 grading scale was adopted. 



The Fj black young were mated inter se and back- 

 crossed with browns to test their gametic composition. 

 When mated inter se they gave 28 black and 11 brown 

 young, nearly a 3:1 ratio (29:10). Back-crossed with 

 browns they gave 37 blacks and 33 brown young, nearly a 

 1 : 1 ratio (35 : 35). F t blacks apparently, then, were sim- 

 ple heterozygotes, not differing from ordinary heterozy- 

 gotes produced by crossing homozygous black with homo- 

 zygous brown. 



Thirteen of the F a sables were tested by mating with 

 browns. One hundred and thirty-three young resulted, of 

 which 70 were yellows of various shades and 63 non-yel- 

 lows. Of this latter group 32 were black and 21 were 



