No. 599] 



THE GENETIC BEHAVIOR OF MICE 



065 



lying yellow areas of the hairs. Yellow-ticked hairs are 

 occasionally seen on flanks and head, encroaching on the 

 black pigmented parts. This peculiarity increases some- 

 what with age, bnt never to such an extent as to make the 

 body color predominantly yellow. 



When bred inter se, they have been found invariably to 

 be heterozygous, no homozygous black-and-tan having 

 been discovered among a dozen individuals tested by suit- 

 able matings. As recessives they have given all-black 

 mice more intense than any we have seen derived from 

 other sources. Forty-two matings inter se of pure-bred 

 black-and-tan parents produced 148 young, an average of 

 3.52 to a litter. Of these young 93 have been black-and- 

 tan and 55 black, a ratio of 1.69 : 1. This approximates a 

 2 : 1 ratio more closely than the 3 : 1 ratio usually given by 

 Mendelian heterozygotes. The black recessives breed true, 

 and when mated to black-and-tans have produced equal 

 numbers of black-and-tan and black young (18 : 18). The 

 approximation of a 2 : 1 ratio in matings of black-and-tans 

 inter se shows their gametic similarity to yellow mice 

 whose unfixable nature was first shown by Cuenot ('03). 

 Figures given by this author combined with those given 

 by Castle and Little ( '10), by Little ( '10 and '11) and by 

 Miss Durham ('11) total 2,673 young produced by yellow 

 X yellow matings. Of these 1,783 were yellow and 890 

 non-yellow, a ratio of almost exactly 2:1. 



The small average size of litters produced by black-and- 

 tan parents mated inter se gives added evidence of their 

 resemblance to yellow. Castle and Little ('10), in con- 

 firmation of Cuenot's observations, showed that yellow X 

 yellow matings produced litters of smaller average size 

 (4.71) than yellow X non-yellow (5.57), and following 

 Cuenot thev attributed the difference to absence of homo- 

 zygous, yellow-yellow zygotes. The 2:1 ratio and the 

 small-sized litters serve also to relate the black-and-tans 

 with Castle's "sooties" and Miss Durham's " sables, 

 both of which were shown to be heterozygous yellows 

 carrying black as a recessive. 



