MENDELISM 63 



show colour. The remaining seven will lack either or 

 both, and will be white. Colour inheritance in sweet- 

 peas is one of the subjects which has been very 

 thoroughly investigated, chiefly by Professor Bateson 

 and his assistants at Cambridge, and many complex 

 relationships have been thoroughly unravelled. 



Other striking cases, which w r ere puzzling when first 

 observed, have turned up in the crossing of differently 

 coloured varieties of mice and rabbits. When an 

 ordinary grey rabbit is crossed with a white or albino, 

 grey is dominant. If the first generation greys be bred 

 together, the usual result is the ratio 3 grey : 1 white in 

 the second generation. But sometimes black animals 

 turn up in the second generation, the ratio in this case 

 being about 9 grey : 3 black : 4 white. 



The explanation which has been suggested for this 

 result is that two factors are responsible for the grey 

 colour. One of these is a general factor for colour, in 

 whose presence colour is produced, and in wiiose ab- 

 sence the animal is white. We may call this factor C. 

 If only C is present the colour will be black, but there 

 is an additional factor G, which changes the black colour 

 into grey. We may represent the result of crossing 

 diagrammatically by the " chess-board " method which 

 we used before. Taking a square of white paper we 

 may blacken three-fourths of it to represent the 

 fact that 75 per cent, of the second generation 

 wi\[ show the dominant character colour. Of these, 

 three-fourths will contain the dominant greying factor. 

 We may therefore divide each of the black squares 

 into four, and dot over three parts of each with white, 

 to indicate that 75 per cent, of the coloured animals 

 will be grey, and 25 per cent, black. This gives us 

 the proportions shown in Fig. 10 viz. 9 grey, 3 black, 

 4 white. 



It is interesting to note that of the four white in- 

 dividuals three will contain the factor G. In other 

 words, three will be " disguised greys " and one a 

 " disguised black," the colour in each case being sup- 

 pressed, owing to the absence of the factor C. What 



