382 The Outline of Science 



quality of tallness dominant to the recessive quality of dwarf- 

 ness, which the hybrid offspring kept, as it were, up their sleeve. 

 The dwarfness is not expressed in the hybrid peas, but it must 

 be part of the inheritance, for it reappears in a quarter of the 

 progeny of the hybrids if these are inbred or allowed to self- 

 fertilise. 



The Japanese have reared a race of peculiar waltzing mice, 

 which have many strange habits, e.g. of dancing round and round. 

 If a Japanese waltzing mouse is crossed with a normal mouse, 

 all the hybrid offspring are normal, the waltzing peculiarity 

 being recessive to normality. But if these hybrid mice are paired 

 together, some of their progeny are waltzers in the proportion 

 of one waltzer to three normals, which is called the Mendelian 

 ratio. If one of the waltzers of the second generation pairs with 

 another waltzer, the progeny are all waltzers, which shows that 

 the factor for normal locomotion has disappeared from the in- 

 heritance along this line. It is a curious fact that one of these 

 second generation waltzers might be conscientiously sold in the 

 market as a pure waltzer, although its parents were normal and 

 one of its grandparents likewise. To return to the beginning, 

 if a waltzing mouse is crossed with a normal mouse, all the off- 

 spring will be normal. Normality is dominant; waltzing is 

 recessive. If these normal hybrids pair, their offspring will be 

 25 per cent, pure waltzers and 75 per cent, apparently normal 

 mice. But of the 75 per cent, apparently normal a third will 

 be pure normals, yielding nothing but normals when bred with 

 others like themselves. But the other two-thirds, though ap- 

 parently normal, have, like their immediate parents, the waltz- 

 ing character up their sleeve, for when they are paired together 

 they yield 25 per cent, pure normals, 50 per cent, apparent nor- 

 mals, and 25 per cent, pure waltzers. It is impossible to keep 

 this clearly in mind without some schematic formulation, such 

 as the above. 



In the case of the mice the character of normal locomotion 



