ALLKN. — THi: HKREDITY OF COAT COLOR IN MICE. 73 



to a heterozygous class similar to the parents, two pure classes, each 

 reproducing respectively the one or the other parent type. On the other 

 hand, when a heterozygote is back-crossed with either of the pure parental 

 types, — formulae (5) and (G), - no new forms result, but there is simply 

 a reproduction of the two which united in fertilization. 



For convenience in reference these formulae of Mendelian inheritance 

 may be brought together as follows : 



To the left of the sign of ecjuality is indicated the character of the 

 parents mated ; to the right, the classes and numerical proportions of the 

 offspring to be expected. 



(1) U XR = 4D(R) 



(2) D(R)x D(R) = D + 2D(R) + R 



(3) D X D = 4D 



(4) Rx R = 4R 



(5) D(R) x D=2D(R) + 2D 

 (G) D(R) X R=2D(R) + 2R 



A similar set of formulae may be deduced for cases in which the dom- 

 inant character is a mosaic of the two characters which, if occurring 

 separately, are to each other as dominant and recessive. Thus the char- 

 acter of a spotted mouse ({)igraented, but with white areas at certain 

 places) is dominant over that of a complete albino, and the pigmented 

 heterozygotes may produce gametes of two sorts, the one bearing the 

 character partial albinism, the other bearing the character complete 

 albinism. The various combinations possible for these two characters 

 may be exj^ressed by the following formulae : 



(la) DRx R = 4DR(R) 



(2a) Dili R) X DR( R) = DR + 2DR(R) + R 



(3a) DRxDR = 4DR 



(4a) R X R = 4R 



(5a) DR(R) X DR = 2DR -f 2DR(R) 



(6a) DRfR) X R =^ 2DR(R) + 2R 



These formulae indicate the results of various matingsin cases of simple 

 dominance and segregation. In cases where compound characters be- 

 come resolved and the elements retniite again in cross-breeding, it is not 

 certain that the relations are quite so simple. Nevertheless, the results 

 so obtained seem to indicate that here, too, segregation does take place in 

 a more or less definite way. 



