MENDELISM 441 



allelomorphic pairs, 7icithcr of zvhich can be manifested in the 

 absence of the other. These factors have been called C and R, 

 their absences c and r respectively. If we write the familiar 

 9:3:3:1 scheme again, we get 



9 C R, 3 cR, 3 Cr, I cr. 



Only in the 9 C R are both C and R present; in the 3 cR and the 

 3 Cr only one of them is present, whilst in the i cr neither of 

 them is. So that only the 9 are coloured and the remainder are 

 all white. 



The actual proportion of purples and reds and whites in 

 F2 in this instance is 27 purple, 9 red, 28 white (= 36 coloured 

 to 28 white, i.e. 9 coloured to 7 white). The existence and 

 proportions of these reds are accounted for by calling in a new 

 pair of allelomorphs, B and b. As already stated, colour is due 

 to the meeting of C with R; if B is absent the colour is red, 

 if present it is purple. But we will not deal further with 

 this now. 



Having now seen how these complicated cases of segregation, 

 together with two instances of reversion, have been elucidated 

 by Mendelian analysis, let us turn to other groups of the animal 

 and vegetable kingdoms, and see what other characters have been 

 dealt with in this way. 



Except for the pair of allelomorphs with which we illustrated 

 the simple Mendelian phenomenon, and for the very super- 

 ficial reference to roundness and wrinkledness in peas, all 

 the characters we have dealt with so far have been colour 

 characters. But Mendelian inheritance is by no means confined 

 to colour characters, as we shall shortly see. Perhaps one of 

 the most striking instances of this is that studied by von Guaita, 

 Weldon, and myself, the waltzing habit in mice. The waltzing 

 habit characterises a race of mice known as Japanese waltzing 

 mice, which have pink eyes and small patches of fawn on the 

 shoulders and rump. A waltzing mouse does not always waltz, 

 but can always be distinguished immediately from a normal 

 mouse. The waltzing itself consists in merely spinning round 

 and round very swiftly in one spot on all-fours. It is not in 

 the least like waltzing. It is simply running round in a circle 

 sometimes at a very great pace for several minutes at a time, 

 so that all that you can see is a sort of nebulous ring of mouse. 

 They do not always go round in the same direction, but sometimes 



