Mr Doncastev, On the Inheritance of Coat-colour in Rats. 219 



There seems to be no doubt that the brown forms (Crampe's 

 types 1, 2, 3) correspond exactly with the similar black forms 7, 

 6, and 5, but are less simple to work with since they may contain 

 recessive black. They also have the disadvantage that the extent 

 of the white area in type 2 is much less easily determined at an 

 early age, since the grey of the underside is not at first distin- 

 guishable from white. I therefore chose the black forms to ex- 

 periment with, and have made only a few matings with the brown 

 varieties. 



It soon became evident that the two sub-varieties of type 

 6 (" Irish ") are distinct from one another, and in the later pairings 

 recorded in the tables these are distinguished as 6 a and 6 b 

 (Irish a and Irish b). Unfortunately these two sub-varieties were 

 not recorded separately in the earlier matings. Where one of the 

 parents was black-and-white (type 5) all the " Irish " offspring were 

 always of the form with a large white patch (6 b) ; this is seen in 

 matings 34 and 45, and was almost certainly also the case in 

 No. 33, where, however, it was recorded only in the last litter. 

 The brown young in No. 33 (type 2) also had much white under- 

 neath. When " Irish " rats were paired together, in all the cases 

 where any black-and-white young were produced (matings Nos. 

 42, 41, 56), both parents had a considerable amount of white on 

 the underside. On the other hand in experiments 35, 35 a, and 55, 

 no black-and-white young were produced, and in each case one 

 parent had only a very small white mark. The other parent in 

 these matings had more white, and the coloured offspring were 

 divided between the two sub-types. In Exp. 63 both parents 

 .were type 6 a, and no young of either 6 b, or 5 (blk.-wh.) were 

 produced. It seems, therefore, that type 6 b ("Irish" with much 

 whiteO; can be produced by mating black-and-white with black 

 (Exp. 34), or with an albino of " Irish " parentage (Exp. 45), and 

 that when two such individuals are paired together, the offspring 

 consist of black-and-white, " Irish " b, and " Irish " a. This points 

 to the conclusion that type Qb is a heterozygous form produced 

 by crossing black or " Irish " a with black-and-white, which then 

 separate in the next generation in the normal Mendelian manner. 

 Type 6 a and black on the other hand clo not contain black-and- 

 white, and therefore in matings in which they are concerned no 

 black-and-white offspring appear. 



The relations between type 6 a and pure black are less clear. 

 Crampe found that blacks (type 7) bred together might give 

 type 6, but they seemed to appear in quite irregular proportions, 

 e.g. from some pairs only blacks appeared, in one mating 26 black 

 to 6 " Irish " and in another G blacks to 27 " Irish." From 

 Crampe's tables it appears that when " Irish " rats arc bred 

 together, pure blacks appear only when they have occurred in the 



